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 vulnerabilit...

Description

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.

Metrics

CVSS 3.1

8.1/10

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Probability

4.73%

90.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-444

Affected Software

Vendor	Product	Versions
Hyper	Hyper	>= 0.12.0, < 0.13.10
Hyper	Hyper	>= 0.14.0, < 0.14.3

References

https://crates.io/crates/hyperProduct, Third Party Advisory
https://github.com/hyperium/hyper/commit/8f93123efef5c1361086688fe4f34c83c89cec02Patch, Third Party Advisory
https://github.com/hyperium/hyper/security/advisories/GHSA-6hfq-h8hq-87mfThird Party Advisory
https://portswigger.net/research/http-desync-attacks-request-smuggling-rebornThird Party Advisory
https://rustsec.org/advisories/RUSTSEC-2021-0020.htmlThird Party Advisory
https://crates.io/crates/hyperProduct, Third Party Advisory
https://github.com/hyperium/hyper/commit/8f93123efef5c1361086688fe4f34c83c89cec02Patch, Third Party Advisory
https://github.com/hyperium/hyper/security/advisories/GHSA-6hfq-h8hq-87mfThird Party Advisory
https://portswigger.net/research/http-desync-attacks-request-smuggling-rebornThird Party Advisory
https://rustsec.org/advisories/RUSTSEC-2021-0020.htmlThird Party Advisory

Timeline

Published: Feb 11, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is 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.

How severe is CVE-2021-21299?

CVE-2021-21299 has a CVSS score of 8.1/10 (HIGH severity). The EPSS model estimates a 4.73% probability of exploitation in the next 30 days.

How do I fix CVE-2021-21299?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.