Incorrect processing of responses to If-None-Modified HTTP conditional requests in Squid HTTP Proxy 3.1.10 through 3.1.23, 3.2.0.3 through 3.5.22, and 4.0.1 through 4.0.16 leads to client-specific Cookie data being leaked to other clients. Attack requests can easily be crafted by a client to probe a cache for this information.
http://www.squid-cache.org/Advisories/SQUID-2016_11.txt
http://www.securitytracker.com/id/1037513
http://www.securityfocus.com/bid/94953
http://www.openwall.com/lists/oss-security/2016/12/18/1
http://www.debian.org/security/2016/dsa-3745