CVE intelligence

CVE-2026-13221

Vulnerability intelligence

Published Jul 13, 2026Sources checked Oct 27, 2026

9.1CRITICALCVSS out of 10

What this means

Review the available evidence

CVE-2026-13221 and is rated Critical severity with a CVSS score of 9.1. It is not in the current CISA KEV record we collected. That does not prove exploitation has not occurred.

What to do next

Remediation and response

Ubuntu Security Notices guidance

Vendor revision: Sep 9, 2026

USN-8675-1 fixed vulnerabilities in Perl.

Ubuntu Security Notices guidance

Vendor revision: Aug 25, 2026

(CVE-2026-12087) It was discovered that Perl incorrectly handled regular expressions containing a large number of fixed string alternatives.

Microsoft Security Response Center guidance

Vendor revision: Jul 15, 2026

CBL-Mariner Releases

Vendor-listed releases

5.38.2-513

CISA KEVNot listedBased on the latest collected catalog

EPSS0.5%Estimated 30-day exploitation probability

Ransomware useNot markedCISA KEV ransomware field

Threat actors0Source-linked actor relationships

Overview

What is CVE-2026-13221?

Perl versions before 5.40.5-RC1, from 5.41.0 before 5.42.3-RC1, from 5.43.0 before 5.43.10 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.

Source: NVD