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.

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Fixes

Solution

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


Workaround

No workaround given by the vendor.

History

Thu, 13 Aug 2026 21:30:00 +0000

Type Values Removed Values Added
References

Thu, 13 Aug 2026 19:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Thu, 13 Aug 2026 18:30:00 +0000

Type Values Removed Values Added
Metrics cvssV3_1

{'score': 5.3, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'}


Thu, 13 Aug 2026 18:15:00 +0000

Type Values Removed Values Added
First Time appeared Perl
Perl perl
Vendors & Products Perl
Perl perl

Thu, 13 Aug 2026 16:00:00 +0000

Type Values Removed Values Added
Description 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.
Title 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
Weaknesses CWE-670
References

Projects

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cve-icon MITRE

Status: PUBLISHED

Assigner: CPANSec

Published:

Updated: 2026-08-13T20:12:11.045Z

Reserved: 2026-08-10T16:35:50.763Z

Link: CVE-2026-19487

cve-icon Vulnrichment

Updated: 2026-08-13T20:12:11.045Z

cve-icon NVD

Status : Received

Published: 2026-08-13T16:17:59.440

Modified: 2026-08-13T21:17:46.147

Link: CVE-2026-19487

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-08-13T19:45:17Z

Weaknesses