Symfony hardened the parser when handling untrusted input
Date: 2026-05
Affected: composer/symfony/yaml, composer/symfony/yaml, composer/symfony/yaml
Attack vector: dos
Category: vulnerability-disclosure
Severity: Low
CVE: CVE-2026-45133
CWE: CWE-1333
CWE-674
CWE-776

### Description `Symfony\Component\Yaml\Parser` is the entry point for parsing YAML strings into PHP values via `Yaml::parse()`. When the parser is exposed to attacker-controlled input, deeply nested mappings or sequences cause both the block-level (`Parser::parseBlock()`) and inline (`Inline::parseSequence()` / `Inline::parseMapping()`) parsers to recurse without a depth limit. A crafted document exhausts the PHP stack and crashes the worker. ### Resolution The `Parser` now tracks recursion depth in a shared `ParserState` object across both block-level and inline parsing, with a default limit of **128**. The limit is configurable via a new `$maxNestingLevel` argument on `Parser::__construct()`, `Yaml::parse()` and `Yaml::parseFile()`. The patch for this issue is available [here](https://github.com/symfony/symfony/commit/914f427ed9630ddb3904dafba763e53d9f133fe3) for branch 5.4. ### Credits Symfony would like to thank Pietro Tirenna (Shielder) for reporting the issue and Nicolas Grekas for fixing it.


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Architectural layers (MAESTRO):