Ubuntu Security Notice 5771-1 - USN-3557-1 fixed vulnerabilities in Squid. This update introduced a regression which could cause the cache log to be filled with many Vary loop messages. This update fixes the problem. Mathias Fischer discovered that Squid incorrectly handled certain long strings in headers. A malicious remote server could possibly cause Squid to crash, resulting in a denial of service. This issue was only addressed in Ubuntu 16.04 LTS.
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