CVE-2026-13221: 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

mediumCVSS 4CVE-2026-13221

CSIRTS triage

vendor: Perl product: PerlOtheraffected: through 5.43.9

What: Produces silently incorrect regular expression matches when an alternation of more than 65535 fixed string branches is compiled into a trie.
Who is affected: Users of Perl versions through 5.43.9.
Urgency: Remediation is advisable due to the medium severity and potential for incorrect behavior.
Action: Upgrade to a version of Perl beyond 5.43.9.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: medium — CVSS 4
Published: 2026-07-14
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-13221

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-132210.43% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 36% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-13221	coverage & exploitation status	NVD · CVE.org

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