HTTP Request Smuggling in waitress

Impact

When using Waitress behind a proxy that does not properly validate the incoming HTTP request matches the RFC7230 standard, Waitress and the frontend proxy may disagree on where one request starts and where it ends.

This would allow requests to be smuggled via the front-end proxy to waitress and later behavior.

There are two classes of vulnerability that may lead to request smuggling that are addressed by this advisory:

• The use of Python's int() to parse strings into integers, leading to +10 to be parsed as 10, or 0x01 to be parsed as 1, where as the standard specifies that the string should contain only digits or hex digits.
• Waitress does not support chunk extensions, however it was discarding them without validating that they did not contain illegal characters

Patches

This has been fixed in Waitress 2.1.1

Workarounds

When deploying a proxy in front of waitress, turning on any and all functionality to make sure that the request matches the RFC7230 standard. Certain proxy servers may not have this functionality though and users are encouraged to upgrade to the latest version of waitress instead.

References

• https://portswigger.net/research/http-desync-attacks-request-smuggling-reborn

For more information

If you have any questions or comments about this advisory:
* Open an issue in the Github issue tracker (if not security related/sensitive)
* Email us at [email protected] (If security related or sensitive)