Affected versions
Symfony YAML versions <5.4.54, >=6, <6.4.45, >=7, <7.4.18, >=8, <8.1.6 are affected by this security issue.
The issue has been fixed in Symfony YAML 5.4.54, 6.4.45, 7.4.18, 8.1.6.
Description
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.
Resolution
The inline lexer now enters and leaves the same nesting level as the rest of the parser, so nested inline collections are bounded by maxNestingLevel, and the parser raises a ParseException beyond it.
The patches for this issue are available here for branch 5.4 (and forward-ported to 6.4, 7.4, 8.1 and 8.2).
The fix was made in #65688.
Credits
We would like to thank Nicolas Grekas for providing the fix.