The "%xx" URL decoding function in Squid 2.5STABLE4 and earlier allows remote attackers to bypass url_regex ACLs via a URL with a NULL ("%00") character, which causes Squid to use only a portion of the requested URL when comparing it against the access control lists.
AV:N/AC:L/Au:N/C:P/I:P/A:P
This vulnerability has a 13.81% probability of being exploited in the next 30 days, ranking higher than 96% of all scored CVEs.