Security

Security Advisory: ZF2013-02

ZF2013-02: Potential Information Disclosure and Insufficient Entropy vulnerabilities in Zend\Math\Rand and Zend\Validate\Csrf Components

In Zend Framework 2, the Zend\Math\Rand component generates random bytes using the OpenSSL or Mcrypt extensions when available but will otherwise use PHP's mt_rand() function as a fallback. All outputs from mt_rand() are predictable for the same PHP process if an attacker can brute force the seed used by the Marsenne-Twister algorithm in a Seed Recovery Attack. This attack can be successfully applied with minimum effort if the attacker has access to either a random number from mt_rand() or a Session ID generated without using additional entropy. This makes mt_rand() unsuitable for generating non-trivial random bytes since it has Insufficient Entropy to protect against brute force attacks on the seed.

The Zend\Validate\Csrf component generates CSRF tokens by SHA1 hashing a salt, random number possibly generated using mt_rand() and a form name. Where the salt is known, an attacker can brute force the SHA1 hash with minimum effort to discover the random number when mt_rand() is utilised as a fallback to the OpenSSL and Mcrypt extensions. This constitutes an Information Disclosure where the recovered random number may itself be brute forced to recover the seed value and predict the output of other mt_rand() calls for the same PHP process. This may potentially lead to vulnerabilities in areas of an application where mt_rand() calls exist beyond the scope of Zend Framework.

Action Taken

Zend Framework have revised the Zend\Math\Rand component to replace the current mt_rand() fallback for OpenSSL/Mcrypt with Anthony Ferrara's RandomLib, incorporating an additional entropy source based on source code published by George Argyros. The new fallback collects entropy from numerous sources other than PHP's internal seed mechanism and extracts random bytes from the resulting mixed entropy pool.

Recommendations

If you are using either Zend\Math\Rand or Zend\Validate\Csrf, do not have either the OpenSSL or Mcrypt extensions installed in PHP, and are on a non-Unix-like system, we recommend upgrading immediately to version 2.0.8 or 2.1.4.

Other Information

Acknowledgments

The Zend Framework team thanks the following for identifying the issues and working with us to help protect its users:

  • Pádraic Brady for identifying and reporting the issue, as well as providing a patch to resolve the issue
  • Enrico Zimuel for collaborating on and reviewing the solution

Released 2013-03-14

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