CVE-2026-45305
Symfony's YAML Parser has a ReDoS via Catastrophic Backtracking in Parser::cleanup() Regex
Description
### Description `Symfony\Component\Yaml\Parser::cleanup()` strips the optional `%YAML` directive header, leading comments, and document start/end markers before parsing. The original regexes contained overlapping quantifiers, most notably `'#^%YAML[: ][\d.]+.*\n#u'`, whose `[\d.]+` and `.*` overlap on the dot, that exhibit catastrophic backtracking on crafted input. A single oversized `%YAML` directive header (or comment / document-marker line) makes the parser hang for an arbitrarily long time, denying service. ### Resolution The four regexes in `Parser::cleanup()` (YAML directive header, leading comments, document-start marker, document-end marker) have been rewritten with possessive quantifiers and unambiguous character classes so backtracking cannot occur. The patch for this issue is available [here](https://github.com/symfony/symfony/commit/9749cd43c5e09b3735093623670b21b9d8a056cb) for branch 5.4. ### Credits Symfony would like to thank Pietro Tirenna (Shielder) for reporting the issue and Nicolas Grekas for fixing it.
How to fix CVE-2026-45305
No fixed version has been published yet. Mitigate by removing the affected package or applying upstream guidance from the references below.
- —no fix listed
Is CVE-2026-45305 being exploited?
Low — EPSS is 0.8%, meaning exploitation activity has not been observed at scale.
Affected packages (1)
- from 0
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | HIGH7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |