RUSTSEC-2021-0020: Multiple Transfer-Encoding headers misinterprets request payload

What to do

1. See which version you have: cargo tree -i hyper (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p hyper. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example hyper = "0.14.3") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

hyper's 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".

References

• Package: crates.io/crates/hyper
• Advisory: rustsec.org/advisories/RUSTSEC-2021-0020.html
• Web: github.com/hyperium/hyper/commit/8f93123efef5c1361086688fe4f34c83c89cec02

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.

Other advisories for hyper

• Vulnerability MediumRUSTSEC-2016-0002 · 2016-05-09HTTPS MitM vulnerability due to lack of hostname verification
• Vulnerability MediumRUSTSEC-2017-0002 · 2017-01-23headers containing newline characters can split messages
• Vulnerability CriticalRUSTSEC-2020-0008 · 2020-03-19Flaw in hyper allows request smuggling by sending a body in GET requests
• Vulnerability MediumRUSTSEC-2021-0078 · 2021-07-07Lenient `hyper` header parsing of `Content-Length` could allow request smuggling
• Vulnerability CriticalRUSTSEC-2021-0079 · 2021-07-07Integer overflow in `hyper`'s parsing of the `Transfer-Encoding` header leads to data loss
• UnsoundRUSTSEC-2022-0022 · 2022-05-10Parser creates invalid uninitialized value

Full security history of hyper