Vulnerability · Published Aug 13, 2026 · Assigned by CPANSec
CVE-2026-19487Perl 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
Medium 5.3
In plain English
A medium-severity security flaw in perl. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 36% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | perl | 5.36.0-7+deb12u4 | DEBIAN-CVE-2026-19487 |
| Debian:13 | perl | 5.40.1-6+deb13u1 | DEBIAN-CVE-2026-19487 |
| Debian:14 | perl | 5.42.2-3 | DEBIAN-CVE-2026-19487 |
| Ubuntu:22.04:LTS | perl | 5.34.0-3ubuntu1.9 | USN-8736-1 |
| Ubuntu:22.04:LTS | perl | 5.34.0-3ubuntu1.9 | UBUNTU-CVE-2026-19487 |
| Ubuntu:24.04:LTS | perl | 5.38.2-3.2ubuntu0.6 | UBUNTU-CVE-2026-19487 |
| Ubuntu:24.04:LTS | perl | 5.38.2-3.2ubuntu0.6 | USN-8736-2 |
| Ubuntu:26.04:LTS | perl | 5.40.1-7ubuntu0.2 | USN-8736-1 |
| Ubuntu:26.04:LTS | perl | 5.40.1-7ubuntu0.2 | UBUNTU-CVE-2026-19487 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| perl | 5.9.4 – before 5.41.9 |
Weakness: CWE-670 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L