CVE-2026-13221
CriticalCVSS 9.1Exploitation Probability (EPSS)
Low risk45th percentile - higher than 45% of all known CVEs
Summary
In Perl versions up to 5.43.9, when compiling an alternation of more than 65535 fixed string branches into a trie in Perl_study_chunk, a 16-bit field storing the delta between the first branch and the shared tail overflows. This truncates the trie's match decision table without warning or error, causing false positive and false negative regular expression matches.
Risk Assessment
The organization may experience incorrect access or filtering decisions when such a regular expression pattern is used for access control or data validation, potentially leading to security breaches or system malfunctions.
Recommendation
Update Perl to version 5.44.0 or later, which includes a fix for this vulnerability. If an update is not possible, avoid using alternations with more than 65535 fixed string branches in regular expressions.
Original NVD description (English source)
Perl versions through 5.43.9 produce silently incorrect regular expression matches when an alternation of more than 65535 fixed string branches is compiled into a trie in Perl_study_chunk. When such branches are combined into a trie, the delta between the first branch and the shared tail is stored in a 16-bit field. A branch count above 65535 overflows the field, and the trie's match decision table is truncated with no warning or error. A pattern of this shape produces false positive matches (matching strings it should not) and false negative matches (failing to match strings it should). When such a pattern gates an access or filtering decision, the result is wrong.