PT-2026-135865

Published

2026-10-23

·

Updated

2026-10-23

Description by

CVSS 4.0
5.5
Medium
MetricsAV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N/E:P/AU:Y/R:U
ANT-2026-KKC4QS4S Str partial-match stale capture registers underflow
Str.string partial match records the beginning and end of a captured group. There was an implicit assumption that beginning <= end. However if it reached the end of the matched string but not the end of the regexp, it was possible for the beginning of the group to be matched at iteration N of the star operator, and the end of the group to be matched at iteration N-1, resulting in beginning > end. Str.matched group is safe, it fails with an Invalid argument on groups with beginning > end, but Str.replace matched can crash, trying to memcpy end - beginning bytes.
The example in the ANT is ab partially matched against ((a)b)+. Group 2 has beginning = 2 and end = 1.

Fix

The fix is to record groups with beginning > end as not matched.

Am I affected?

All of the following conditions are necessary:
  • you need to use Str.string partial match
  • if you don't use this function you are not affected
  • you need to use one of Str.{replace matched,global replace,replace first} on the same regexp after a call to Str.string partial match.
  • If you use Str.group beginning and Str.group end and do your own string manipulation that is safe, because the String module would raise on out of bounds accesses. Similarly Str.matched group is safe (uses String functions in its OCaml implementation)
  • the match replacement must reference a group > 0
  • if the replacement string argument is fixed, and doesn't reference any groups, or references only group 0, then you are not affected
  • the referenced replacement group must be under a repetition operator in the original regexp
  • if there is no repetition operator in the regexp, or there are no groups under the repetition operator then you are not affected
For example {|((a)b)+|} as the regular expression in a call to Str.string partial match, followed by a reference to 2 in the argument of Str.replace matched could be affected.

Timeline

  • 2026-08-24: vulnerability was found
  • 2026-08-27: forwarded report to security@ocaml.org
  • 2026-09-29: fix merged to trunk
Exploitability
Fix
Vendors
Affected products
Related identifiers
OSEC-2026-21
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