H3 has an Open Redirect via Protocol-Relative Path in redirectBack() Referer Validation

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.4 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N

EPSS

• CVE-2026-86205: 0.27% estimated exploitation activity in the next 30 days; percentile 19.39 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The redirectBack() utility in h3 validates that the Referer header shares the same origin as the request before using its pathname as the redirect Location. However, the pathname is not sanitized for protocol-relative paths (starting with //). An attacker can craft a same-origin URL with a double-slash path segment that passes the origin check but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain. Details The vulnerable code is in src/utils/response.ts:89-97: The root cause is a discrepancy between how the WHATWG URL parser and browsers handle double-slash paths: 1. new URL("http://target.com//evil.com/path").origin → "http://target.com" — origin check passes 2. new URL("http://target.com//evil.com/path").pathname → "//evil.com/path" — extracted as redirect location 3. Browser receives Location: //evil.com/path → interprets as protocol-relative URL → redirects to evil.com Attack scenario: The attacker shares a link like…

Published by the source

23 Mar 2026, 21:48 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 12:46 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-fp4x-ggrf-wmc6

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-fp4x-ggrf-wmc6

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# Patch Next advisory brief: CVE-2026-86205

Advisory: H3 has an Open Redirect via Protocol-Relative Path in redirectBack() Referer Validation
IDs: CVE-2026-86205, GHSA-fp4x-ggrf-wmc6
Record: https://patchnext.com/advisories/cve-2026-86205 (revision 2, source checked 26 Oct 2026, 12:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: h3 2.0.1-rc.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm h3: affected >= 2.0.1-rc.17, <= 2.0.1-rc.17; fixed in 2.0.1-rc.18

## Suggested inspection (read-only)
- `npm ls h3 --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on h3.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-fp4x-ggrf-wmc6 (OSV record GHSA-fp4x-ggrf-wmc6 lists affected ranges: npm h3 >=2.0.1-rc.17 <2.0.1-rc.18 (fixed))
- Verified: https://github.com/advisories/GHSA-fp4x-ggrf-wmc6 (GitHub-reviewed advisory GHSA-fp4x-ggrf-wmc6 lists affected ranges: npm h3 >=2.0.1-rc.17 <=2.0.1-rc.17)

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