The MS11-100 fixes a total of four security vulnerabilities (one publicly disclosed) and Microsoft is urging all Windows users to treat this as a "critical" issue because of the risk of privilege escalation attacks if an unauthenticated attacker sends a specially crafted web request to the target site.
An attacker who successfully exploited this vulnerability could take any action in the context of an existing account on the ASP.NET site, including executing arbitrary commands. In order to exploit this vulnerability, an attacker must be able to register an account on the ASP.NET site, and must know an existing user name.
The problem is not unique to Microsoft. As explained by German security outfit n.runs AG (.pdf), some programming language implementations do not sufficiently randomize their hash functions or provide means to limit key collision attacks. This can be be leveraged by an unauthenticated attacker to cause a denial-of-service (DoS) condition.
More details in this oCERT advisory:
The issue finds particular exposure in web server applications and/or frameworks. In particular, the lack of sufficient limits for the number of parameters in POST requests in conjunction with the predictable collision properties in the hashing functions of the underlying languages can render web applications vulnerable to the DoS condition. The attacker, using specially crafted HTTP requests, can lead to a 100% of CPU usage which can last up to several hours depending on the targeted application and server performance, the amplification effect is considerable and requires little bandwidth and time on the attacker side.
In addition to Microsoft .NET, the following programming languages are affected:
- Java, all versions
- JRuby <= 1.6.5
- PHP <= 5.3.8, <= 5.4.0RC3
- Python, all versions
- Rubinius, all versions
- Ruby <= 1.8.7-p356
- Apache Geronimo, all versions
- Apache Tomcat <= 5.5.34, <= 6.0.34, <= 7.0.22
- Oracle Glassfish <= 3.1.1
- Jetty, all versions
- Plone, all versions
- Rack <= 1.3.5, <= 1.2.4, <= 1.1.2