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CRITICAL severity

CVE-2020-35863 — hyper

CRITICAL

CVE-2020-35863 is a critical-severity (CVSS 9.8) CWE-444 vulnerability in hyper. A fix is available for hyper — see the affected versions and patch details below.

HTTP Request Smuggling in hyper

Also known asGHSA-h3qr-rq2j-74w4RUSTSEC-2020-0008
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
3.0%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs87th percentile — riskier than 87% of all scored CVEsHighest risk
1.51%2.16%2.82%3.47%2.0%3.0%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2020-35863 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🦀hyper

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.

Description

Vulnerable versions of hyper allow GET requests to have bodies, even if there is no Transfer-Encoding or Content-Length header. As per the HTTP 1.1 specification, such requests do not have bodies, so the body will be interpreted as a separate HTTP request.

This allows an attacker who can control the body and method of an HTTP request made by hyper to inject a request with headers that would not otherwise be allowed, as demonstrated by sending a malformed HTTP request from a Substrate runtime. This allows bypassing CORS restrictions. In combination with other vulnerabilities, such as an exploitable web server listening on loopback, it may allow remote code execution.

The flaw was corrected in hyper version 0.12.34.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🦀crates.iohyper≥ 0.11.0&&< 0.12.340.12.34cargo update -p hyper --precise 0.12.34

Affected Products

1 product · 1 configurations
Application
hyperhyper
< 0.12.34
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for hyper, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update hyper to 0.12.34 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-35863 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

Vulnerable versions of hyper allow GET requests to have bodies, even if there is no Transfer-Encoding or Content-Length header. As per the HTTP 1.1 specification, such requests do not have bodies, so the body will be interpreted as a separate HTTP request. This allows an attacker who can control the body and method of an HTTP request made by hyper to inject a request with headers that would not otherwise be allowed, as demonstrated by sending a malformed HTTP request from a Substrate runtime. This allows bypassing CORS restrictions. In combination with other vulnerabilities, such as an exploi
O3 Security · Impact-Aware SCA

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CVE-2020-35863: hyper RCE — Fixed in 0.12.34