Microsoft has released an out-of-cycle patch designed to address a serious flaw in its ASP.Net web application development toolkit.
The vulnerability, which has been under active attack for several weeks, creates a mechanism for attackers to read any file on a web application server. Microsoft rates the flaw as only "important", while independent security watchers such the the SANS Institute's Internet Storm Centre say that rating underestimates the risk posed by the flaw to online shops built using Microsoft's developer tools. The ISC has raised the InfoCon status of the flaw from green to yellow.
Microsoft's advisory provides more detail on the "information disclosure" flaw. It explains that "in Microsoft .NET Framework 3.5 Service Pack 1 and above, this vulnerability can be used by an attacker to retrieve the contents of any file within the ASP.NET application, including web.config" and that "this vulnerability can also be used for data tampering, which, if successfully exploited, could be used to decrypt and tamper with the data encrypted by the server".
Redmond decided the problem was too severe to wait for the next scheduled Patch Tuesday, 12 October, meriting an unusual but not unprecedented decision to release an out-of-cycle patch (MS10-070) instead.
The vulnerability stems from a cryptographic weakness, specifically involving improper error handling during encryption padding verification.
Attacks based on this weakness could allow a hacker to decrypt sniffed cookies or forge authentication tickets, among other attacks. Researchers Thai Duong and Juliano Rizzo discovered the flaw and presented a Padding Oracle Exploit Tool that illustrated the risk during a presentation at the Ekoparty conference in Argentina last month. ®