CVE-2026-19487 Perl versions from 5.9.4 before 5.41.9 produce incorrect regular expression match results when a stale failure flag ends the Aho-Corasick pr
Perl regex engine incorrectly matches patterns due to a stale flag, potentially causing security filtering or access control decisions to fail.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-19487?
CVE-2026-19487 is a medium-severity vulnerability, published on 13 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Perl versions from 5.9.4 before 5.41.9 produce incorrect regular expression match results when a stale failure flag ends the Aho-Corasick prescan early in S_find_byclass.
The prescan walks the subject for positions where the full pattern could match, and the engine tries it from the leftmost one recorded. A failing transition sets the failed flag, and a later successful transition does not clear it, so the prescan reads the stale flag as a failure and stops before it can record a candidate that starts earlier. It takes a subject where one candidate is recorded and a later character then forces a fallback through a fail link that succeeds.
Example:
"ABCDE" =~ m/ABCF|BCDE|C/; # matches C at offset 2, not BCDE "ABCDE" =~ m/ABCF|BCDE|C(G)/; # no match, BCDE missed
An alternation like this can miss input it should match, or match it on the wrong branch, so an access or filtering decision made from the result can be wrong.
Weakness: CWE-670
Which versions are affected by CVE-2026-19487?
How do I fix CVE-2026-19487?
Upgrade to a fixed version.
Is CVE-2026-19487 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-19487 exists.
Sources
Also known as AZL-95756, BELL-CVE-2026-19487, CLSA-2026-1789043015, CLSA-2026-1789204224, DEBIAN-CVE-2026-19487, ECHO-f6c7-844b-6a34 and 9 more.