XML Threat Prevention

Where should security live?

The divide between operations and application developers is pretty wide, especially when it comes to defining who should be ultimately responsible for application security.

Mike Fratto and I have often had lively discussions (read: arguments) about whether security is the responsibility of the developer or the network and security administrators. It's wholly inappropriate to recreate any of these discussions here, as they often devolve to including the words your mother said not to use in public. 'Nuff said.

The truth is that when XML enters the picture then the responsibility for securing that traffic has to be borne by both the network/security administrators and the developers. While there is certainly good reason to expect that developers are doing simply security checks for buffer overflows, length restrictions on incoming data, and strong typing, the fact is that there are some attacks in XML that make it completely impractical to check for in the code.

Let's take a couple of attack types as examples.

XML Entity Expansion

This attack is a million laughs, or at least a million or more bytes of memory.

Applications need to parse XML in order to manipulate it, so the first thing that happens when XML hits an application is that it is parsed - before the developer even has a chance to check it. In an application server this is generally done before the arguments to the specific operation being invoked are marshaled - meaning it is the application server, not the developer that is responsible for handling this type of attack.

These messages can be used to force recursive entity expansion or other repeated processing that exhausts server resources. The most common example of this type of attack is the "billion laughs" attack, which is widely available. The CPU is monopolized while the entities are being expanded, and each entity takes up X amount of memory - eventually consuming all available resources and effectively preventing legitimate traffic from being processed. It's essentially a DoS (Denial of Service) attack.

  
  
  
  
  
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It is accepted that almost all traditional DoS attacks (ping of death, teardrop, etc...) should be handled by a perimeter security device - a firewall or an application delivery controller - so why should a DoS attack that is perpetrated through XML be any different?

It shouldn't. This isn't a developer problem, it's a parser problem and for the most part developers have little or no control over the behavior of the parser used by the application server. The application admin, however, can configure most modern parsers to prevent this type of attack, but that's assuming that their parser is modern and can be configured to handle it. Of course then you have to wonder what happens if that arbitrary limit inhibits processing of valid traffic? Yeah, it's a serious problem.

SQL Injection

SQL Injection is one of the most commonly perpetrated attacks via web-based applications. It consists basically of inserting SQL code into string-based fields which the attacks thinks (or knows) will be passed to a database as part of an SQL query. This type of attack can easily be accomplished via XML as well simply by inserting the appropriate SQL code into a string element.

Aha! The developer can stop this one, you're thinking. After all, the developer has the string and builds the SQL that will be executed, so he can just check for it before he builds the string and sends it off for execution.

While this is certainly true, there are myriad combinations of SQL commands that might induce the database to return more data than it should, or to return sensitive data not authorized to the user. This extensive list of commands and combinations of commands would need to be searched for in each and every parameter used to create an SQL query and on every call to the database. That's a lot of extra code and a lot of extra processing - which is going to slow down the application and impede performance.

And when a new attack is discovered, each and every function and application needs to be updated, tested, and re-deployed. I'm fairly certain developers have better ways to spend their time than updating parameter checking in every function in every application they have in production. And we won't even talk about third-party applications and the dangers inherent in that scenario.

One of the goals of SOA is engendering reuse, and this is one of the best examples of taking advantage of reuse in order to ensure consistent behavior between applications and to reduce the lengthy development cycle required to update, test, and redeploy whenever a new attack is discovered. By placing the onus for keeping this kind of attack from reaching the server on an edge device such as an application firewall like F5's application firewall, updates to address new attacks are immediately applied to all applications and there is no need to recode and redeploy applications.

Although there are some aspects of security that are certainly best left to the developer, there are other aspects of security that are better deployed in the network. It's the most effective plan in terms of effort, cost, and consistent behavior where applications are concerned.