HTTP Request Smuggling: Invalid whitespace characters in headers in Waitress

Type github
Reporter GitHub Advisory Database
Modified 2019-12-26T16:34:38



If a proxy server is used in front of waitress, an invalid request may be sent by an attacker that bypasses the front-end and is parsed differently by waitress leading to a potential for HTTP request smuggling.

Content-Length: 10 Transfer-Encoding: [\x0b]chunked

For clarity:

0x0b == vertical tab

Would get parsed by Waitress as being a chunked request, but a front-end server would use the Content-Length instead as the Transfer-Encoding header is considered invalid due to containing invalid characters.

If a front-end server does HTTP pipelining to a backend Waitress server this could lead to HTTP request splitting which may lead to potential cache poisoning or unexpected information disclosure.


Please upgrade to Waitress 1.4.1 which fixes this issue with stricter HTTP field validation.

Waitress 1.4.1 due to this change has become much more strict in what is allowed in header values, while the maintainers don't believe that these changes will cause any issues, it may cause failures with non-conformist reverse proxies or clients, and it is highly recommend that users validate the changes in their environment and make sure it won't cause any unacceptable failures.


You may enable additional protections on front-end servers, those that follow RFC7230 correctly would drop the request with a 400 Bad Request.

Waitress will now correctly responds to the request with a 400 Bad Request, and will drop the connection to avoid any potential HTTP pipelining issues.


This was mentioned in and was specifically mentioned as being an issue in HAProxy which did not properly filter it in this article:


The Pylons Project would like to thank ZeddYu Lu for doing extended testing against Waitress 1.4.0 and bringing this to our attention!

For more information

If you have any questions or comments about this advisory:

  • open an issue at (if not sensitive or security related)
  • email the Pylons Security mailing list: (if security related)