Symfony's YAML Parser Vulnerable to Exponential Memory Allocation via Recursive Collection-Alias Expansion ("Billion Laughs")GHSA-4qpc-3hr4-r2p4
Fix: symfony/symfony@e77391bGHSA-4qpc-3hr4-r2p4 is a CWE-776 vulnerability in symfony/yaml. A fix is available for symfony/yaml — see the affected versions and patch details below.
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-4qpc-3hr4-r2p4.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
Real-World Exposure
symfony/yaml🐘symfony/symfony🐘symfony/symfony🐘symfony/symfony🐘symfony/symfony🐘symfony/yaml🐘symfony/yaml🐘symfony/yamlReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
Description
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.
Resolution
The Parser now counts collection alias resolutions in a shared ParserState object, with a default limit of 128, following the SnakeYAML model. Scalar aliases remain unrestricted since they cannot drive exponential growth. The limit is configurable via a new $maxAliasesForCollections argument on Parser::__construct(), Yaml::parse() and Yaml::parseFile(). A new Yaml::PARSE_EXCEPTION_ON_ALIAS flag also rejects all aliases outright when parsing fully untrusted input.
The patch for this issue is available here 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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | symfony/yaml | all versions | 5.4.52composer require symfony/yaml:^5.4.52 |
| 🐘Packagist | symfony/symfony | all versions | 5.4.52composer require symfony/symfony:^5.4.52 |
| 🐘Packagist | symfony/symfony | ≥ 6.0.0&&< 6.4.40 | 6.4.40composer require symfony/symfony:^6.4.40 |
| 🐘Packagist | symfony/symfony | ≥ 7.0.0&&< 7.4.12 | 7.4.12composer require symfony/symfony:^7.4.12 |
| 🐘Packagist | symfony/symfony | ≥ 8.0.0&&< 8.0.12 | 8.0.12composer require symfony/symfony:^8.0.12 |
| 🐘Packagist | symfony/yaml | ≥ 6.0.0&&< 6.4.40 | 6.4.40composer require symfony/yaml:^6.4.40 |
Affected Products
symfonysensiolabsDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for symfony/yaml, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update symfony/yaml to 5.4.52 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-4qpc-3hr4-r2p4 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is GHSA-4qpc-3hr4-r2p4 in your dependencies?
Find it across Packagist, including transitive dependencies.