Perl EUVD-2026-58110

Perl versions from 5.9.4 before 5.41.9 produce incorrect regular expression match results when a stale failure flag ends the Aho-Corasick prescan early in S_find_byclass.

The prescan walks the subject for positions where the full pattern could match, and the engine tries it from the leftmost one recorded. A failing transition sets the failed flag, and a later successful transition does not clear it, so the prescan reads the stale flag as a failure and stops before it can record a candidate that starts earlier. It takes a subject where one candidate is recorded and a later character then forces a fallback through a fail link that succeeds.

Example:

"ABCDE" =~ m/ABCF|BCDE|C/;

matches C at offset 2, not BCDE

"ABCDE" =~ m/ABCF|BCDE|C(G)/;

no match, BCDE missed

An alternation like this can miss input it should match, or match it on the wrong branch, so an access or filtering decision made from the result can be wrong.

Perl interpreter versions 5.9.4 through 5.41.8 (all releases before 5.41.9) are affected across all platforms. No CPE strings were provided in the intelligence data; affected software is the core perl5 interpreter distributed by The Perl Foundation and available via CPAN, Linux distribution packages, and vendor-bundled Perl installations. The vulnerability is tracked at https://github.com/Perl/perl5/issues/22892 and the upstream fix commit is at https://github.com/Perl/perl5/commit/1a21abacaf6f684928bae8baaa153733c8c238eb.