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From the simple handling of file names and path names, to more complex multipart and unicode parsing, different Web servers and security software handle the HTTP protocol in different ways. By exploiting the disconnect between a Web server and its WAF, an attacker could bypass defenses to exploit vulnerabilities in a Web server, says Ivan Ristic, director of engineering for vulnerability management firm Qualys, who will present his research at Black Hat.
"This is about attacking the way that a Web application firewall interprets data streams," says Ristic, who has developed WAFs for more than a decade, including the creation of the open-source WAF ModSecurity. "There has not been a good public discussion and disclosure of these issues, aside for the occasional vulnerability."
While there has not been a large number of attackers that have used evasion techniques to date, the increasing use of WAFs means that attackers will have to adapt and find ways around them. To help improve the ability of customers and penetration testers to gauge the security of WAFs, Ristic plans to release a set of nearly 150 tests for a variety of security weaknesses that he had identified in current WAFs.
Being able to test a vendor's product before deploying it could help immensely, says Paul Sop, technology evangelist for attack-mitigation firm Prolexic. His company tested a handful of systems and had little trouble bypassing all of them. For most companies, however, that level of evaluation will be out of their capabilities.
"There are many, many different attack vectors, and you have to kind of somehow know which attack vector maps to which features and how do you test it," he says. "How do you certify and prove that this control that you just activated actually works?"
[ Two researchers release a hacking tool called Waldo to test for WebSocket vulnerabilities and potential attack vectors, which will be demonstrated at Black Hat USA. See 'Waldo' Finds Ways To Abuse HTML5 WebSockets. ]
A strong set of tests could help customers check on vendors' claims and help vendors improve their systems, he says. In addition, many companies only turn on the features necessary to be PCI-compliant rather than tuning the WAF to their environment.
"To turn on a certain abstract feature of a WAF, you have to know a lot about the HTTP protocol and the application that you are protecting," Sop says. "If you don't turn on a protection for something, then it's not going to do anything."
Luckily, the run-of-the-mill attacker will likely not be dodging their target's WAF any time soon, says Qualys' Ristic. Designing evasions for WAFs requires a lot of knowledge about the systems, he says.
"These are advanced attacks, so the obvious attackers won't use them," he says. "Developing these types of attacks takes a lot of dedication. I would expect these evasion techniques to be used when a high-value target is involved."
The security technologies add an extra layer of security in front of a public website catching attacks against vulnerabilities that developers may have missed or that have gone unpatched. Overall, WAFs are a good security technology, but need a great deal more research, Ristic argues. In addition, vendors need to be more transparent regarding their technologies and products, he says.
"Users, in general, should make it clear to the vendors that they care about the quality in WAFs," he says. "I have been involved in WAF development for 10 years, but I'm disappointed by the current state of the market."
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