Buffer Overflow in ext_lm_group_acl helper

## Summary Due to incorrect buffer management ext_lm_group_acl is vulnerable to a denial of service attack when processing NTLM Authentication credentials. This problem is limited to installations using the ext_lm_group_acl binary. ## Affected Versions Squid 2.x -> 2.7.STABLE9 Squid 3.x -> 3.5.28 Squid 4.x -> 4.9 ## Severity Due to incorrect input validation the NTLM authentication credentials parser in ext_lm_group_acl may write to memory outside the credentials buffer. On systems with memory access protections this can result in the helper process being terminated unexpectedly. Resulting in the Squid process also terminating and a denial of service for all clients using the proxy. ## Supporting Material/References: Advisory : http://www.squid-cache.org/Advisories/SQUID-2020_3.txt ## Remediation An official patch is available from the Squid archives for both Squid 3.5 and Squid 4. ## Timeline 2019-11-11 : I reported the issue 2019-11-18 : I made a PR on GitHub with a fix 2019-11-22 : Fix was merged ## Impact See 'Severity' section of report.