A squid3 security update has been released for Debian 6 LTS

Package : squid3
Version : 3.1.6-1.2+squeeze5
CVE ID : CVE-2015-5400
Debian Bug : 793128

Alex Rousskov discovered that Squid configured with cache_peer and
operating on explicit proxy traffic does not correctly handle CONNECT
method peer responses. In some configurations, it allows remote clients to
bypass security in an explicit gateway proxy.

For Debian 6 Squeeze, this problem has been fixed in squid3 version
3.1.6-1.2+squeeze5. We recommend that you upgrade your squid3 packages.

  Squid3 security update for Debian 6 LTS