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 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.

Perl 5 is a highly capable, feature-rich programming language with over 30 years of development. Perl 5 runs on over 100 platforms from portables to mainframes and is suitable for both rapid prototyping and large scale development projects.

Security Fix(es):

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.(CVE-2026-19487)

Perl 5 is a highly capable, feature-rich programming language with over 30 years of development. Perl 5 runs on over 100 platforms from portables to mainframes and is suitable for both rapid prototyping and large scale development projects.

Security Fix(es):

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.(CVE-2026-19487)

Perl 5 is a highly capable, feature-rich programming language with over 30 years of development. Perl 5 runs on over 100 platforms from portables to mainframes and is suitable for both rapid prototyping and large scale development projects.

Security Fix(es):

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.(CVE-2026-19487)

Perl 5 is a highly capable, feature-rich programming language with over 30 years of development. Perl 5 runs on over 100 platforms from portables to mainframes and is suitable for both rapid prototyping and large scale development projects.

Security Fix(es):

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.(CVE-2026-19487)

Perl 5 is a highly capable, feature-rich programming language with over 30 years of development. Perl 5 runs on over 100 platforms from portables to mainframes and is suitable for both rapid prototyping and large scale development projects.

Security Fix(es):

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.(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 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.

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.

• SECURITY UPDATE: incorrect regular expression match when the Aho-Corasick prescan resumes after following a fail link
  • debian/patches/CVE-2026-19487.patch: reset the "failed" flag on a successful trie transition in S_find_byclass() in regexec.c, and add the upstream regression tests to t/re/re_tests
  • CVE-2026-19487

• SECURITY UPDATE: incorrect regex match in Aho-Corasick prescan (GH#22892)
  • debian/patches/fixes/CVE-2026-19487.patch: reset the stale failed flag after a successful trie transition in the AHOCORASICK arm of S_find_byclass() in regexec.c, so the prescan is no longer aborted on a worse or non-matching alternative; adds the upstream regression vectors to t/re/re_tests
  • CVE-2026-19487

It was discovered that Perl incorrectly handled certain large inputs during regular expression matching. An attacker could possibly use this issue to trigger out-of-bounds heap reads or writes, resulting in a denial of service or arbitrary code execution. (CVE-2026-15534)

It was discovered that Perl incorrectly handled certain regular expression containing alternative matching branches. An attacker could possibly use this issue to cause incorrect regular expression matches, resulting in security restrictions being bypassed. (CVE-2026-19487)

USN-8736-1 fixed vulnerabilities in Perl. This update provides the corresponding fix for Perl on Ubuntu 24.04 LTS.

Original advisory details:

It was discovered that Perl incorrectly handled certain large inputs during regular expression matching. An attacker could possibly use this issue to trigger out-of-bounds heap reads or writes, resulting in a denial of service or arbitrary code execution. (CVE-2026-15534)

It was discovered that Perl incorrectly handled certain regular expression containing alternative matching branches. An attacker could possibly use this issue to cause incorrect regular expression matches, resulting in security restrictions being bypassed. (CVE-2026-19487)