In a rare move, Microsoft is breaking its normal procedures and will issue an emergency out-of-band security update on Thursday to address a recently disclosed vulnerability that affects various Web platforms industry-wide.

Update: Microsoft has released the security bulletin as planned. The patches are available here.

The vulnerability comes in the way attackers could exploit hash tables to cause a denial of service (DoS) condition. “Attacks targeting this type of vulnerability are generically known as hash collision attacks,” Microsoft notes. “Attacks such as these are not specific to Microsoft technologies and affect other web service software providers.”

According to n.runs AG, the research firm who discovered the issue, the vulnerability has been discovered to impact PHP 5, Java, .NET, and Google’s v8, while PHP 4, Ruby, and Python are somewhat vulnerable. The Ruby security team has addressed the issue, as well as Tomcat.

According to Microsoft, the vulnerability affects all versions of Microsoft .NET Framework and could allow for an unauthenticated denial of service attack on servers that serve ASP.NET pages. Sites that only serve static content or disallow dynamic content types listed in the mitigation factors below are not vulnerable.

Microsoft released the bulletin on December 29, 2011, at 10:00 AM Pacific Time, and said it addresses vulnerabilities in all supported releases of Microsoft Windows.

“Hash tables are a commonly used data structure in most programming languages. 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,” n.runs AG's report explains.

“The impact of this vulnerability is similar to other Denial of Service attacks that have been released in the past, such as the Slowloris DoS or the HTTP POST DoS,” Chris Eng, Vice President of Research at Veracode told SecurityWeek. “Unlike traditional DoS attacks, they could be conducted with very small amounts of bandwidth. This hash table multi-collision bug shares that property. What’s particularly unique about this bug is that it affects a broader range of platforms and technologies in a virtually identical way.”

Microsoft is hosting a webcast which will provide additional information and address questions on the out-of-band security bulletin on December 29, 2011, at 1:00 PM Pacific Time. You can register for the webcast here.

The research from n.runs AG explaining the vulnerability in detail is available here. Information on mitigation and patches are available for the following: PHP | Ruby | Microsoft | Tomcat