HTTP::Parser::XS versions before 0.18 for Perl truncate PATH_INFO at an encoded NUL byte in parse_http_request. The XS parser URL-decodes the request path and measures the result with strlen(), so PATH_INFO ends at the first byte decoded from %00. The pure-Perl backend is not affected. A remote attacker can send a path that a proxy or filter checks in full while the application routes on a shorter one.