Huge portions of the Web vulnerable to hashing denial-of-service attack

Researchers have shown how a flaw that is common to most popular Web programming languages can be used to launch denial-of-service attacks by exploiting hash tables. Announced publicly on Wednesday at the Chaos Communication Congress event in Germany, the flaw affects a long list of technologies, including PHP, ASP.NET, Java, Python, Ruby, Apache Tomcat, Apache Geronimo, Jetty, and Glassfish, as well as Google's open source JavaScript engine V8. The vendors and developers behind these technologies are working to close the vulnerability, with Microsoft warning of "imminent public release of exploit code" for what is known as a hash collision attack.

Researchers Alexander Klink and Julian Wälde explained that the theory behind such attacks has been known since at least 2003, when it was described in a paper for the Usenix security conference, and influenced the developers of Perl and CRuby to "change their hash functions to include randomization."

But Klink and Wälde showed that "PHP 5, Java, ASP.NET as well as V8 are fully vulnerable to this issue and PHP 4, Python and Ruby are partially vulnerable, depending on version or whether the server running the code is a 32-bit or 64-bit machine."

"This attack is mostly independent of the underlying Web application and just relies on a common fact of how Web application servers typically work," the team wrote, noting that such attacks would force Web application servers "to use 99% of CPU for several minutes to hours for a single HTTP request."

"Hash tables are a commonly used data structure in most programming languages," they explained. "Web application servers or platforms commonly parse attacker-controlled POST form data into hash tables automatically, so that they can be accessed by application developers. If the language does not provide a randomized hash function or the application server does not recognize attacks using multi-collisions, an attacker can degenerate the hash table by sending lots of colliding keys. The algorithmic complexity of inserting n elements into the table then goes to O(n**2), making it possible to exhaust hours of CPU time using a single HTTP request."

Prior to going public, Klink and Wälde contacted vendors and developer groups such as PHP, Oracle, Python, Ruby, Google, and Microsoft. The researchers noted that the Ruby security team and Tomcat have already released fixes, and that "Oracle has decided there is nothing that needs to be fixed within Java itself, but will release an updated version of Glassfish in a future CPU (critical patch update)."

On Wednesday, Microsoft published its own security advisory, recommending that all ASP.NET website admins evaluate their risk and implement a workaround that Microsoft has posted while it works on a patch to be released in a future security update.

All versions of the .NET Framework are affected by the vulnerability, according to Microsoft. "The vulnerability exists due to the way that ASP.NET processes values in an ASP.NET form post causing a hash collision," Microsoft wrote in its advisory. "It is possible for an attacker to send a small number of specially crafted posts to an ASP.NET server, causing performance to degrade significantly enough to cause a denial of service condition. Microsoft is aware of detailed information available publicly that could be used to exploit this vulnerability but is not aware of any active attacks."

The potential for hash collision attack is garnering quite a bit of discussion on Twitter under the hash tag #hashdos and video of the researchers' presentation is posted on YouTube.

Listing image by Photograph by Elliott Brown