Tor before 0.1.1.20 allows remote attackers to identify hidden services via a malicious Tor server that attempts a large number of accesses of the hidden service, which eventually causes a circuit to be built through the malicious server.
https://exchange.xforce.ibmcloud.com/vulnerabilities/24285
http://www.securityfocus.com/bid/19795
http://www.securityfocus.com/bid/18323
http://tor.eff.org/cvs/tor/ChangeLog
http://security.gentoo.org/glsa/glsa-200606-04.xml
http://secunia.com/advisories/20514