aiohttp's ClientSession is vulnerable to CRLF injection via method

Summary

Improper validation makes it possible for an attacker to modify the HTTP request (e.g. insert a new header) or even create a new HTTP request if the attacker controls the HTTP method.

Details

The vulnerability occurs only if the attacker can control the HTTP method (GET, POST etc.) of the request.

Previous releases performed no validation on the provided value. If an attacker controls the HTTP method it will be used as is and can lead to HTTP request smuggling.

PoC

A minimal example can be found here: https://gist.github.com/jnovikov/7f411ae9fe6a9a7804cf162a3bdbb44b

Impact

If the attacker can control the HTTP version of the request it will be able to modify the request (request smuggling).

Workaround

If unable to upgrade and using user-provided values for the request method, perform manual validation of the user value (e.g. by restricting it to a few known values like GET, POST etc.).

Patch: https://github.com/aio-libs/aiohttp/pull/7806/files