What is this vulnerability?

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.

How it works

This is classified as Integer Overflow. In plain terms, an attacker could use this flaw in Perl to gain access, disrupt service, or steal data — the exact impact depends on how it’s deployed in your environment.

CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Mitigations

• Apply the vendor’s security patch or update as soon as it’s available and tested.
• If no patch exists yet, apply the vendor’s temporary workaround.
• Limit network exposure of the affected system until it’s patched.
• Track the vendor advisory — disclosed vulnerabilities can move to active exploitation quickly.

Recommendations

• Confirm whether Perl Perl is used anywhere in your environment.
• Patch internet-facing systems first, then internal ones.
• Prioritise by exposure and exploitability while a fix is scheduled.
• Subscribe to the vendor’s security advisories for earlier warning next time.