Perl Regex Stale Flag Causing Wrong Matches before v5.41.9
CVE-2026-19487 Published on August 13, 2026
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
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.
Vulnerability Analysis
CVE-2026-19487 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. An automatable proof of concept (POC) exploit exists. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a small impact on availability.
Weakness Type
Always-Incorrect Control Flow Implementation
The code contains a control flow path that does not reflect the algorithm that the path is intended to implement, leading to incorrect behavior any time this path is navigated. This weakness captures cases in which a particular code segment is always incorrect with respect to the algorithm that it is implementing. For example, if a C programmer intends to include multiple statements in a single block but does not include the enclosing braces (CWE-483), then the logic is always incorrect. This issue is in contrast to most weaknesses in which the code usually behaves correctly, except when it is externally manipulated in malicious ways.
Products Associated with CVE-2026-19487
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