Impact
Undici allows duplicate HTTP Content-Length headers when they are provided in an array with case-variant names (e.g., Content-Length and content-length). This produces malformed HTTP/1.1 requests with multiple conflicting Content-Length values on the wire.
Who is impacted:
- Applications using
undici.request(), undici.Client, or similar low-level APIs with headers passed as flat arrays
- Applications that accept user-controlled header names without case-normalization
Potential consequences:
- Denial of Service: Strict HTTP parsers (proxies, servers) will reject requests with duplicate
Content-Length headers (400 Bad Request)
- HTTP Request Smuggling: In deployments where an intermediary and backend interpret duplicate headers inconsistently (e.g., one uses the first value, the other uses the last), this can enable request smuggling attacks leading to ACL bypass, cache poisoning, or credential hijacking
Patches
Patched in the undici version v7.24.0 and v6.24.0. Users should upgrade to this version or later.
Workarounds
If upgrading is not immediately possible:
- Validate header names: Ensure no duplicate
Content-Length headers (case-insensitive) are present before passing headers to undici
- Use object format: Pass headers as a plain object (
{ 'content-length': '123' }) rather than an array, which naturally deduplicates by key
- Sanitize user input: If headers originate from user input, normalize header names to lowercase and reject duplicates
References
mcollina Remediation developer
UlisesGascon Remediation reviewer
Impact
Undici allows duplicate HTTP
Content-Lengthheaders when they are provided in an array with case-variant names (e.g.,Content-Lengthandcontent-length). This produces malformed HTTP/1.1 requests with multiple conflictingContent-Lengthvalues on the wire.Who is impacted:
undici.request(),undici.Client, or similar low-level APIs with headers passed as flat arraysPotential consequences:
Content-Lengthheaders (400 Bad Request)Patches
Patched in the undici version v7.24.0 and v6.24.0. Users should upgrade to this version or later.
Workarounds
If upgrading is not immediately possible:
Content-Lengthheaders (case-insensitive) are present before passing headers to undici{ 'content-length': '123' }) rather than an array, which naturally deduplicates by keyReferences