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

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-19487
StatePUBLISHED
AssignerCPANSec
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-13 16:17:59 UTC
Updated2026-08-13 16:17:59 UTC
DescriptionPerl 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.

Risk And Classification

Problem Types: CWE-670 | CWE-670 CWE-670 Always-Incorrect Control Flow Implementation

References

ReferenceSourceLinkTags
github.com/Perl/perl5/issues/22892 9b29abf9-4ab0-4765-b253-1875cd9b441e github.com
github.com/Perl/perl5/commit/1a21abacaf6f684928bae8baaa153733c8c238eb.patch 9b29abf9-4ab0-4765-b253-1875cd9b441e github.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Additional Advisory Data

Solutions

CNA: Upgrade to Perl 5.42.0 or later, or apply the upstream patch.

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