CVE-2026-45070

### Description `Symfony\Component\Mime\Header\ParameterizedHeader` (and the related parameter handling reachable from `Symfony\Component\Mime\Header\Headers`) is responsible for serializing structured headers such as `Content-Type` and `Content-Disposition`, which carry `key=value` parameters (e.g. `Content-Disposition: attachment; filename="x"`). RFC 2045 / RFC 5322 require parameter *names* to be `tokens`: a restricted ASCII subset that excludes whitespace, CR/LF, and the `tspecials` set. Symfony's parameter handling validates and properly encodes parameter *values*, but does not validate parameter *names*: the supplied name is emitted verbatim into the serialized header. A caller that derives a parameter name from untrusted input, e.g. an application that lets a user influence a `Content-Disposition` parameter name, can include `\r\n` or other non-token bytes inside the name, terminating the current header and injecting additional headers in the rendered message. This is the classic CRLF / header-injection primitive applied to the parameter-name slot. ### Resolution `ParameterizedHeader` now rejects parameter names that contain bytes outside the RFC `token` character class. The patch for this issue is available [here](https://github.com/symfony/symfony/commit/e62ea217f8b4ca8ae922ad0f949e0c4dc1f9b613) for branch 5.4. ### Credits Symfony would like to thank Fabian Fleischer for reporting the issue and Alexandre Daubois for fixing it.