Hash Collision DoS Vulnerability Fixed in PHP 5.3.9

The PHP development team has released version 5.3.9 of the popular Web development platform in order to address a recently disclosed denial-of-service (DoS) vulnerability, as well as other security issues and bugs.

The DoS vulnerability was disclosed in December 2011 at the Chaos Communication Congress, Europe's largest hacker conference, by security researchers Alexander Klink and Julian Wälde. It affects a number of development platforms including PHP, ASP.NET, Java and Python.

Identified as CVE-2011-4885, the vulnerability allows an attacker to perform what is known as a hash collision attack by forcing the server to process a specially crafted form that contains thousands of values.

This type of HTTP request can trigger a computationally intensive routine, resulting in a denial-of-service condition. For ASP.NET, a request of approximately 100KB in size can consume 100 percent of a CPU core for 90 to 110 seconds.

Sending multiple requests can have an impact on the responsiveness of even a cluster of servers that use multicore processors. The newly released PHP 5.3.9 fixes this issue by adding a max_input_vars directive to the configuration file.

A separate DoS vulnerability that can be exploited to read arbitrary memory locations was also addressed in this release. Identified as CVE-2011-4566, the flaw stems from a bug in the PHP function that parses exif headers. Attackers can exploit the vulnerability by tricking the server into processing JPEG files with specially crafted offset_val values in their Exif headers.

In addition to patching these two vulnerabilities, PHP 5.3.9 contains fixes for a large number of non-security-related bugs, as well as various enhancements.

This stable release comes after the development team made available the fifth release candidate for the upcoming PHP 5.4. "The next and probably last release candidate will be released in 14 days," the developers said at the time.

Earlier this week, a user affiliated with the Anonymous hacktivist movement released working attack code for the hash collision DoS vulnerability in ASP.NET. A similar exploit for the PHP version would not be hard to produce. "All users are strongly encouraged to upgrade to PHP 5.3.9," the platform's developers said.

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