CVE-2026-45305

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

To remediate CVE-2026-45305, upgrade the affected package to a fixed version below.

• —no fix listed
• —upgrade to 5.4.52 or later
• —upgrade to 5.4.52 or later

Is CVE-2026-45305 being exploited?

No exploitation signal available. Neither CISA KEV nor a current EPSS score has been published for CVE-2026-45305.

Affected packages (3)

• from 0
• from 0, < 5.4.52
• from 0, < 5.4.52

CVSS scores

References (7)

• ADVISORYsecurity-tracker.debian.org/tracker/CVE-2026-45305
• PATCHgithub.com/symfony/symfony
• WEBgithub.com/FriendsOfPHP/security-advisories/blob/master/symfony/symfony/CVE-2026-45305.yaml
• WEBgithub.com/FriendsOfPHP/security-advisories/blob/master/symfony/yaml/CVE-2026-45305.yaml