Bug 2515438 (CVE-2026-19487)

Summary: CVE-2026-19487 perl: Perl: Incorrect regular expression matching can lead to wrong access or filtering decisions.
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, crizzo, dpaolell, gtanzill, jbuscemi, jdelft, jupierce, kshier, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sdawley, sghai, sidsharm, stcannon, suppawar, teagle, vlaad, watson-tool-maintainers, yguenane
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Perl. The regular expression engine may produce incorrect match results due to a stale failure flag in the Aho-Corasick prescan, which causes it to end prematurely. This can lead to the engine missing valid matches or matching on the wrong branch. As a result, applications that use regular expressions for critical functions like access control or data filtering could make erroneous decisions, potentially leading to security bypasses or unintended information disclosure.
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Bug Depends On: 2529177    
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Description OSIDB Bzimport 2026-08-13 16:27:17 UTC 
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.