Vulnerability GHSA-36hh-x5p5-jgc8

Impact

The two parsers resolved duplicates inconsistently and silently:

• Content.disposition() retained the last occurrence of each parameter.
• Content.type() retained the first occurrence of charset and boundary.

Either behavior creates a parameter-smuggling primitive when another component in the request-processing chain (a WAF, reverse proxy, security filter, or alternate parser) resolves duplicates the opposite way. The primary attack vector is upload filename allowlist bypass:

Content-Disposition: form-data; name="file"; filename="safe.txt"; filename="shell.php"

Patches

The issue has been patched in 6.0.2.

Workarounds

Pre or post validate headers looking for duplicates.

Resources

• RFC 6266 §4.1 — Content-Disposition syntax
• RFC 7231 §3.1.1.1 — Content-Type syntax
• RFC 7230 §3.2.6 — token character set