Wisdom of Squid for More Security Sensitive Applications

CVE-2009-2621 was released last week. The issues seem to include a buffer-overflow, which implies a potentially easy remote root shell exploit1. So it’s perhaps worth repeating my previous claim that Squid really should not be used for security sensitive applications.

This is not to take away from Squid. As the swiss-army knife of web-caching/proxying software it’s awesomely valuable software. It’s just that building for security is somewhat at odds with software intended to be highly flexible and featureful.

Even if Squid’s code were 100% defect free managing its default feature-set remains a significant security problem. E.g. my own ISPs’ Squid-using Pædosieve still honours CONNECT to anywhere, proving just how hard it is to configure such software to disable unneeded features – and they were made aware of the problem many months ago. Prior to that they had forgotten to lock-down access to the internal in-band management protocol (accessible by using CONNECT proxying to localhost). They only recently disabled SMTP on the Pædo-sieve, possibly because they received email sent through it via the Squid proxy.

1. The bug report mentions this is triggerable via ESI, which I don’t think is relevant to a Pædo-sieve, but as it was in some generic string storage code there may well be many other paths to the bug.

Also, bugs were found in the HTTP header parser, where it will assert if fed a very large header. Relying on assert() is somewhat fragile, as these can be disabled by the compiler. Indeed, the fix for the ESI buffer overlow is to add an assert, it seems. Not confidence inspiring, security wise at least.

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