CVE-2026-13221 in Perl
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.
Quick answer
Perl should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- Perl through 5.43.9
Fixed versions
- Perl 5.43.10 or later
How to fix it
CVE-2026-13221 affects Perl. In simple terms, an attacker may be able to misuse the affected system if it is exposed or if a user opens unsafe input. Update to Perl 5.43.10 or later. If you cannot update right away, limit access until the fix is installed.
- Find every place where Perl is installed or used.
- Check the installed version against the affected versions on this page.
- Update to Perl 5.43.10 or later.
- If an update is not possible today, turn off the risky feature or restrict it to trusted users only.
- Review recent logs for strange access, failed logins, changed files, or unexpected admin actions.
- Rotate passwords, API keys, and tokens if you think the system may have been abused.
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Verify the fix
- Confirm Perl now runs a fixed version or has the vendor mitigation in place.
- Test the affected feature again and make sure normal users can still work.
- Check that public or untrusted access is blocked where it should be blocked.
- Review logs again after the fix and confirm the same suspicious activity does not continue.
- Run a new Fixnx scan or your normal security check and save the result.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-13221?
Perl versions listed as affected should be reviewed: Perl through 5.43.9.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.