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.
Affected packages (3)
- Debian/symfonyfrom 0
- Packagist/symfony/symfonyfrom 0, < 5.4.52
- Packagist/symfony/yamlfrom 0, < 5.4.52
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U |
References (7)
- ADVISORYhttps://security-tracker.debian.org/tracker/CVE-2026-45305
- PATCHhttps://github.com/symfony/symfony
- WEBhttps://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/symfony/CVE-2026-45305.yaml
- WEBhttps://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/yaml/CVE-2026-45305.yaml
- WEBhttps://github.com/symfony/symfony/commit/9749cd43c5e09b3735093623670b21b9d8a056cb
- WEBhttps://github.com/symfony/symfony/security/advisories/GHSA-9frc-8383-795m
- WEBhttps://symfony.com/cve-2026-45305