Affected versions
Symfony versions <5.4.52, >=6, <6.4.40, >=7, <7.4.12, >=8, <8.0.12 of the Symfony YAML component are affected by this security issue.
The issue has been fixed in Symfony 5.4.52, 6.4.40, 7.4.12, 8.0.12.
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
We would like to thank Pietro Tirenna (Shielder) for reporting the issue and Nicolas Grekas for fixing it.