Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion') Affecting symfony/yaml package, versions <5.4.52>=6.0.0-BETA1, <6.4.40>=7.0.0-BETA1, <7.4.12>=8.0.0-BETA1, <8.0.12
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-PHP-SYMFONYYAML-16798664
- published22 May 2026
- disclosed20 May 2026
- creditPietro Tirenna
Introduced: 20 May 2026
NewCVE-2026-45304 (opens in a new tab)How to fix?
Upgrade symfony/yaml to version 5.4.52, 6.4.40, 7.4.12, 8.0.12 or higher.
Overview
Affected versions of this package are vulnerable to Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion') via Recursive Collection-Alias Expansion ("Billion Laughs"). Symfony\Component\Yaml\Parser resolves YAML aliases (*anchor) during parsing. Aliases that reference collections (arrays, stdClass, TaggedValue-wrapped collections) can themselves point to other collections containing aliases, creating exponential expansion at resolution time. A small input can blow up into a multi-gigabyte structure and exhaust memory: the classic "Billion Laughs" denial-of-service against any parser exposed to untrusted YAML.