A vulnerability in h11's HTTP/1.1 chunked transfer encoding parsing allows malformed chunked bodies to be accepted. This leniency can cause request smuggling when combined with a buggy proxy that parses chunked bodies differently. The issue is fixed in h11 version 0.15.0. Exploitation requires both a vulnerable h11 and a buggy proxy, but can lead to request smuggling attacks that may allow attackers to bypass protections or steal credentials.