PerlApplication
CVE-2026-13221
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedPerl 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidencePerl versions through 5.43.9 contain a critical flaw in the regex engine where alternations exceeding 65535 fixed string branches cause a 16-bit field overflow when compiling into a trie structure. This silently truncates the match decision table without any warning, resulting in both false positive and false negative matches that can bypass or block access controls.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
PerlApplication
<= 5.43.9CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Determine installed Perl versionRun 'perl -v' or 'perl --version' and locate the version number in the outputAffected if Version is 5.43.9 or lower (the vulnerable range)
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Identify regex patterns using alternationsSearch codebase for regex patterns containing multiple '|' alternation operators, especially in configuration files, application code, or data-driven regex definitionsAffected if Any regex contains more than 65535 distinct alternation branches
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Count alternation branches in suspect patternsFor each regex with many alternations, parse or count the number of fixed string branches to determine if the count exceeds 65535Affected if Any single regex pattern has more than 65535 alternation branches (the overflow threshold)
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Test regex behavior if potentially affectedCreate a test regex with exactly 65536 alternation branches and verify match behavior matches expected results, or inspect trie compilation warningsAffected if Matches produce unexpected results or the trie structure silently truncates without warning
A user is affected if they have Perl version 5.43.9 or lower AND are using regex patterns with more than 65535 alternation branches in their application or configuration.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpgrade Perl to a version beyond 5.43.9 that addresses this overflow, or refactor regex patterns to avoid having more than 65535 alternation branches.
Fix this in Perl
Scoped from the published advisory
- Consultation8.0 h
- Implementation12.0 h
- Testing6.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $8,608.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-13221 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-13221 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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