Summary

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

Details

The vulnerability only occurs if the attacker can control the HTTP version of the request (including its type).
For example if an unvalidated JSON value is used as a version and the attacker is then able to pass an array as the version parameter.
Furthermore, the vulnerability only occurs when the Connection header is passed to the headers parameter.

At this point, the library will use the parsed value to create the request. If a list is passed, then it bypasses validation and it is possible to perform CRLF injection.

PoC

The POC below shows an example of providing an unvalidated array as a version:
https://gist.github.com/jnovikov/184afb593d9c2114d77f508e0ccd508e

Impact

CRLF injection leading to Request Smuggling.

Workaround

If these specific conditions are met and you are unable to upgrade, then validate the user input to the version parameter to ensure it is a str.

Patch: https://github.com/aio-libs/aiohttp/pull/7835/files