The tor_realloc function in Tor before 0.2.1.29 and 0.2.2.x before 0.2.2.21-alpha does not validate a certain size value during memory allocation, which might allow remote attackers to cause a denial of service (daemon crash) via unspecified vectors, related to "underflow errors."
https://trac.torproject.org/projects/tor/ticket/2324
https://gitweb.torproject.org/tor.git/blob/refs/heads/release-0.2.2:/ChangeLog
https://exchange.xforce.ibmcloud.com/vulnerabilities/64888
http://www.securityfocus.com/bid/45953
http://blog.torproject.org/blog/tor-02129-released-security-patches