Symfony: Unbounded Recursion in the YAML Inline Lexer Lets a Single Document Crash the Process

Summary

In Symfony YAML prior to versions 5.4.54, 6.4.45, 7.4.18, and 8.1.6, Yaml\Parser::lexInlineStructure() recursed through lexInlineMapping() and lexInlineSequence() for every { and [ of an inline collection, without entering the nesting level that the rest of the parser bounds with maxNestingLevel. A document made of deeply nested inline collections, such as [[[[...]]]], exhausted the stack while it was being lexed: depending on the PHP version and its stack settings, the process crashed or the request ended with a fatal error. Any code parsing untrusted YAML is affected. That includes #[MapRequestPayload] when the Yaml component is installed and the endpoint does not restrict acceptFormat: a single unauthenticated request with a YAML content type is enough. This issue affects symfony/yaml through v5.4.53, v6.0.0-BETA1 through v6.4.44, v7.0.0-BETA1 through v7.4.17, and v8.0.0-BETA1 through v8.1.5.

Timeline

1. Oct 15, 2026, 2:57 PM UTCCVE published
2. Oct 15, 2026, 6:06 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Oct 15, 2026, 6:06 PM UTCRecord last updated

Affected versions

• symfony/yaml

  fixed in v5.4.54; From v6.0.0-BETA1 · fixed in v6.4.45; From v7.0.0-BETA1 · fixed in v7.4.18; From v8.0.0-BETA1 · fixed in v8.1.6

• symfony/symfonyPackagist

  From 2.0.0 · fixed in 5.4.54

• symfony/symfonyPackagist

  From 6.0.0 · fixed in 6.4.45

• symfony/symfonyPackagist

  From 7.0.0 · fixed in 7.4.18

• symfony/symfonyPackagist

  From 8.0.0 · fixed in 8.1.6

• symfony/yamlPackagist

  From 2.0.0 · fixed in 5.4.54

• symfony/yamlPackagist

  From 6.0.0 · fixed in 6.4.45

• symfony/yamlPackagist

  From 7.0.0 · fixed in 7.4.18

• symfony/yamlPackagist

  From 8.0.0 · fixed in 8.1.6

References 3