GHSA-f3pg-qwvg-p99c is a low-severity (CVSS 3.1) CWE-444 vulnerability in hyper. 1 public exploit reference exists, so weaponization risk is real. A fix is available for hyper — see the affected versions and patch details below.
Lenient Parsing of Content-Length Header When Prefixed with Plus Sign
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-f3pg-qwvg-p99c plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
hyperReal-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
Summary
hyper's HTTP/1 server code had a flaw that incorrectly parses and accepts requests with a Content-Length header with a prefixed plus sign, when it should have been rejected as illegal. This combined with an upstream HTTP proxy that doesn't parse such Content-Length headers, but forwards them, can result in "request smuggling" or "desync attacks".
Vulnerability
The flaw exists in all prior versions of hyper, if built with rustc v1.5.0 or newer.
Example:
GET / HTTP/1.1
Host: example.com
Content-Length: +3
abc
This request gets accepted and hyper reads the body as abc. The request should be rejected, according to RFC 7230, since the ABNF for Content-Length only allows for DIGITs. This is due to using the FromStr implementation for u64 in the standard library. By differing from the spec, it is possible to send requests like these to endpoints that have different HTTP implementations, with different interpretations of the payload semantics, and cause "desync attacks".
In this particular case, an upstream proxy would need to error when parsing the Content-Length, but not reject the request (swallowing its own error), and forwarding the request as-is with the Content-Length still included. Then the upstream proxy and hyper would disagree on the length of the request body. The combination of these factors would be extremely rare.
Read more about desync attacks: https://portswigger.net/research/http-desync-attacks-request-smuggling-reborn
Impact
To determine if vulnerable, all these things must be true:
- Using hyper as an HTTP server. While the lenient decoder also exists in the client, a vulnerability does not exist around responses.
- Using HTTP/1. The HTTP/2 code uses a stricter parser.
- Using a vulnerable HTTP proxy upstream to hyper. If an upstream proxy correctly rejects the illegal
Content-Lengthheader, OR can parse the length with the plus sign, the desync attack cannot succeed.
Patches
We have released the following patch versions:
- v0.14.10 (to be released when this advisor is published)
Workarounds
Besides upgrading hyper, you can take the following options:
- Reject requests manually that contain a plus sign prefix in the
Content-Lengthheader. - Ensure any upstream proxy handles
Content-Lengthheaders with a plus sign prefix.
Credits
This issue was initially reported by Mattias Grenfeldt and Asta Olofsson.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | hyper | all versions | 0.14.10cargo update -p hyper --precise 0.14.10 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
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.
Fix
Update hyper to 0.14.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-f3pg-qwvg-p99c is resolved across your whole dependency graph.
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-f3pg-qwvg-p99c can be triaged on real exposure rather than presence alone.
Tailored to GHSA-f3pg-qwvg-p99c. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-f3pg-qwvg-p99c in your dependencies?
O3 Security finds GHSA-f3pg-qwvg-p99c across crates.io dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.