DoS and NTLM Brute force protection for SIP traffic

Problem this snippet solves:

This snippet has been designed to mainly protect against NTLM's downgrade attacks present in a widely used Instant Messaging solution. This solution authenticate users on the SIP/TLS protocol. This irule block brute forced users and source IP address.

How to use this snippet:

This snippet should be applied on the Virtual Server that handle SIP/TLS traffic. SSL bridging is required to make this irule work properly. Moreover, Skype for Business may require that you change your cipher suite to a weak one.

The internal domain should be define in "

" and "
" variables. The script will focus on those domains.

The max attempts before blocking is defined in the "

" variable. This setting should be under the max attempts allowed on the Active Directory.

The blocking duration is configured in the "

" variable. (in seconds)

The "

" variable define the window that we increase the attempt counter. After reaching the end of this duration, the entry is removed until a new invalid attempt occurs.

External links

Github :

Related Articles

Code :

when RULE_INIT {
    array set NTLMFlags {
        unicode        0x00000001
        oem            0x00000002
        req_target     0x00000004
        unknown1       0x00000008
        sign           0x00000010
        seal           0x00000020
        datagram       0x00000040
        lmkey          0x00000080
        netware        0x00000100
        ntlm           0x00000200
        unknown2       0x00000400
        unknown3       0x00000800
        ntlm_domain    0x00001000
        ntlm_server    0x00002000
        ntlm_share     0x00004000
        NTLM2          0x00008000
        targetinfo     0x00800000
        128bit         0x20000000
        keyexch        0x40000000
        56bit          0x80000000
    set static::email_domain     ""
    set static::user_domain      "DOMAIN"
    set static::log_pri          "local0."
    set static::fail_tab         "NTLMfails"
    set static::blacklist_tab      "NTLMblackhole"
    set static::userfail_tab         "NTLMUserfails"
    set static::userblacklist_tab  "NTLMUserblackhole"
    set static::max_failures   5
    set static::fail_memory    300
    set static::block_duration 300

    if {[table lookup -subtable $static::blacklist_tab [IP::client_addr]] == 1} {
        log $static::log_pri "[virtual] - BLACKHOLED IPADDR [IP::client_addr]:[TCP::client_port] (Reputation=[IP::reputation [IP::client_addr]])"

set payload [SSL::payload]
if { ($payload contains "3 REGISTER") } {
regexp -nocase {gssapi-data=\"([A-Za-z0-9+\/=]*)\",} $payload match gssapi garbage
if { [info exists match] } {
unset match
unset garbage
if { $gssapi != "" } {
set ntlm_msg [ b64decode [string trim $gssapi]]
binary scan $ntlm_msg a7ci protocol zero type
if { $type eq 3} {
binary scan $ntlm_msg @12ssissississississii \
lmlen lmlen2 lmoff \
ntlen ntlen2 ntoff \
dlen  dlen2  doff  \
ulen  ulen2  uoff \
hlen  hlen2  hoff \
slen  slen2  soff \
set ntlm_domain {}; binary scan $ntlm_msg @${doff}a${dlen} ntlm_domain
set ntlm_user {};   binary scan $ntlm_msg @${uoff}a${ulen} ntlm_user
set ntlm_host {};   binary scan $ntlm_msg @${hoff}a${hlen} ntlm_host
set unicode [expr {$flags & 0x00000001}]
if {$unicode} {
set ntlm_domain_convert ""
foreach i [ split $ntlm_domain ""] {
scan $i %c c
if {$c>1} {
append ntlm_domain_convert $i
} elseif {$c<128} {
set ntlm_domain_convert $ntlm_domain_convert
} else {
append ntlm_domain_convert \\u[format %04.4X $c]
set ntlm_domain $ntlm_domain_convert
set ntlm_user_convert ""
foreach i [ split $ntlm_user ""] {
scan $i %c c
if {$c>1} {
append ntlm_user_convert $i
} elseif {$c<128} {
set ntlm_user_convert $ntlm_user_convert
} else {
append ntlm_user_convert \\u[format %04.4X $c]
set ntlm_user   $ntlm_user_convert
set ntlm_host_convert ""
foreach i [ split $ntlm_host ""] {
scan $i %c c
if {$c>1} {
append ntlm_host_convert $i
} elseif {$c<128} {
set ntlm_host_convert $ntlm_host_convert
} else {
append ntlm_host_convert \\u[format %04.4X $c]
set ntlm_host   $ntlm_host_convert
binary scan $ntlm_msg @${ntoff}a${ntlen} ntdata
binary scan $ntlm_msg @${lmoff}a${lmlen} lmdata
binary scan $ntdata H* ntdata_h
binary scan $lmdata H* lmdata_h
set interesting 1

if { ($ntlm_domain equals $static::user_domain or $ntlm_user ends_with $static::email_domain) } {
set attack 1
if {[table lookup -subtable $static::userblacklist_tab $ntlm_user] == 1} {
log $static::log_pri "[virtual] - BLACKHOLED $ntlm_domain\\$ntlm_user from $ntlm_host at [IP::client_addr]:[TCP::client_port] (Reputation=[IP::reputation [IP::client_addr]])"
} else {
log $static::log_pri "[virtual] - Login attempt by $ntlm_domain\\$ntlm_user from $ntlm_host for SIP."
} else {
set attack 0
log $static::log_pri "[virtual] - Not a valid user - Login attempt by $ntlm_domain\\$ntlm_user from $ntlm_host for SIP."
# Release the payload

   SSL::release 0

set payload [SSL::payload]

if {[info exists interesting] && $interesting == 1} {
        set client [IP::client_addr]:[TCP::client_port]
        set node [IP::server_addr]:[TCP::server_port]

        if { $payload contains "401 Unauthorized ms-user-logon-data" and ([info exists attack] and $attack == 1) } {
            table set -subtable $static::fail_tab -notouch -excl [IP::client_addr] 0 indef $static::fail_memory
            table incr -subtable $static::fail_tab [IP::client_addr]

set now [clock seconds]
            set now_date [split [clock format $now -format {%X %x}] " "]

set later [expr {$now + $static::block_duration}]
set later_date [split [clock format $later -format {%X %x}] " "]

if {[info exists ntlm_user]} {
table set -subtable $static::userfail_tab -notouch -excl $ntlm_user 0 indef $static::fail_memory
table incr -subtable $static::userfail_tab $ntlm_user

if {[table lookup -subtable $static::userfail_tab $ntlm_user] >= $static::max_failures} {
log $static::log_pri "[virtual] - BLACKHOLING USER - $ntlm_user at $now_date until $later_date"
table set -subtable $static::userblacklist_tab -excl $ntlm_user 1 indef $static::block_duration


            if {[table lookup -subtable $static::fail_tab [IP::client_addr]] >= $static::max_failures} {
                log $static::log_pri "[virtual] - BLACKHOLING IPADDR - [IP::client_addr] (Reputation=[IP::reputation [IP::client_addr]]) at $now_date until $later_date"
                table set -subtable $static::blacklist_tab -excl [IP::client_addr] 1 indef $static::block_duration


Tested this on version:

Updated Jun 06, 2023
Version 2.0

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