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F5 SIRT

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on 27-Feb-2017 18:19

It seems like every time you turn around there is a new vulnerability to deal with, and some of them, such as Sweet32, have required altering cipher configurations for mitigation. Still other users may tweak their cipher suite settings to meet requirements for PCI compliance, regulatory issues, local compatibility needs, etc.

However, once you start modifying your cipher suite settings you must take great care, as it is very easy to shoot yourself in the foot. Many misconfigurations will silently fail – seeming to achieve the intended result while opening up new, even worse, vulnerabilities. Let's take a look at cipher configuration on the F5 BIG-IP products to try stay on the safe path.

Before we talk about how they're configured, let's define exactly what we mean by '**cipher suite**', how it differs from just a '**cipher**', and the components of the suite.

Wikipedia had a good summary, so rather than reinvent the wheel:

A cipher suite is a named combination of authentication, encryption, message authentication code (MAC) and key exchange algorithms used to negotiate the security settings for a network connection using the Transport Layer Security (TLS) / Secure Sockets Layer (SSL) network protocol.

When we talk about configuring *ciphers* on BIG-IP we're really talking about configuring *cipher suites*. More specifically the configured list of cipher suites is a menu of options available to be negotiated. Each cipher suite specifies the key exchange algorithm, authentication algorithm, cipher, cipher mode, and MAC that will be used.

I recommend reading **K15194: Overview of the BIG-IP SSL/TLS cipher suite** for more information. But as a quick overview, let's look at a couple of example cipher suites.

The cipher suite is in the format:

**Key Exchange-Authentication-Cipher-Cipher Mode-MAC**

Note that not all of these components may be explicitly present in the cipher suite, but they are still implicitly part of the suite.

Let's consider this cipher suite:

**ECDHE-RSA-AES256-GCM-SHA384**

This breaks down as follows:

- Key Exchange Algorithm: ECDHE (Elliptic Curve Diffie-Hellman Ephemeral)
- Authentication Algorithm: RSA
- Cipher: AES256 (aka AES with a 256-bit key)
- Cipher Mode: GCM (Galois/Counter Mode)
- MAC: SHA384 (aka SHA-2 (Secure Hash Algorithm 2) with 384-bit hash)

This is arguably the strongest cipher suite we have on BIG-IP at this time. Let's compare that to a simpler cipher suite:

**AES128-SHA**

- Key Exchange Algorithm: RSA (Implied) – When it isn't specified, presume RSA.
- Authentication Algorithm: RSA (Implied) – When it isn't specified, presume RSA.
- Cipher: AES128 (aka AES with a 128-bit key)
- Cipher Mode: CBC (Cipher Block Chaining) (Implied) – When it isn't specified, presume CBC.
- MAC: SHA1 (Secure Hash Algorithm 1; SHA-1 always produces a 160-bit hash.)

This example illustrates that the cipher suite may not always explicitly specify every parameter, but they're still there. There are 'default' values that are fairly safe to presume when not otherwise specified. If an algorithm isn't specified, it is RSA. That's a safe bet. And if a cipher mode isn't specified it is CBC. Always CBC.

Note that *all* ciphers currently supported on BIG-IP are CBC mode *except* for AES-GCM and RC4. ** ALL.** I stress this as it has been a recurring source of confusion amongst customers. It isn't only the cipher suites which explicitly state 'CBC' in their name.

Let's examine each of these components. This article is primarily about cipher suite configuration and ciphers, and not the SSL/TLS protocol, so I won't dive too deeply here, but I think it helps to have a basic understanding. Forgive me if I simplify a bit.

As a quick review of the difference between *asymmetric key *(aka public key) cryptography and *symmetric key *cryptography:

With the asymmetric key you have two keys – K_{public} and K_{private }– which have a mathematical relationship. Since you can openly share the public key there is no need to pre-share keys with anyone. The downside is that these algorithms are computationally expensive. Key lengths for a common algorithm such as RSA are at least 1024-bit, and 2048-bit is really the minimally acceptable these days.

Symmetric key has only K_{private}. Both ends use the same key, which poses the problem of key distribution. The advantage is higher computational performance and common key sizes are 128-bit or 256-bit.

SSL/TLS, of course, uses both public and private key systems – the Key Exchange Algorithm is the public key system used to exchange the symmetric key. Examples you'll see in cipher suites include ECDHE, DHE, RSA, ECDH, and ADH.

The Authentication Algorithm is sometimes grouped in with the Key Exchange Algorithm for configuration purposes; 'ECDHE_RSA' for example. But we'll consider it as a separate component.

This is the algorithm used in the SSL/TLS handshake for the server to sign (using the server's private key) elements sent to the client in the negotiation. The client can authenticate them using the server's public key.

Examples include: RSA, ECDSA, DSS (aka DSA), and Anonymous.

Anonymous means **no** authentication; this is generally bad. The most common way users run into this is by accidentally enabling an 'ADH' cipher suite. More on this later when we talk about pitfalls.

Note that when RSA is used for the key exchange, authentication is inherent to the scheme so there really isn't a separate authentication step. However, most tools will list it out for completeness.

To borrow once again from Wikipedia:

In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, 'cipher' is synonymous with 'code', as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.

This is what most of us mean when we refer to 'configuring ciphers'. We're primarily interested in controlling the cipher used to protect our information through encryption. There are many, many examples of ciphers which you may be familiar with: DES (Data Encryption Standard), 3DES (Triple DES), AES (Advanced Encryption Standard), RC4 (Rivest Cipher 4), Camellia, RC6, RC2, Blowfish, Twofish, IDEA, SEED, GOST, Rijndael, Serpent, MARS, etc.

For a little cipher humor, I recommend **RFC2410: The NULL Encryption Algorithm and Its Use With IPsec.**

Roughly speaking, ciphers come in two types – block ciphers and stream ciphers.

**Block Ciphers**

Block ciphers operate on fixed-length chunks of data, or blocks. For example, DES operates on 64-bit blocks while AES operates on 128-bit blocks. Most of the ciphers you'll encounter are block ciphers.

Examples: DES, 3DES, AES, Blowfish, Twofish, etc.

**Stream Ciphers**

Stream ciphers mathematically operate on each bit in the data flow individually. The most commonly encountered stream cipher is RC4, and that's deprecated. So we're generally focused on block ciphers, not that it really changes anything for the purposes of this article.

All of the secrecy in encryption comes from the key that is used, not the cipher itself. Obtain the key and you can unlock the ciphertext. The cipher itself – the algorithm, source code, etc. – not only *can* be, but *should* be, openly available. History is full of examples of private cryptosystems failing due to weaknesses missed by their creators, while the most trusted ciphers were created via open processes (AES for example).

Keys are of varying lengths and, generally speaking, the longer the key the more secure the encryption. DES only had 56-bits of key data, and thus is considered insecure. We label 3DES as 168-bit, but it is really only equivalent to 112-bit strength. (More on this later.) Newer ciphers, such as AES, often offer options – 128-bits, 192-bits, or 256-bits of key.

Remember, a 256-bit key is far more than twice as strong as a 128-bit key. It is 2^{128} vs. 2^{256} - 3.4028237e+38 vs. 1.1579209e+77

Cipher mode is the mode of operation used by the cipher when encrypting plaintext into ciphertext, or decrypting ciphertext into plaintext. The most common mode is CBC – Cipher Block Chaining. In cipher block chaining the ciphertext from block n feeds into the process for block n+1 – the blocks are chained together.

To ~~steal~~ borrow an image from Wikipedia:

As I mentioned previously, ** all** ciphers on BIG-IP are CBC mode

GCM stands for Galois/Counter Mode, a more advanced mode of operation than CBC. In GCM the blocks are not chained together. GCM runs in an Authenticated Encryption with Associated Data (AEAD) mode which eliminates the separate per-message hashing step, therefore it can achieve higher performance than CBC mode on a given HW platform. It is also immune to classes of attack that have harried CBC, such as the numerous padding attacks (BEAST, Lucky 13, etc.)

Via Wikipedia:

The main drawback to AES-GCM is that it was only added in TLSv1.2, so any older clients which don't support TLSv1.2 cannot use it.

There are other cipher suites officially supported in TLS which have other modes, but F5 does not currently support those ciphers so we won't get too deep into that. Other ciphers include AES-CCM (CTR mode with a CBC MAC; CTR is Counter Mode), CAMELLIA-GCM (CAMELLIA as introduced in 12.0.0 is CBC), and GOST CNT (aka CTR). We may see these in the future.

What did we ever do before Wikipedia?

A hash function is any function that can be used to map data of arbitrary size to data of fixed size. The values returned by a hash function are called hash values, hash codes, digests, or simply hashes. One use is a data structure called a hash table, widely used in computer software for rapid data lookup. Hash functions accelerate table or database lookup by detecting duplicated records in a large file. An example is finding similar stretches in DNA sequences. They are also useful in cryptography. A cryptographic hash function allows one to easily verify that some input data maps to a given hash value, but if the input data is unknown, it is deliberately difficult to reconstruct it (or equivalent alternatives) by knowing the stored hash value. This is used for assuring integrity of transmitted data, and is the building block for HMACs, which provide message authentication.

In short, the MAC provides message integrity. Hash functions include MD5, SHA-1 (aka SHA), SHA-2 (aka SHA128, SHA256, & SHA384), and AEAD (Authenticated Encryption with Associated Data). MD5 has long since been rendered completely insecure and is deprecated. SHA-1 is now being 'shamed', if not blocked, by browsers as it is falling victim to advances in cryptographic attacks. While some may need to continue to support SHA-1 cipher suites for legacy clients, it is encouraged to migrate to SHA-2 as soon as possible – especially for digital certificates.

Now that we've covered what cipher suites *are, *let's look at where we use them.

There are two distinct and separate areas where cipher suites are used – the host, or control plane, and TMM, or the data plane. On the host side SSL/TLS is handled by OpenSSL and the configuration follows the standard OpenSSL configuration options.

The primary use of SSL/TLS on the control plane is for httpd. To see the currently configured cipher suite, use '`tmsh list sys http ssl-ciphersuite`

'. The defaults may vary depending on the version of TMOS. For example, these were the defaults in 12.0.0:

`tmsh list sys http ssl-ciphersuite`

sys httpd {

ssl-ciphersuite DEFAULT:!aNULL:!eNULL:!LOW:!RC4:!MD5:!EXP

}

As of 12.1.2 these have been updated to a more explicit list:

`tmsh list sys http ssl-ciphersuite`

sys httpd {

ssl-ciphersuite ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA:ECDHE-ECDSA-AES128-SHA256:ECDHE-ECDSA-AES256-SHA384:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA:AES256-SHA:AES128-SHA256:AES256-SHA256:ECDHE-RSA-DES-CBC3-SHA:ECDHE-ECDSA-DES-CBC3-SHA:DES-CBC3-SHA

}

You can change this configuration via '

`tmsh modify sys http ssl-ciphersuite <value>`

'.One important thing to note is that the default is not just 'DEFAULT' as it is on the data plane. This is one thing that users have been caught by; thinking that setting the keyword to 'DEFAULT' will reset the configuration.

As OpenSSL provides SSL/TLS support for the control plane, if you want to see which ciphers will actually be supported you can use '`openssl ciphers -v <cipherstring>`

'.

For example:

`openssl ciphers -v 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA:ECDHE-ECDSA-AES128-SHA256:ECDHE-ECDSA-AES256-SHA384:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA:AES256-SHA:AES128-SHA256:AES256-SHA256:ECDHE-RSA-DES-CBC3-SHA:ECDHE-ECDSA-DES-CBC3-SHA:DES-CBC3-SHA'`

ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH Au=RSA Enc=AESGCM(128) Mac=AEAD ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AESGCM(256) Mac=AEAD ECDHE-RSA-AES128-SHA SSLv3 Kx=ECDH Au=RSA Enc=AES(128) Mac=SHA1 ECDHE-RSA-AES256-SHA SSLv3 Kx=ECDH Au=RSA Enc=AES(256) Mac=SHA1 ECDHE-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH Au=RSA Enc=AES(128) Mac=SHA256 ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AES(256) Mac=SHA384 ECDHE-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AESGCM(128) Mac=AEAD ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AESGCM(256) Mac=AEAD ECDHE-ECDSA-AES128-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=AES(128) Mac=SHA1 ECDHE-ECDSA-AES256-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=AES(256) Mac=SHA1 ECDHE-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AES(128) Mac=SHA256 ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AES(256) Mac=SHA384 AES128-GCM-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AESGCM(128) Mac=AEAD AES256-GCM-SHA384 TLSv1.2 Kx=RSA Au=RSA Enc=AESGCM(256) Mac=AEAD AES128-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA1 AES256-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(256) Mac=SHA1 AES128-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA256 AES256-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AES(256) Mac=SHA256 ECDHE-RSA-DES-CBC3-SHA SSLv3 Kx=ECDH Au=RSA Enc=3DES(168) Mac=SHA1 ECDHE-ECDSA-DES-CBC3-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=3DES(168) Mac=SHA1 DES-CBC3-SHA SSLv3 Kx=RSA Au=RSA Enc=3DES(168) Mac=SHA1

Now let's see what happens if you use 'DEFAULT':

`openssl ciphers -v 'DEFAULT'`

ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AESGCM(256) Mac=AEAD ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AESGCM(256) Mac=AEAD ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AES(256) Mac=SHA384 ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AES(256) Mac=SHA384 ECDHE-RSA-AES256-SHA SSLv3 Kx=ECDH Au=RSA Enc=AES(256) Mac=SHA1 ECDHE-ECDSA-AES256-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=AES(256) Mac=SHA1 DHE-DSS-AES256-GCM-SHA384 TLSv1.2 Kx=DH Au=DSS Enc=AESGCM(256) Mac=AEAD DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(256) Mac=AEAD DHE-RSA-AES256-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(256) Mac=SHA256 DHE-DSS-AES256-SHA256 TLSv1.2 Kx=DH Au=DSS Enc=AES(256) Mac=SHA256 DHE-RSA-AES256-SHA SSLv3 Kx=DH Au=RSA Enc=AES(256) Mac=SHA1 DHE-DSS-AES256-SHA SSLv3 Kx=DH Au=DSS Enc=AES(256) Mac=SHA1 DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(256) Mac=SHA1 DHE-DSS-CAMELLIA256-SHA SSLv3 Kx=DH Au=DSS Enc=Camellia(256) Mac=SHA1 ECDH-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AESGCM(256) Mac=AEAD ECDH-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AESGCM(256) Mac=AEAD ECDH-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AES(256) Mac=SHA384 ECDH-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AES(256) Mac=SHA384 ECDH-RSA-AES256-SHA SSLv3 Kx=ECDH/RSA Au=ECDH Enc=AES(256) Mac=SHA1 ECDH-ECDSA-AES256-SHA SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=AES(256) Mac=SHA1 AES256-GCM-SHA384 TLSv1.2 Kx=RSA Au=RSA Enc=AESGCM(256) Mac=AEAD AES256-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AES(256) Mac=SHA256 AES256-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(256) Mac=SHA1 CAMELLIA256-SHA SSLv3 Kx=RSA Au=RSA Enc=Camellia(256) Mac=SHA1 PSK-AES256-CBC-SHA SSLv3 Kx=PSK Au=PSK Enc=AES(256) Mac=SHA1 ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH Au=RSA Enc=AESGCM(128) Mac=AEAD ECDHE-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AESGCM(128) Mac=AEAD ECDHE-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH Au=RSA Enc=AES(128) Mac=SHA256 ECDHE-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AES(128) Mac=SHA256 ECDHE-RSA-AES128-SHA SSLv3 Kx=ECDH Au=RSA Enc=AES(128) Mac=SHA1 ECDHE-ECDSA-AES128-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=AES(128) Mac=SHA1 DHE-DSS-AES128-GCM-SHA256 TLSv1.2 Kx=DH Au=DSS Enc=AESGCM(128) Mac=AEAD DHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(128) Mac=AEAD DHE-RSA-AES128-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(128) Mac=SHA256 DHE-DSS-AES128-SHA256 TLSv1.2 Kx=DH Au=DSS Enc=AES(128) Mac=SHA256 DHE-RSA-AES128-SHA SSLv3 Kx=DH Au=RSA Enc=AES(128) Mac=SHA1 DHE-DSS-AES128-SHA SSLv3 Kx=DH Au=DSS Enc=AES(128) Mac=SHA1 DHE-RSA-SEED-SHA SSLv3 Kx=DH Au=RSA Enc=SEED(128) Mac=SHA1 DHE-DSS-SEED-SHA SSLv3 Kx=DH Au=DSS Enc=SEED(128) Mac=SHA1 DHE-RSA-CAMELLIA128-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(128) Mac=SHA1 DHE-DSS-CAMELLIA128-SHA SSLv3 Kx=DH Au=DSS Enc=Camellia(128) Mac=SHA1 ECDH-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AESGCM(128) Mac=AEAD ECDH-ECDSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AESGCM(128) Mac=AEAD ECDH-RSA-AES128-SHA256 TLSv1.2 Kx=ECDH/RSA Au=ECDH Enc=AES(128) Mac=SHA256 ECDH-ECDSA-AES128-SHA256 TLSv1.2 Kx=ECDH/ECDSA Au=ECDH Enc=AES(128) Mac=SHA256 ECDH-RSA-AES128-SHA SSLv3 Kx=ECDH/RSA Au=ECDH Enc=AES(128) Mac=SHA1 ECDH-ECDSA-AES128-SHA SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=AES(128) Mac=SHA1 AES128-GCM-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AESGCM(128) Mac=AEAD AES128-SHA256 TLSv1.2 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA256 AES128-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA1 SEED-SHA SSLv3 Kx=RSA Au=RSA Enc=SEED(128) Mac=SHA1 CAMELLIA128-SHA SSLv3 Kx=RSA Au=RSA Enc=Camellia(128) Mac=SHA1 PSK-AES128-CBC-SHA SSLv3 Kx=PSK Au=PSK Enc=AES(128) Mac=SHA1 ECDHE-RSA-RC4-SHA SSLv3 Kx=ECDH Au=RSA Enc=RC4(128) Mac=SHA1 ECDHE-ECDSA-RC4-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=RC4(128) Mac=SHA1 ECDH-RSA-RC4-SHA SSLv3 Kx=ECDH/RSA Au=ECDH Enc=RC4(128) Mac=SHA1 ECDH-ECDSA-RC4-SHA SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=RC4(128) Mac=SHA1 RC4-SHA SSLv3 Kx=RSA Au=RSA Enc=RC4(128) Mac=SHA1 RC4-MD5 SSLv3 Kx=RSA Au=RSA Enc=RC4(128) Mac=MD5 PSK-RC4-SHA SSLv3 Kx=PSK Au=PSK Enc=RC4(128) Mac=SHA1 ECDHE-RSA-DES-CBC3-SHA SSLv3 Kx=ECDH Au=RSA Enc=3DES(168) Mac=SHA1 ECDHE-ECDSA-DES-CBC3-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=3DES(168) Mac=SHA1 EDH-RSA-DES-CBC3-SHA SSLv3 Kx=DH Au=RSA Enc=3DES(168) Mac=SHA1 EDH-DSS-DES-CBC3-SHA SSLv3 Kx=DH Au=DSS Enc=3DES(168) Mac=SHA1 ECDH-RSA-DES-CBC3-SHA SSLv3 Kx=ECDH/RSA Au=ECDH Enc=3DES(168) Mac=SHA1 ECDH-ECDSA-DES-CBC3-SHA SSLv3 Kx=ECDH/ECDSA Au=ECDH Enc=3DES(168) Mac=SHA1 DES-CBC3-SHA SSLv3 Kx=RSA Au=RSA Enc=3DES(168) Mac=SHA1 PSK-3DES-EDE-CBC-SHA SSLv3 Kx=PSK Au=PSK Enc=3DES(168) Mac=SHA1 EDH-RSA-DES-CBC-SHA SSLv3 Kx=DH Au=RSA Enc=DES(56) Mac=SHA1 EDH-DSS-DES-CBC-SHA SSLv3 Kx=DH Au=DSS Enc=DES(56) Mac=SHA1 DES-CBC-SHA SSLv3 Kx=RSA Au=RSA Enc=DES(56) Mac=SHA1 EXP-EDH-RSA-DES-CBC-SHA SSLv3 Kx=DH(512) Au=RSA Enc=DES(40) Mac=SHA1 export EXP-EDH-DSS-DES-CBC-SHA SSLv3 Kx=DH(512) Au=DSS Enc=DES(40) Mac=SHA1 export EXP-DES-CBC-SHA SSLv3 Kx=RSA(512) Au=RSA Enc=DES(40) Mac=SHA1 export EXP-RC2-CBC-MD5 SSLv3 Kx=RSA(512) Au=RSA Enc=RC2(40) Mac=MD5 export EXP-RC4-MD5 SSLv3 Kx=RSA(512) Au=RSA Enc=RC4(40) Mac=MD5 export

As you can see that enables far, far more ciphers, including a number of unsafe ciphers – export, MD5, DES, etc. This is a good example of why you *always* want to confirm your cipher settings and check exactly what is being enabled before placing new settings into production. Many security disasters could be avoided if everyone doublechecked their settings first.

Let’s take a closer look at how OpenSSL represents one of the cipher suites:

**ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AESGCM(256) Mac=AEAD**

The columns are:

- Cipher Suite: ECDHE-RSA-AES256-GCM-SHA384
- Protocol: TLSv1.2
- Key Exchange Algorithm (Kx): ECDH
- Authentication Algorithm (Au): RSA
- Cipher/Encryption Algorithm (Enc): AESGCM(256)
- MAC (Mac): AEAD

Since the control plane uses OpenSSL you can use the standard OpenSSL documentation, so I won't spend a lot of time on that.

In TMM the cipher suites are configured in the *Ciphers* field of the **Client SSL** or **Server SSL** profiles. See **K14783: Overview of the Client SSL profile (11.x - 12.x)** & **K14806: Overview of the Server SSL profile (11.x - 12.x)**, respectively for more details. It is important to keep in mind that these are two different worlds with their own requirements and quirks.

As most of the configuration activity, and security concerns, occur on the public facing side of the system, we'll focus on the Client SSL Profile. Most of the things we'll cover here will also apply to the Server SSL profile.

In the GUI it appears as an editable field:

Presuming the profile was created with the name 'Test':

`tmsh list ltm profile client-ssl Test`

ltm profile client-ssl Test { app-service none cert default.crt cert-key-chain { default { cert default.crt key default.key } } chain none ciphers DEFAULT defaults-from clientssl inherit-certkeychain true key default.key passphrase none }

Modifying the cipher configuration from the command line is simple.

`tmsh list ltm profile client-ssl Test ciphers`

ltm profile client-ssl Test { ciphers DEFAULT }

`tmsh modify ltm profile client-ssl Test ciphers 'DEFAULT:!3DES'`

`tmsh list ltm profile client-ssl Test ciphers`

ltm profile client-ssl Test { ciphers DEFAULT:!3DES }

Just remember the '`tmsh save sys config`

' when you're happy with the configuration.

Note here the default is just 'DEFAULT'. What that expands to will vary depending on the version of TMOS. **K13156: SSL ciphers used in the default SSL profiles (11.x - 12.x)** defines the default values for each version of TMOS. Or you can check it locally from the command line:

`tmm --clientciphers 'DEFAULT'`

On 12.1.2 that would be:

`tmm --clientciphers 'DEFAULT'`

ID SUITE BITS PROT METHOD CIPHER MAC KEYX 0: 159 DHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 EDH/RSA 1: 158 DHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 EDH/RSA 2: 107 DHE-RSA-AES256-SHA256 256 TLS1.2 Native AES SHA256 EDH/RSA 3: 57 DHE-RSA-AES256-SHA 256 TLS1 Native AES SHA EDH/RSA 4: 57 DHE-RSA-AES256-SHA 256 TLS1.1 Native AES SHA EDH/RSA 5: 57 DHE-RSA-AES256-SHA 256 TLS1.2 Native AES SHA EDH/RSA 6: 57 DHE-RSA-AES256-SHA 256 DTLS1 Native AES SHA EDH/RSA 7: 103 DHE-RSA-AES128-SHA256 128 TLS1.2 Native AES SHA256 EDH/RSA 8: 51 DHE-RSA-AES128-SHA 128 TLS1 Native AES SHA EDH/RSA 9: 51 DHE-RSA-AES128-SHA 128 TLS1.1 Native AES SHA EDH/RSA 10: 51 DHE-RSA-AES128-SHA 128 TLS1.2 Native AES SHA EDH/RSA 11: 51 DHE-RSA-AES128-SHA 128 DTLS1 Native AES SHA EDH/RSA 12: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1 Native DES SHA EDH/RSA 13: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1.1 Native DES SHA EDH/RSA 14: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1.2 Native DES SHA EDH/RSA 15: 22 DHE-RSA-DES-CBC3-SHA 168 DTLS1 Native DES SHA EDH/RSA 16: 157 AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 RSA 17: 156 AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 RSA 18: 61 AES256-SHA256 256 TLS1.2 Native AES SHA256 RSA 19: 53 AES256-SHA 256 TLS1 Native AES SHA RSA 20: 53 AES256-SHA 256 TLS1.1 Native AES SHA RSA 21: 53 AES256-SHA 256 TLS1.2 Native AES SHA RSA 22: 53 AES256-SHA 256 DTLS1 Native AES SHA RSA 23: 60 AES128-SHA256 128 TLS1.2 Native AES SHA256 RSA 24: 47 AES128-SHA 128 TLS1 Native AES SHA RSA 25: 47 AES128-SHA 128 TLS1.1 Native AES SHA RSA 26: 47 AES128-SHA 128 TLS1.2 Native AES SHA RSA 27: 47 AES128-SHA 128 DTLS1 Native AES SHA RSA 28: 10 DES-CBC3-SHA 168 TLS1 Native DES SHA RSA 29: 10 DES-CBC3-SHA 168 TLS1.1 Native DES SHA RSA 30: 10 DES-CBC3-SHA 168 TLS1.2 Native DES SHA RSA 31: 10 DES-CBC3-SHA 168 DTLS1 Native DES SHA RSA 32: 49200 ECDHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 ECDHE_RSA 33: 49199 ECDHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 ECDHE_RSA 34: 49192 ECDHE-RSA-AES256-SHA384 256 TLS1.2 Native AES SHA384 ECDHE_RSA 35: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1 Native AES SHA ECDHE_RSA 36: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.1 Native AES SHA ECDHE_RSA 37: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.2 Native AES SHA ECDHE_RSA 38: 49191 ECDHE-RSA-AES128-SHA256 128 TLS1.2 Native AES SHA256 ECDHE_RSA 39: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1 Native AES SHA ECDHE_RSA 40: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.1 Native AES SHA ECDHE_RSA 41: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.2 Native AES SHA ECDHE_RSA 42: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1 Native DES SHA ECDHE_RSA 43: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1.1 Native DES SHA ECDHE_RSA 44: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1.2 Native DES SHA ECDHE_RSA

Some differences when compared to OpenSSL are readily apparent. For starters, TMM kindly includes a column label header, and actually aligns the columns. The first column is simply a 0-ordinal numeric index, the rest are as follows:

- ID: The official SSL/TLS ID assigned to that cipher suite.
- SUITE: The cipher suite.
- BITS: The size of the key in bits.
- PROT: The protocol supported.
- METHOD: NATIVE (in TMM) vs. COMPAT (using OpenSSL code).
- CIPHER: The cipher.
- MAC: The hash function.
- KEYX: The Key Exchange
*and*Authentication Algorithms

Note that the MAC is a little misleading for AES-GCM cipher suites. There is no separate MAC as they're AEAD. But the hashing algorithm is used in the Pseudo-Random Function (PRF) and a few other handshake related places.

Now we know how to look at the current configuration, modify it, and list the actual ciphers that will be enabled by the listed suites. But what do we put into the configuration?

Most users won't have to touch this. The default values are carefully selected by F5 to meet the needs of the majority of our customers. That's the good news. The bad news is that some customers *will* need to get in there and change the configuration – be it for regulatory compliance, internal policies, legacy client support, etc.

Once you begin modifying them, the configuration is truly custom for each customer. Every customer who modifies the configuration, and uses a custom cipher configuration, needs to determine what the proper list is for their needs.

Let's say we have determined that we need to support only AES & AES-GCM, 128-bit or 256-bit, and only ECDHE key exchange. Any MAC or Authentication is fine. OK, let's proceed from there.

On 12.1.2 there are six cipher suites that fit those criteria. We could list them all explicitly:

`tmm --clientciphers 'ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES256-CBC-SHA:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES128-CBC-SHA'`

That will work, but it gets unwieldy fast. Not only that, but in versions up to 11.5.0 the ciphers configuration string was truncated at 256bytes. Starting in 11.5.0 that was increased to 768bytes, but that can still truncate long configurations. We'll revisit this when we get to the pitfalls section.

Fortunately, there is an alternative – keywords! This will result in the same list of cipher suites:

`tmm --clientciphers 'ECDHE+AES-GCM:ECDHE+AES'`

That specifies the ECDHE key exchange with AES-GCM ciphers, and ECDHE with AES ciphers. Let's take a closer look to help understand what is happening here.

Keywords are extremely important when working with cipher suite configuration, so we'll spend a little time on those. Most of these apply to both the control plane (OpenSSL) and the data plane (TMM), unless otherwise noted, but we're focused on the data plane as that's F5 specific.

Keywords organize into different categories.

Note the 'Protocol' keywords in the cipher configuration control the *ciphers* associated with that protocol, and *not* the protocol itself! More on this in pitfalls.

Note the Authentication Algorithms don't work as standalone keywords in TMM. You can't use 'ECDSA' or 'DSS' for example. And you might think ECDHE or DHE includes all such cipher suites – note that they don't if you read carefully.

Note that DEFAULT does change periodically as F5 updates the configuration to follow the latest best practices. **K13156: SSL ciphers used in the default SSL profiles (11.x - 12.x)** documents these changes.

On older TMOS versions when using the **COMPAT** keyword it also enables two additional keywords:

So, let's go back to our example:

`tmm --clientciphers 'ECDHE+AES-GCM:ECDHE+AES'`

Note that you can combine keywords using '**+**' (plus sign). And multiple entries in the ciphers configuration line are separated with '**:**' (colon). You may also need to wrap the string in single quotes on the command line – I find it is a good habit to just always do so.

We can also *exclude* suites or keywords. There are two ways to do that:

'**!**' (exclamation point) is a *hard* exclusion. Anything excluded this way cannot be implicitly or explicitly re-enabled. It is disabled, period.

'**-**' (minus sign or dash) is a *soft* exclusion. Anything excluded this way *can* be explicitly re-enabled later in the configuration string. (Note: The dash is also used *in* the names of many cipher suites, such as ECDHE-RSA-AES256-GCM-SHA384 or AES128-SHA. Do not confuse the dashes that are part of the cipher suite names with a soft exclusion, which always *precedes, or prefixes,* the value being excluded. 'AES128-SHA': AES128-SHA cipher suite. '-SHA': SHA is soft excluded. '-AES128-SHA': the AES128-SHA cipher suite is soft excluded. Position matters.)

Let's look at the difference in hard and soft exclusions.

We'll start with our base example:

`tmm --clientciphers 'ECDHE+AES-GCM:DHE+AES-GCM'`

ID SUITE BITS PROT METHOD CIPHER MAC KEYX 0: 49200 ECDHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 ECDHE_RSA 1: 49199 ECDHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 ECDHE_RSA 2: 159 DHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 EDH/RSA 3: 158 DHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 EDH/RSA

Now let's look at a *hard* exclusion:

`tmm --clientciphers 'ECDHE+AES-GCM:!DHE:DHE+AES-GCM'`

ID SUITE BITS PROT METHOD CIPHER MAC KEYX 0: 49200 ECDHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 ECDHE_RSA 1: 49199 ECDHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 ECDHE_RSA

And lastly a *soft* exclusion:

`tmm --clientciphers 'ECDHE+AES-GCM:-DHE:DHE+AES-GCM'`

ID SUITE BITS PROT METHOD CIPHER MAC KEYX 0: 49200 ECDHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 ECDHE_RSA 1: 49199 ECDHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 ECDHE_RSA 2: 159 DHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 EDH/RSA 3: 158 DHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 EDH/RSA

Note that in the second example, the hard exclusion, we used '**!DHE**' and even though we then explicitly added '**DHE+AES-GCM**' those ciphers were not enabled. This is because, once excluded with a hard exclusion, ciphers cannot be re-enabled.

In the third example, the soft exclusion, we used '**-DHE**' and then '**DHE+AES-GCM**'. This time it *did* enable those ciphers, which is possible with a soft exclusion.

You might be wondering what soft disabling is useful for; why would you ever want to remove ciphers only to add them again? Reordering the ciphers is a common use case. As an example, DEFAULT orders ciphers differently in different versions, but mainly based on strength – bit size. Let's say we know 3DES is really 112-bit equivalent strength and not 168-bit as it is usually labeled. For some reason, maybe legacy clients, we can't disable them, but we want them to be last on the list.

One way to do this is to first configure the DEFAULT list, then *remove* all of the 3DES ciphers. But then add the 3DES ciphers back explicitly – *at the end of the list.* Let's try it – compare the following:

`tmm --clientciphers 'DEFAULT'`

ID SUITE BITS PROT METHOD CIPHER MAC KEYX 0: 159 DHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 EDH/RSA 1: 158 DHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 EDH/RSA 2: 107 DHE-RSA-AES256-SHA256 256 TLS1.2 Native AES SHA256 EDH/RSA 3: 57 DHE-RSA-AES256-SHA 256 TLS1 Native AES SHA EDH/RSA 4: 57 DHE-RSA-AES256-SHA 256 TLS1.1 Native AES SHA EDH/RSA 5: 57 DHE-RSA-AES256-SHA 256 TLS1.2 Native AES SHA EDH/RSA 6: 57 DHE-RSA-AES256-SHA 256 DTLS1 Native AES SHA EDH/RSA 7: 103 DHE-RSA-AES128-SHA256 128 TLS1.2 Native AES SHA256 EDH/RSA 8: 51 DHE-RSA-AES128-SHA 128 TLS1 Native AES SHA EDH/RSA 9: 51 DHE-RSA-AES128-SHA 128 TLS1.1 Native AES SHA EDH/RSA 10: 51 DHE-RSA-AES128-SHA 128 TLS1.2 Native AES SHA EDH/RSA 11: 51 DHE-RSA-AES128-SHA 128 DTLS1 Native AES SHA EDH/RSA 12: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1 Native DES SHA EDH/RSA 13: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1.1 Native DES SHA EDH/RSA 14: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1.2 Native DES SHA EDH/RSA 15: 22 DHE-RSA-DES-CBC3-SHA 168 DTLS1 Native DES SHA EDH/RSA 16: 157 AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 RSA 17: 156 AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 RSA 18: 61 AES256-SHA256 256 TLS1.2 Native AES SHA256 RSA 19: 53 AES256-SHA 256 TLS1 Native AES SHA RSA 20: 53 AES256-SHA 256 TLS1.1 Native AES SHA RSA 21: 53 AES256-SHA 256 TLS1.2 Native AES SHA RSA 22: 53 AES256-SHA 256 DTLS1 Native AES SHA RSA 23: 60 AES128-SHA256 128 TLS1.2 Native AES SHA256 RSA 24: 47 AES128-SHA 128 TLS1 Native AES SHA RSA 25: 47 AES128-SHA 128 TLS1.1 Native AES SHA RSA 26: 47 AES128-SHA 128 TLS1.2 Native AES SHA RSA 27: 47 AES128-SHA 128 DTLS1 Native AES SHA RSA 28: 10 DES-CBC3-SHA 168 TLS1 Native DES SHA RSA 29: 10 DES-CBC3-SHA 168 TLS1.1 Native DES SHA RSA 30: 10 DES-CBC3-SHA 168 TLS1.2 Native DES SHA RSA 31: 10 DES-CBC3-SHA 168 DTLS1 Native DES SHA RSA 32: 49200 ECDHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 ECDHE_RSA 33: 49199 ECDHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 ECDHE_RSA 34: 49192 ECDHE-RSA-AES256-SHA384 256 TLS1.2 Native AES SHA384 ECDHE_RSA 35: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1 Native AES SHA ECDHE_RSA 36: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.1 Native AES SHA ECDHE_RSA 37: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.2 Native AES SHA ECDHE_RSA 38: 49191 ECDHE-RSA-AES128-SHA256 128 TLS1.2 Native AES SHA256 ECDHE_RSA 39: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1 Native AES SHA ECDHE_RSA 40: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.1 Native AES SHA ECDHE_RSA 41: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.2 Native AES SHA ECDHE_RSA 42: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1 Native DES SHA ECDHE_RSA 43: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1.1 Native DES SHA ECDHE_RSA 44: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1.2 Native DES SHA ECDHE_RSA

`tmm --clientciphers 'DEFAULT:-3DES:!SSLv3:3DES+ECDHE:3DES+DHE:3DES+RSA'`

ID SUITE BITS PROT METHOD CIPHER MAC KEYX 0: 159 DHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 EDH/RSA 1: 158 DHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 EDH/RSA 2: 107 DHE-RSA-AES256-SHA256 256 TLS1.2 Native AES SHA256 EDH/RSA 3: 57 DHE-RSA-AES256-SHA 256 TLS1 Native AES SHA EDH/RSA 4: 57 DHE-RSA-AES256-SHA 256 TLS1.1 Native AES SHA EDH/RSA 5: 57 DHE-RSA-AES256-SHA 256 TLS1.2 Native AES SHA EDH/RSA 6: 57 DHE-RSA-AES256-SHA 256 DTLS1 Native AES SHA EDH/RSA 7: 103 DHE-RSA-AES128-SHA256 128 TLS1.2 Native AES SHA256 EDH/RSA 8: 51 DHE-RSA-AES128-SHA 128 TLS1 Native AES SHA EDH/RSA 9: 51 DHE-RSA-AES128-SHA 128 TLS1.1 Native AES SHA EDH/RSA 10: 51 DHE-RSA-AES128-SHA 128 TLS1.2 Native AES SHA EDH/RSA 11: 51 DHE-RSA-AES128-SHA 128 DTLS1 Native AES SHA EDH/RSA 12: 157 AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 RSA 13: 156 AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 RSA 14: 61 AES256-SHA256 256 TLS1.2 Native AES SHA256 RSA 15: 53 AES256-SHA 256 TLS1 Native AES SHA RSA 16: 53 AES256-SHA 256 TLS1.1 Native AES SHA RSA 17: 53 AES256-SHA 256 TLS1.2 Native AES SHA RSA 18: 53 AES256-SHA 256 DTLS1 Native AES SHA RSA 19: 60 AES128-SHA256 128 TLS1.2 Native AES SHA256 RSA 20: 47 AES128-SHA 128 TLS1 Native AES SHA RSA 21: 47 AES128-SHA 128 TLS1.1 Native AES SHA RSA 22: 47 AES128-SHA 128 TLS1.2 Native AES SHA RSA 23: 47 AES128-SHA 128 DTLS1 Native AES SHA RSA 24: 49200 ECDHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 ECDHE_RSA 25: 49199 ECDHE-RSA-AES128-GCM-SHA256 128 TLS1.2 Native AES-GCM SHA256 ECDHE_RSA 26: 49192 ECDHE-RSA-AES256-SHA384 256 TLS1.2 Native AES SHA384 ECDHE_RSA 27: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1 Native AES SHA ECDHE_RSA 28: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.1 Native AES SHA ECDHE_RSA 29: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.2 Native AES SHA ECDHE_RSA 30: 49191 ECDHE-RSA-AES128-SHA256 128 TLS1.2 Native AES SHA256 ECDHE_RSA 31: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1 Native AES SHA ECDHE_RSA 32: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.1 Native AES SHA ECDHE_RSA 33: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.2 Native AES SHA ECDHE_RSA 34: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1 Native DES SHA ECDHE_RSA 35: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1.1 Native DES SHA ECDHE_RSA 36: 49170 ECDHE-RSA-DES-CBC3-SHA 168 TLS1.2 Native DES SHA ECDHE_RSA 37: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1 Native DES SHA EDH/RSA 38: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1.1 Native DES SHA EDH/RSA 39: 22 DHE-RSA-DES-CBC3-SHA 168 TLS1.2 Native DES SHA EDH/RSA 40: 22 DHE-RSA-DES-CBC3-SHA 168 DTLS1 Native DES SHA EDH/RSA 41: 10 DES-CBC3-SHA 168 TLS1 Native DES SHA RSA 42: 10 DES-CBC3-SHA 168 TLS1.1 Native DES SHA RSA 43: 10 DES-CBC3-SHA 168 TLS1.2 Native DES SHA RSA 44: 10 DES-CBC3-SHA 168 DTLS1 Native DES SHA RSA

I added something else in there which I'll come back to later.

As should be clear by now cipher configuration is a powerful tool, but as the song says, every tool is a weapon if you hold it right. And weapons are dangerous. With a little careless handling it is easy to lose a toe – or a leg. Whenever you are working with cipher suite configuration the old rule of 'measure twice, cut once' applies – and then double-check the work to be certain.

There are several common pitfalls which await you.

Perhaps the most common pitfall is simply misuse – using cipher suite configuration for that which it is not intended. And the single most common example of this comes from using *cipher* configuration to manipulate *protocols.*

Given the keywords, as described above, it seems common for users to presume that if they want to disable a *protocol*, such as TLSv1.0, then the way to do that is to use a *cipher suite* keyword, such as !TLSv1. And, indeed, this may *seem* to work – but it isn't doing what is desired. The *protocol* is not disabled, only the ciphers that are supported for that protocol are.

The *protocol* is configured on the VIP independently of the *ciphers*. !TLSv1 would disable all *ciphers* supported under the TLSv1.0 *protocol*, but not the protocol itself. Note that the protocol negotiation and the cipher negotiation in the SSL/TLS handshake are independent. What happens if the VIP only supports TLSv1.0/v1.1/v1.2 and the client only supports SSLv3 & TLSv1.0? Well, they'd agree on TLSv1.0 as the common protocol.

The cipher list the client sends in the Client Hello is independent of the protocol that is eventually negotiated. Say the client sends AES128-SHA and the server has that in its list, so it is selected. OK, we've agreed on a protocol and a cipher suite – only the server won't do any ciphers on TLSv1.0 because of '!TLSv1' in the ciphers configuration, and the connection will fail.

That may seem like splitting hairs, but it makes a difference. If a scanner is looking for protocols that are enabled, and not the full handshake, it may still flag a system which has been configured this way. The protocol is negotiated during the SSL/TLS handshake before the cipher is selected.

This also means the system is doing more work, as the handshake continues further before failing, and the log messages may be misleading. Instead of logging a protocol incompatibility the logs will reflect the failure to find a viable cipher, which can be a red herring when it comes time to debug the configuration.

The right way to do this is to actually disable the protocol, which doesn't involve the cipher suite configuration at all. For the control plane this is done through the **ssl-protocol **directive:

`tmsh list sys http ssl-protocol`

`sys httpd {`

` ssl-protocol "all -SSLv2 -SSLv3"`

`}`

For example, if we wanted to disable TLSv1.0:

`tmsh modify sys http ssl-protocol 'all -SSLv2 -SSLv3 -TLSv1'`

`tmsh list sys http ssl-protocol`

`sys httpd {`

` ssl-protocol "all -SSLv2 -SSLv3 -TLSv1"`

`}`

For the data plane this can be done via the Options List in the SSL Profile GUI, via the No SSL, No TLSv1.1, etc. directives:

Or via the command line:

`tmsh list ltm profile client-ssl Test options`

`ltm profile client-ssl Test {`

` options { dont-insert-empty-fragments }`

`}`

`tmsh modify ltm profile client-ssl Test options {dont-insert-empty-fragments no-tlsv1}`

`tmsh list /ltm profile client-ssl Test options`

`ltm profile client-ssl Test {`

` options { dont-insert-empty-fragments no-tlsv1 }`

`}`

The values are slightly different on the command line, use this command to see them all:

`tmsh modify ltm profile client-ssl <profile-name> options ?`

Use the right tool for the job and you'll be more likely to succeed.

As I previously mentioned, in versions up to 11.5.0 the ciphers configuration string was truncated at 256 bytes. Starting in 11.5.0 that was increased to 768 bytes (see **K11481: The SSL profile cipher lists have a 256 character limitation** for more information), but that can still silently truncate long configurations.

This is not a theoretical issue, we've seen users run into this in the real world. For example, little over a year ago I worked with a customer who was then using 11.4.1 HF8. They were trying to very precisely control which ciphers were enabled, and their order. In order to do this they'd decided to enumerate every individual cipher in their configuration – resulting in this cipher suite configuration string:

**TLSv1_2+ECDHE-RSA-AES256-CBC-SHA:TLSv1_1+ECDHE-RSA-AES256-CBC-SHA:TLSv1_2+ECDHE-RSA-AES128-CBC-SHA:TLSv1_1+ECDHE-RSA-AES128-CBC-SHA:TLSv1_2+DHE-RSA-AES256-SHA:TLSv1_1+DHE-RSA-AES256-SHA:TLSv1_2+DHE-RSA-AES128-SHA:TLSv1_1+DHE-RSA-AES128-SHA:TLSv1_2+AES256-SHA256:TLSv1_1+AES256-SHA:TLSv1_2+AES128-SHA256:TLSv1_1+AES128-SHA:TLSv1+ECDHE-RSA-AES256-CBC-SHA:TLSv1+ECDHE-RSA-AES128-CBC-SHA:TLSv1+DHE-RSA-AES256-SHA:TLSv1+DHE-RSA-AES128-SHA:TLSv1+AES256-SHA:TLSv1+AES128-SHA:TLSv1+DES-CBC3-SHA**

That string would save in the configuration and it was there if you looked at the bigip.conf file, but it was *silently truncated* when the configuration was loaded. Since this was 11.4.1, only the first 256 bytes were loaded successfully, which made the running configuration:

**TLSv1_2+ECDHE-RSA-AES256-CBC-SHA:TLSv1_1+ECDHE-RSA-AES256-CBC-SHA:TLSv1_2+ECDHE-RSA-AES128-CBC-SHA:TLSv1_1+ECDHE-RSA-AES128-CBC-SHA:TLSv1_2+DHE-RSA-AES256-SHA:TLSv1_1+DHE-RSA-AES256-SHA:TLSv1_2+DHE-RSA-AES128-SHA:TLSv1_1+DHE-RSA-AES128-SHA:TLSv1_2+AES256-S**

Note the last suite is truncated itself, which means it was invalid and therefore ignored. If their configuration had worked they would've had nineteen protocol+suite combinations – instead they had *eight*. Needless to say, this caused some problems.

This customer was *missing* ciphers that they expected to have working. That is bad enough – but it could be worse.

Let's imagine a customer who wants to specify several specific ciphers first, then generally enable a number of other TLSv1.2 & TLSv1.1 ciphers. And, of course, they are careful to disable dangerous ciphers!

**TLSv1_2+ECDHE-RSA-AES256-CBC-SHA:TLSv1_1+ECDHE-RSA-AES256-CBC-SHA:TLSv1_2+ECDHE-RSA-AES128-CBC-SHA:TLSv1_1+ECDHE-RSA-AES128-CBC-SHA:TLSv1_2+DHE-RSA-AES256-SHA:TLSv1_1+DHE-RSA-AES256-SHA:TLSv1_2+DHE-RSA-AES128-SHA:TLSv1_1+DHE-RSA-AES128-SHA:TLSv1_2:TLSv1_1:!RC4:!MD5:!ADH:!DES:!EXPORT**

OK, that looks fairly solid, right? What do you suppose the problem with this is?

This is the problem; in 11.4.1 and earlier it would truncate to this:

**TLSv1_2+ECDHE-RSA-AES256-CBC-SHA:TLSv1_1+ECDHE-RSA-AES256-CBC-SHA:TLSv1_2+ECDHE-RSA-AES128-CBC-SHA:TLSv1_1+ECDHE-RSA-AES128-CBC-SHA:TLSv1_2+DHE-RSA-AES256-SHA:TLSv1_1+DHE-RSA-AES256-SHA:TLSv1_2+DHE-RSA-AES128-SHA:TLSv1_1+DHE-RSA-AES128-SHA:TLSv1_2:TLSv1_1:**

All of the exclusions were truncated off! Now we have the opposite problem – there are a number of ciphers *enabled* which the customer expects to be *disabled!* And they're ** BAD** ciphers – ADH, DES, MD5, RC4. So this customer would be at high risk without realizing it.

Be aware of this; it is very sneaky. The configuration will look fine; the truncation happens in the code when it loads the configuration. This is also one reason why I always recommend listing your exclusions *first* in the configuration string. Then you can never accidentally enable something.

Let's say a new CVE is announced which exposes a very serious vulnerability in SSLv3 & TLSv1.0. There is no way to mitigate it, and the only solution is to limit connections to only TLSv1.1 & TLSv1.2.

You want a cipher configuration to accomplish this. It seems straight-forward – just configure it to use only ciphers on TLSv1.1 & TLSv1.2:

`tmsh modify ltm profile client-ssl <profile> ciphers 'TLSv1_2:TLSv1_1'`

Congratulations, you've solved the problem. You are no longer vulnerable to this CVE.

You know there is a but coming, right?

What's wrong? Well, you just enabled *all* TLSv1.2 & TLSv1.1 ciphers. That includes such gems as RC4-MD5, RC4-SHA, DES, and a few ADH (Anonymous Diffie-Hellman) suites which have no authentication. As recently as 11.3.0 you'd even be enabling some 40-bit EXPORT ciphers. (We pulled them out of NATIVE in 11.4.0.)

So you just leapt out of the frying pan and into the fire.

Always, always, always check the configuration before using it. Running that through `tmm --clientciphers 'TLSv1_2:TLSv1_1'`

would've raised red flags.

Instead, this configuration would work without causing those problems:

`tmsh modify ltm profile client-ssl <profile> ciphers 'DEFAULT:!TLSv1:!SSLv3'`

Another option, and probably the better one, is to disable the SSLv3 and TLSv1.0 *protocols* on the VIP. As I discussed above. Of course, you can do both – belt and suspenders.

And just to show you how easy it is to make such a mistake, F5 did this! In **K13400: SSL 3.0/TLS 1.0 BEAST vulnerability CVE-2011-3389 and TLS protocol vulnerability CVE-2012-18... **we originally had the following in the mitigation section:

Note: Alternatively, to configure an SSL profile to use only TLS 1.1-compatible, TLS 1.2-compatible, AES-GCM, or RC4-SHA ciphers using the tmsh utility, use the following syntax:

tmsh create /ltm profile client-ssl <name> ciphers TLSv1_1:TLSv1_2:AES-GCM:RC4-SHA

Yes, I had this fixed long ago.

Remember back in the section on keywords I had this comparison example:

`tmm --clientciphers 'DEFAULT'`

`tmm --clientciphers 'DEFAULT:-3DES:!SSLv3:3DES+ECDHE:3DES+DHE:3DES+RSA'`

Who caught the '!SSLv3' in the second line? Why do you think I added that? Did I need to?

Hint: What do you think the side effect of blanket enabling all of those 3DES ciphers would be if I didn't explicitly disable SSLv3?

In SSL/TLS there are two main models to the cipher suite negotiation – Server Cipher Preference or Client Cipher Preference. What does this mean? In SSL/TLS the client sends the list of cipher suites it is willing and able to support in the Client Hello. The server also has its list of cipher suites that it is willing and able to support.

In Client Cipher Preference the server will select the first cipher on the *client's* list that is also in the server's list. Effectively this gives the client influence over which cipher is selected based on the order of the list it sends.

In Server Cipher Preference the server will select the first server on *its own* list that is also on the client's list. So the server gives the order of its list precedence.

BIG-IP always operates in Server Cipher Preference, so be very careful in how you order your cipher suites. Preferred suites should go at the top of the list. How you order your cipher suites will directly affect which ciphers are used. It doesn't matter if a stronger cipher is available if a weak cipher is matched first.

How is HTTP/2 a pitfall? The HTTP/2 RFC7540 includes a blacklist of ciphers that are valid in TLS, but should not be used in HTTP/2. This can cause a problem on a server where the TLS negotiation is decoupled from the ALPN exchange for the higher level protocol. The server might select a cipher which is on the blacklist, and then when the connection attempts to step up to HTTP/2 via ALPN the client may terminate the connection with extreme prejudice. It is well known enough to be called out in the RFC – Section 9.2.2.

F5 added support for HTTP/2 in 12.0.0 – and we fell into this trap. Our DEFAULT ciphers list was ordered such that it was almost certain a blacklisted cipher would be selected.; This was fixed in 12.0.0 HF3 and 12.1.0, but serves as an example.

On 12.0.0 FINAL through 12.0.0 HF2 a simple fix was to configure the ciphers to be 'ECDHE+AES-GCM:DEFAULT'. ECDHE+AES-GCM is guaranteed to be supported by any client compliant with RFC7540 (HTTP/2). Putting it first ensures it is selected before any blacklisted cipher.

Back in the section on ciphers I mentioned that we label 3DES as being 168-bit, but that it only provides the equivalent of 112-bit strength. So, what did I mean by that?

DES operates on 64-bit data blocks, using 56-bits of key. So it has a strength of 2^{56}. 3DES, aka Triple DES, was a stop-gap designed to stretch the life of DES once 56-bits was too weak to be safe, until AES became available. 3DES use the exact same DES cipher, it just uses it three times – hence the name. So you might think 3x56-bits = 168-bits. 2^{168} strong. Right? No, not really.

The standard implementation of 3DES is known as EDE – for Encrypt, Decrypt, Encrypt. (For reasons we don't need to get into here.) You take the 64-bit data block, run it through DES once to encrypt it with K_{1}, then run it through again to decrypt it using K_{2}, then encrypt it once again using K_{3}.

Three keys, that's still 168-bits, right?

Well, you'd think so. But the devil is in the (implementation) details. First of all there are three keying options for 3DES:

- Keying option 1: K1, K2, K3 – 168 unique bits (but only 112-bit strength!)

- Keying option 2: K1, K2, K1 – 112 unique bits (but only 80-bit strength!)

- Keying option 3: K1, K1, K1 – 56 unique bits, 56-bit strength (Equivalent to DES due to EDE!)

F5 uses keying option one, so we have 168-bits of unique key. However, 3DES with keying option one is subject to a meet-in-the-middle cryptographic attack which only has a cost of 2^{112}. It has even been reduced as low as 2^{108}, as described in this paper. So it does not provide the expected 168-bits of security, and is in fact weaker than AES128.

To add some confusion, due to an old issue we used to describe 3DES as being 192-bit. See: **K17296: The BIG-IP system incorrectly reports a 192-bit key length for cipher suites using 3DES (DES...** for more details.

Of course, with the appearance of the Sweet32 attack last fall I would encourage everyone to disable 3DES completely whenever possible. We're also seeing a growing number of scanners and audit tools recategorizing 3DES as a 'Medium' strength cipher, down from 'High', and correspondingly lowering the grade for any site still supporting it. If you don't need it, turn it off. See **K13167034: OpenSSL vulnerability CVE-2016-2183** for more information.

Believe it or not, that's the quick overview of cipher suite configuration on BIG-IP. There are many areas where we could dig in further and spend some time in the weeds, but I hope that this article helps at least one person understand cipher suite configuration better, and to avoid the pitfalls that commonly claim those who work with them.

This article is by no means comprehensive, and for those interested I'd encourage additional reading:

BIG-IP SSL Cipher History by David Holmes, here on DevCentral

Cipher Rules And Groups in BIG-IP v13 by Chase Abbott, also on DevCentral

K8802: Using SSL ciphers with BIG-IP Client SSL and Server SSL profiles

K15194: Overview of the BIG-IP SSL/TLS cipher suite

K13163: SSL ciphers supported on BIG-IP platforms (11.x - 12.x)

K13156: SSL ciphers used in the default SSL profiles (11.x - 12.x)

K17370: Configuring the cipher strength for SSL profiles (12.x)

K13171: Configuring the cipher strength for SSL profiles (11.x)

Labels:

Comments

Barny_Riches

Nimbostratus

adam88

Cirrus

Oh wow, this is such a fantastic article. I was actually attempting to block protocols using the cipher string, definitely changing my approach now.

One of my LTMs runs BIG-IP v11.4.1 and on this I wanted to purely use ECDHE key exchange with only TLS1.2. I was thinking that I would use ECDHE+AES then use the Options list to block TLS1, TLS1.1, DTLS, SSLv2 and SSLv3.

`tmm --clientciphers 'ECDHE+AES'`

```
ID SUITE BITS PROT METHOD CIPHER MAC KEYX
0: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1 Native AES SHA ECDHE_RSA
1: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.1 Native AES SHA ECDHE_RSA
2: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.2 Native AES SHA ECDHE_RSA
3: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1 Native AES SHA ECDHE_RSA
4: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.1 Native AES SHA ECDHE_RSA
5: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.2 Native AES SHA ECDHE_RSA
```

And to see what happens when I disable TLS1 and TLS1.1:

`tmm --clientciphers 'ECDHE+AES:!TLSv1:!TLSv1_1'`

```
ID SUITE BITS PROT METHOD CIPHER MAC KEYX
0: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.2 Native AES SHA ECDHE_RSA
1: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.2 Native AES SHA ECDHE_RSA
```

I noticed that I could also do:

`tmm --clientciphers 'ECDHE+AES+TLSv1_2'`

```
ID SUITE BITS PROT METHOD CIPHER MAC KEYX
0: 49171 ECDHE-RSA-AES128-CBC-SHA 128 TLS1.2 Native AES SHA ECDHE_RSA
1: 49172 ECDHE-RSA-AES256-CBC-SHA 256 TLS1.2 Native AES SHA ECDHE_RSA
```

Is this a good idea to add +TLSv1_2 to the string? I can't see any pitfalls to this but I'm not super experienced with cipher strings.

MegaZone

F5 SIRT

I'm glad you found it useful. When you chain together keywords using '+' cipher suites must match *all* of the keywords, so that should be safe. As you see in the results you're getting cipher suites that are ECDHE, AES, AND TLSv1.2 *only*.

I would strongly encourage upgrading that 11.4.1 TMOS though - that's out of support and hasn't received any updates, including security patches, in quite a while. I'd look at going to something newer if possible - 11.5.7, 11.6.3.2, 12.1.3.6, etc.

Car4los_13384

Nimbostratus

Here is an oddity. Needed to get off TLSv1.0 on an LTM at 11.5 version. I googled around and found this: https://support.f5.com/csp/article/K13400ssl_p1, so I tried the recommended change. There were no errors reported by TMSH using the command shown in the article. However Firefox barfs with SSL_ERROR_NO_CYPHER_OVERLAP. Tweaking the settings in the about:config did not help. Chrome works, even Edge works. Has anyone else hit this issue? A test of the URL to the configuration utility shows: upported cipher suites (ORDER IS NOT SIGNIFICANT): TLSv1.2 RSA_WITH_AES_128_CBC_SHA256 RSA_WITH_AES_256_CBC_SHA256 DHE_RSA_WITH_AES_128_CBC_SHA256 DHE_RSA_WITH_AES_256_CBC_SHA256 TLS_RSA_WITH_AES_128_GCM_SHA256 TLS_RSA_WITH_AES_256_GCM_SHA384 TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 TLS_DHE_RSA_WITH_AES_256_GCM_SHA384

```
I'm not sure what Firefox is puzzled by. Any thoughts?
```

MegaZone

F5 SIRT

There's another article that might be helpful: https://support.f5.com/csp/article/K31320003

As for this specific situation, can you do look at the traffic? Specifically look at the ClientHello coming from Firefox, as that will contain a list of the cipher suites that the browser is offering to support. Make sure there is at least one cipher suite shared between FF and what the BIG-IP is offering.

You can see what the BIG-IP will support with 'tmm --clientciphers [your cipher suite config]'

Chris_Olson

Nimbostratus

MegaZone

F5 SIRT

CBC ciphers aren't inherently vulnerable (unlike RC4), the vulnerabilities are implementation specific. You can have CBC ciphers without being vulnerable.

On BIG-IP *all* ciphers are CBC *except* AES-GCM and RC4. So those are the only ciphers you can use if you really don't want any CBC enabled - and RC4 has its own issues, so AES-GCM is really the only option. As AES-GCM was only added in TLSv1.2, this also means you can't support earlier protocols without CBC.

Note that 'ECDSA' ciphers only work if you have a digital certificate signed with ECDSA - and that's not typical. Even if the ciphers would be enabled by the config string they will NOT be enabled on the VIP if the cert doesn't support them.

So you're basically looking at ECDHE-RSA-AES256-GCM-SHA384 and ECDHE-RSA-AES128-GCM-SHA256 as your only two ciphers that satisfy PFS and not CBC.

DM706_346160

Altostratus

Hi MegaZone,

This is a great article, I really appreciate you taking the time to explain the individual components of a cipher suite.

I've been working on tightening of some of my SSL profiles and have come to the conclusion that if I want Qualys to give me no "weak" ciphers that I have to drop CBC support. However, doing so, with the other requirement that I need 2048bit DHE or ECDHE as well, that I drop support for Windows 7 and IE11. In a year or two that will obviously be less of a problem, however, today I think it is. With these cipher requirements, I get literally 2 ciphers.... "ECDHE+AES-GCM:@STRENGTH"

In your last comment you stated "CBC ciphers aren't inherently vulnerable (unlike RC4), the vulnerabilities are implementation specific. You can have CBC ciphers without being vulnerable"

Can you expand on that statement a little more? I'd like to know if, in my scenario, that CBC is actually weak or if its just ssllabs.com doing a blanket "if cipher.instr('CBC') then 'WEAK'"...

Chris_Olson

Nimbostratus

MegaZone

F5 SIRT

BIG-IP does not currently support DHE beyond 1024-bit. The short version of the reason why is that there was no way for the client and server to negotiate the key size, and if the two ends used a different size it doesn't work. An extension to TLS, FFDHE, was proposed to allow for negotiation of DHE key sizes, and this was incorporated into TLSv1.3. FFDHE isn't in our initial implementation of TLSv1.3, but it is on the roadmap (I'm not sure of when we can expect it yet). I'm pushing to support it on earlier versions of TLS as well, for any client that does. (The initial standalone proposal was as an extension to TLS in general, not specific to 1.3.)

As for CBC ciphers - there isn't a fundamental weakness in CBC itself. The problem is that CBC ciphers, as a group, are more susceptible to implementation issues which lead to things like oracle vulnerabilities. Basically it is easier to get something wrong and create a vulnerability when implementing CBC. Because of the recurring issues with CBC implementations suffering from padding attacks, timing attacks, etc., the use of CBC has been discouraged - and CBC is removed completely from TLSv1.3. But, again, the vulnerabilities are in the implementations, not the cipher itself. It is possible to have an AES-CBC implementation with no known weaknesses, just as you can with AES-GCM - but in the real world there are more ways to get it wrong with CBC.

As for SSLLabs, I'd have to know what exactly it is flagging you for. You could be running a version of TMOS which is actually vulnerable - we have had our share of issues as have most vendors. If it is giving you a CVE or attack name that you're vulnerable to, check AskF5 for a Security Advisory on it. There have also been cases of false positives in the past, and that's always a possibility. Sometimes it is an ordering thing - having PFS+non-CBC ciphers first but still allowing CBC ciphers as an option for older clients may improve the score.

Also, make sure you don't have any 3DES ciphers enabled - those will get dinged for strength (Medium at best).