CVE-2026-13221

Description

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.

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Severity score breakdown

CVSS version: CVSS v3.0

References

• MITRE
• NVD
• Launchpad
• Debian

Related Ubuntu Security Notices (USN)

• USN-8675-1
• Perl vulnerabilities
• 25 August 2026
• USN-8684-1
• Perl vulnerabilities
• 27 August 2026

Other references

• https://www.cve.org/CVERecord?id=CVE-2026-13221
• https://lists.security.metacpan.org/cve-announce/msg/41780104/
• https://github.com/Perl/perl5/issues/23388