Squid < 3.5.6 Squid Cache Peer CONNECT Remote Access Bypass
Low Nessus Plugin ID 84674
SynopsisThe remote proxy server is potentially affected by an authentication bypass vulnerability.
DescriptionAccording to its banner, the version of Squid running on the remote host is prior to 3.5.6. It is, therefore, potentially affected by an authentication bypass vulnerability due to a flaw in file tunnel.cc, which is triggered whenever cache peer CONNECT responses are blindly forwarded in a hierarchy of two or more proxies, resulting in unrestricted access to a back-end proxy through its gateway proxy. A remote, unauthenticated attacker, using a specially crafted request, can exploit this vulnerability to bypass authentication or gain access to protected resources. This issue occurs in configurations with cache_peer enabled, and exploitation would require that the two proxies have differing levels of security.
Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number. The patch released to address this issue does not update the version in the banner. If the patch has been applied properly, and the service has been restarted, consider this to be a false positive.
SolutionUpgrade to Squid versions 3.5.6 or later, or apply the vendor-supplied patch.