CVE-2021-21299

hyper is an open-source HTTP library for Rust (crates.io). In hyper from
version 0.12.0 and before versions 0.13.10 and 0.14.3 there is a
vulnerability that can enable a request smuggling attack. The HTTP server
code had a flaw that incorrectly understands some requests with multiple
transfer-encoding headers to have a chunked payload, when it should have
been rejected as illegal. This combined with an upstream HTTP proxy that
understands the request payload boundary differently can result in “request
smuggling” or “desync attacks”. To determine if vulnerable, all these
things must be true: 1) Using hyper as an HTTP server (the client is not
affected), 2) Using HTTP/1.1 (HTTP/2 does not use transfer-encoding), 3)
Using a vulnerable HTTP proxy upstream to hyper. If an upstream proxy
correctly rejects the illegal transfer-encoding headers, the desync attack
cannot succeed. If there is no proxy upstream of hyper, hyper cannot start
the desync attack, as the client will repair the headers before forwarding.
This is fixed in versions 0.14.3 and 0.13.10. As a workaround one can take
the following options: 1) Reject requests that contain a
transfer-encoding header, 2) Ensure any upstream proxy handles
transfer-encoding correctly.