Bug 2529177

Disclaimer: Community trackers are created by Red Hat Product Security team on a best effort basis. Package maintainers are required to ascertain if the flaw indeed affects their package, before starting the update process.

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.