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.
Analysis
Analysis and contextual insights are available on OpenCVE Cloud.
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | ||||||
|---|---|---|---|---|---|---|---|---|---|
| unaffected |
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| Vendor | Product | Confidence | Versions | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Perl | Perl | 80% |
|
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Vendor Solution
Upgrade to Perl 5.42.0 or later, or apply the upstream patch.
Tracking
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No advisories yet.
Thu, 13 Aug 2026 19:30:00 +0000
| Type | Values Removed | Values Added |
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| Metrics |
ssvc
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Thu, 13 Aug 2026 18:30:00 +0000
| Type | Values Removed | Values Added |
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| Metrics |
cvssV3_1
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Thu, 13 Aug 2026 18:15:00 +0000
| Type | Values Removed | Values Added |
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| First Time appeared |
Perl
Perl perl |
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| Vendors & Products |
Perl
Perl perl |
Thu, 13 Aug 2026 16:00:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| 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. | |
| Title | 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 | |
| Weaknesses | CWE-670 | |
| References |
|
MITRE
Status: PUBLISHED
Assigner: CPANSec
Published:
Updated: 2026-08-13T17:48:30.123Z
Reserved: 2026-08-10T16:35:50.763Z
Link: CVE-2026-19487
Vulnrichment
Updated: 2026-08-13T17:46:59.130Z
NVD
Status : Received
Published: 2026-08-13T16:17:59.440
Modified: 2026-08-13T18:17:24.750
Link: CVE-2026-19487
Redhat
No data.
OpenCVE Enrichment
Updated: 2026-08-13T18:00:04Z