CVE-2026-33131: h3 middleware bypass via Host header spoofing

CVE-2026-33131 · Severity: low · CVSS 3.1 · Published 2026-03-18

h3 is a minimal HTTP framework used to build web services and APIs. An attacker can bypass authentication and authorization middleware by crafting a malicious Host header, allowing them to access protected routes that should be restricted. This could enable unauthorized access to sensitive internal endpoints and functions.

Technologies: h3 (npm), Uber H3-Js. Vendors: npm, Uber.

Quick answers

Is CVE-2026-33131 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-33131 being exploited in the wild.
How severe is CVE-2026-33131?
Low severity, with a CVSS score of 3.1, published 18 March 2026.
Which products and versions are affected?
h3 (npm) SEMVER:introduced=2.0.0-0,fixed=2.0.1-rc.15; Uber H3-Js >= 2.0.0-0, < 2.0.1-rc.15.
Is there a fix for CVE-2026-33131?
Yes. h3 (npm): upgrade to 2.0.1-rc.15 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-33131 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-33131?
Check whether you run h3 (npm) or Uber H3-Js at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • h3 (npm): upgrade to 2.0.1-rc.15 or later
    npm install h3@2.0.1-rc.15

Technical details

The vulnerability exists in h3's NodeRequestUrl class, which extends FastURL. When accessing event.url or event.url.hostname in middleware (e.g., during logging), the _url() getter is triggered, which reconstructs the URL from untrusted components including the Host header. An attacker can pollute the Host header with a path-like string (e.g., "localhost:3000/abchehe?") that causes the href property to be malformed. Since route handlers are resolved before middleware execution, but middleware path matching uses the spoofed event.url, an attacker can bypass middleware authentication checks by requesting a protected route with a crafted Host header. The vulnerability affects h3 versions 2.0.0-0 through 2.0.1-rc.14, and a fix is available in version 2.0.1-rc.15 or later. Network reachability is required; no authentication or user interaction is needed.

Affected products

  • h3js h3 >=2.0.0-0, <2.0.1-rc.15

Identifiers

  • CVE-2026-33131
  • GHSA-3vj8-jmxq-cgj5

Timeline

  • 2026-03-18: disclosed
  • 2026-03-18: patched: Fixed in version 2.0.1-rc.15

References

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