A variety of programming languages suffer from a denial-of-service (DoS) condition against storage functions of key/value pairs in hash data structures, the condition can be leveraged by exploiting predictable collisions in the underlying hashing algorithms.
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.
The condition for predictable collisions in the hashing functions has been reported for the following language implementations: Java, JRuby, PHP, Python, Rubinius, Ruby. In the case of the Ruby language, the 1.9.x branch is not affected by the predictable collision condition since this version includes a randomization of the hashing function.
The vulnerability outlined in this advisory is practically identical to the one reported in 2003 and described in the paper Denial of Service via Algorithmic Complexity Attacks which affected the Perl language.
The reporters own advisory can be found at http://www.nruns.com/_downloads/advisory28122011.pdf
Affected version: 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
Fixed version: Java, N/A JRuby >= 184.108.40.206 PHP >= 5.3.9, >= 5.4.0RC4 Python, N/A Rubinius, N/A Ruby >= 1.8.7-p357, 1.9.x
Credit: vulnerability report and PoC code received from Alexander Klink <alexander.klink AT nruns.com> and Julian Waelde <jwaelde AT cdc.informatik.tu-darmstadt.de>.
CVE: CVE-2011-4461 (Jetty), CVE-2011-4838 (JRuby), CVE-2011-4885 (PHP), CVE-2011-4462 (Plone), CVE-2011-4815 (Ruby)
2011-09-25: vulnerability report received, reporters set embargo date to December 27th 2011-10-18: contacted maintainers of Apache Tomcat, Apache Geronimo, Jetty, Java, Plone, Zope, V8 2011-11-01: contacted maintainers of Ruby on Rails, Ruby, Python, PHP 2011-11-01: contacted affected distributions 2011-11-02: contacted JRuby maintainer 2011-12-13: contacted Ruby Installer maintainer 2011-12-14: assigned CVE for Ruby 2011-12-15: assigned CVE for JRuby 2011-12-13: contacted Rack maintainer 2011-12-16: assigned CVE for Apache Tomcat 2011-12-21: assigned CVE for PHP 2011-12-28: advisory release
References: http://www.nruns.com/_downloads/advisory28122011.pdf http://www.cs.rice.edu/~scrosby/hash/CrosbyWallach_UsenixSec2003.pdf http://svn.php.net/viewvc?view=revision&revision=321003 (unstable, not final) http://svn.php.net/viewvc?view=revision&revision=321040 (unstable, not final) https://gist.github.com/52bbc6b9cc19ce330829
-- Andrea Barisani | Founder & Project Coordinator oCERT | OSS Computer Security Incident Response Team
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