CVE-2026-19487

Perl versions from 5.9.4 before 5.41.9 can produce incorrect regular-expression match results because a stale failure flag in the Aho-Corasick prescan (S_find_byclass) is not cleared when a later transition succeeds, causing the prescan to stop before it records an earlier valid match position. The flaw is triggered by alternation patterns where one candidate position is recorded and a later character then forces a fallback through a fail link that succeeds; for example, 'ABCDE' =~ m/ABCF|BCDE|C/ matches C at offset 2 instead of BCDE, and m/ABCF|BCDE|C(G/) fails to match at all. An attacker who controls input to such a regex can make an access-control, validation, or filtering decision resolve incorrectly, either matching the wrong alternative branch or missing a match entirely, with impact rated low (CVSS 5.3, medium). Any application running an affected Perl interpreter whose security-relevant regexes use alternations is exposed, and because Perl ships by default with virtually all Linux and Unix systems the installed base is enormous even though only specific pattern shapes trigger the bug. No public proof-of-concept is known, the issue is not in CISA KEV, and EPSS puts the 30-day exploitation probability at 0.4%, so no exploitation is currently known.

What to do: Upgrade to Perl 5.41.9 or later, or apply vendor backports such as Ubuntu's USN-8736-1 for affected distribution packages. Audit regex-based access-control, validation, and filtering rules that use alternations, and test them against inputs where a later alternative should match after an earlier one fails. No reliable workaround exists because the defect is inside the regex engine itself, so patching is the primary remediation.