HTTP::Headers::ActionPack versions before 0.10 for Perl allow CPU exhaustion via Accept-Language ranges matched as regular expressions. _language_match interpolates each Accept-Language range into the pattern "/^$range\-/i" without escaping it, so regular expression syntax in the header reaches the regex engine. A 12-byte range such as "(a?|b?){26}z" backtracks for seconds, and each increment of the quantifier doubles the time. An invalid range such as "(" fails to compile. A remote attacker can exhaust CPU, or make choose_language die, in any application that passes an untrusted Accept-Language header to it.