---
id: CVE-2026-13221
title: >-
  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…
summary: >-
  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…
severity: critical
cvss: 9.1
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H'
cwe:
  - CWE-190
  - CWE-625
vendor: perl
product: perl
affected:
  - perl <= 5.43.9
published: '2026-07-13'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T22:17:37.217'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-13221'
references:
  - url: >-
      https://github.com/Perl/perl5/commit/03f74bbbd3a68350d926ee93d56ee4808c28c4c7.patch
    label: 9b29abf9-4ab0-4765-b253-1875cd9b441e
  - url: 'https://github.com/Perl/perl5/issues/23388'
    label: 9b29abf9-4ab0-4765-b253-1875cd9b441e
  - url: 'http://www.openwall.com/lists/oss-security/2026/07/13/5'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-13221.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-13221'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2499727'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-13221'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-13221'
  - url: 'https://access.redhat.com/errata/RHSA-2026:67156'
  - url: 'https://access.redhat.com/errata/RHSA-2026:67162'
  - url: 'https://access.redhat.com/errata/RHSA-2026:67278'
  - url: 'https://access.redhat.com/errata/RHSA-2026:67155'
  - url: 'https://access.redhat.com/errata/RHSA-2026:39997'
  - url: 'https://access.redhat.com/errata/RHSA-2026:67956'
tags:
  - nvd
  - cve.org
  - csaf
  - vex
  - red-hat
  - score-dispute
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-07-14T13:33:13.127997Z'
epss: 0.00432
epssPercentile: 0.34924
ingestedAt: '2026-09-08T22:12:31.038Z'
patched:
  - enterprise_linux_appstream_v_10
  - enterprise_linux_appstream_v_8
  - enterprise_linux_appstream_v_9
  - enterprise_linux_baseos_v_8
  - hardened_images
scores:
  nvd: 9.1
  vendor: 5.9
---

## Overview

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.

## Affected

- `perl <= 5.43.9`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

## Vendor advisories

- **RHSA-2026:67156** · Red Hat · fixed in: Red Hat Enterprise Linux AppStream (v. 10) · released 2026-09-14 · [advisory](https://access.redhat.com/errata/RHSA-2026:67156)
- **RHSA-2026:67162** · Red Hat · fixed in: Red Hat Enterprise Linux AppStream (v. 8), Red Hat Enterprise Linux BaseOS (v. 8) · released 2026-09-14 · [advisory](https://access.redhat.com/errata/RHSA-2026:67162)
- **RHSA-2026:67278** · Red Hat · fixed in: Red Hat Enterprise Linux AppStream (v. 8) · released 2026-09-14 · [advisory](https://access.redhat.com/errata/RHSA-2026:67278)
- **RHSA-2026:67155** · Red Hat · fixed in: Red Hat Enterprise Linux AppStream (v. 9) · released 2026-09-14 · [advisory](https://access.redhat.com/errata/RHSA-2026:67155)
- **RHSA-2026:39997** · Red Hat · fixed in: Red Hat Hardened Images · released 2026-07-15 · [advisory](https://access.redhat.com/errata/RHSA-2026:39997)
- **Red Hat VEX** · Moderate · affected: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat OpenShift Container Platform 4, Red Hat OpenShift AI (RHOAI), Red Hat OpenShift Dev Spaces · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat OpenShift Container Platform 4 · updated 2026-09-21 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-13221.json)
- **RHSA-2026:67956** · Red Hat · fixed in: Red Hat Update Infrastructure 5 · released 2026-09-16 · [advisory](https://access.redhat.com/errata/RHSA-2026:67956)