Security researcher Alexander Klink and Julian Wälde revealed a serious vulnerability that until recently affected the vast majority of web server. The attack only requires a single HTTP request that is specially designed to create hash code collisions in POST form data. When first discovered this attack affected Python, Ruby, PHP, Java, and ASP.NET, but vendors have been working with the researchers to produce patches. Tomcat  updates 7.0.23 and 6.0.35 address this issue by limiting the number of POST form fields to 10,000. The  change log  says that this is configurable, but no details were provided. The patch for  ASP.NET  was released on December 29. The patch will be automatically applied for Windows Azure customers with the default servicing policy. The patch works by limiting the number of POST form fields in a single request to 1,000, which is well below the number needed for a denial of service attack.  This value is configurable  using the appSettings key “aspnet:MaxHttpCollectionKeys”. Currently this can only be applied site-wide, but there have been requests for page-specific overrides. A fix was also added for related flaws in the JSON input and deserialization logic. PHP  5.4.0, which is only a release candidate also offers a max_input_vars directive. The  release notes  do not state what the default value is. So far every vendor we’ve discussed has addressed the issue at the web server level by limiting the number of fields in a single request. Another option is the use of a randomized hash code formula for strings.  Ruby  is one such language. .NET does this as well, but only for internal builds. Production releases currently have a set formula, but given the severity of this issue that may change the next time the CLR is updated. For Java it is not quite so easy; the JVM specifies the hash code formula for strings, which means developers may be relying on it to be consistent across all versions. An update for  Oracle Glassfish  is supposedly complete, but not yet available. There is no information of the method used to address the issue. More information about this issue is available on  Ars Technica  and the  Chaos Communication Congress  website.

Excerpt from:
Major Denial of Service Vulnerability Affects Most Web Servers