Tor before 0.1.1.20 creates "internal circuits" primarily consisting of nodes with "useful exit nodes," which allows remote attackers to conduct unspecified statistical attacks.
https://exchange.xforce.ibmcloud.com/vulnerabilities/26798
http://www.securityfocus.com/bid/18323
http://tor.eff.org/cvs/tor/ChangeLog
http://security.gentoo.org/glsa/glsa-200606-04.xml