{"cve":"CVE-2021-21299","nvd":{"published":"2021-02-11T18:15:16.800","modified":"2026-06-17T03:35:14.627","status":"Modified","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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H","score":8.1},"cwes":["CWE-444"],"references":["https://crates.io/crates/hyper","https://github.com/hyperium/hyper/commit/8f93123efef5c1361086688fe4f34c83c89cec02","https://github.com/hyperium/hyper/security/advisories/GHSA-6hfq-h8hq-87mf","https://portswigger.net/research/http-desync-attacks-request-smuggling-reborn","https://rustsec.org/advisories/RUSTSEC-2021-0020.html"],"products":["hyper:hyper"],"source":"nvd","synced_at":"2026-10-19T21:14:10.479542811+00:00"},"epss":{"cve":"CVE-2021-21299","score":0.04732,"percentile":0.91852,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-6hfq-h8hq-87mf","url":"https://osv.dev/vulnerability/GHSA-6hfq-h8hq-87mf","summary":"HTTP Request Smuggling in hyper","modified":"2026-07-08T06:28:31.977759336Z","packages":[{"ecosystem":"crates.io","name":"hyper","purl":"pkg:cargo/hyper","ranges":[">=0.14.0 <0.14.3"],"fixed":["0.14.3"]},{"ecosystem":"crates.io","name":"hyper","purl":"pkg:cargo/hyper","ranges":[">=0.13.0 <0.13.10"],"fixed":["0.13.10"]},{"ecosystem":"crates.io","name":"hyper","purl":"pkg:cargo/hyper","ranges":[">=0.12.0 <0.12.36"],"fixed":["0.12.36"]}]},{"id":"RUSTSEC-2021-0020","url":"https://osv.dev/vulnerability/RUSTSEC-2021-0020","summary":"Multiple Transfer-Encoding headers misinterprets request payload","modified":"2026-10-12T18:00:04.410731844Z","packages":[{"ecosystem":"crates.io","name":"hyper","purl":"pkg:cargo/hyper","ranges":[">=0.12.0 <0.12.36",">=0.13.0-0 <0.13.10",">=0.14.0-0 <0.14.3"],"fixed":["0.12.36","0.13.10","0.14.3"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}