Improper Check for Unusual or Exceptional Conditions vulnerability in the pattern and patternProperties keywords in lud jsv allows an unauthenticated remote attacker to bypass schema validation via a string that matches a schema regex only after heavy backtracking. JSV.Helpers.RegexExt.bounded_match?/2 runs schema regexes with a backtracking budget but calls :re.run/3 without :report_errors, so a match that exhausts the budget returns the same :nomatch as a real non-match. A key that matches a patternProperties regex is then checked against additionalProperties or not at all, and a string that matches a not subschema with a pattern is accepted. Before the budget existed, Regex.match?/2 failed the same way at Erlang's default match limit.
Only schemas whose regex explores a catastrophic branch before the branch that matches are affected. The attacker controls the data, not the schema, and linear patterns never exhaust the budget.
This issue affects jsv: from 0.1.0 before 0.26.2.
The schema must contain a pattern or patternProperties regex that PCRE can match only after exploring a catastrophic branch first, such as a nested quantifier in an earlier alternative. Linear patterns never exhaust the budget. The attacker controls the validated data but not the schema.
Pair patternProperties with additionalProperties: false. A key whose match exhausts the budget is then rejected instead of accepted unchecked. Valid keys that need heavy backtracking are rejected as well.
Write schema regexes without nested quantifiers or overlapping alternatives, and do not express deny-lists as a not subschema with a pattern on untrusted input.