# CVE-2026-86205: H3 open redirect via protocol-relative path in redirectBack()

Source: https://junglewise.ai/threats/cve-2026-86205-h3-open-redirect-via-protocol-relative-path-in-redirectback  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-06. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-86205 · Severity: medium · CVSS 5.4 · Published 2026-09-06

H3 is a popular HTTP framework used to build web applications and APIs. The redirectBack() function, which safely redirects users back to their referrer after form submission, contains a flaw that allows attackers to redirect users to external phishing sites by crafting malicious URLs. This can lead to credential theft and compromise of user accounts through phishing attacks.

Technologies: [h3 (npm)](https://junglewise.ai/threats/technologies/h3). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-86205 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-86205 being exploited in the wild.

**How severe is CVE-2026-86205?** Medium severity, with a CVSS score of 5.4, published 6 September 2026.

**Which products and versions are affected?** h3 (npm) SEMVER:introduced=2.0.1-rc.17,fixed=2.0.1-rc.18.

**Is there a fix for CVE-2026-86205?** Yes. h3 (npm): upgrade to 2.0.1-rc.18 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-86205 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-86205?** Check whether you run h3 (npm) 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.18 or later

  ```sh
  npm install h3@2.0.1-rc.18
  ```


## Technical details

The vulnerability is an open redirect (CWE-601) in the redirectBack() utility function located in src/utils/response.ts. The function validates that a Referer header shares the same origin as the current request before using its pathname for the Location header. However, it fails to sanitize protocol-relative paths (starting with //). An attacker can craft a URL like http://target.com//evil.com/path that passes the origin check (since the origin is target.com) but extracts a pathname of //evil.com/path, which browsers interpret as a protocol-relative redirect to an external domain. The vulnerability requires user interaction (the user must follow an attacker-crafted link and then trigger a redirectBack() call), is network-accessible, and requires no authentication. A fix is available in version 2.0.1-rc.18 and later, which sanitizes the pathname to prevent protocol-relative paths.

## Affected products

- H3 H3 2.0.1-rc.17

## Identifiers

- CVE-2026-86205
- GHSA-fp4x-ggrf-wmc6

## Timeline

- 2026-03-23: disclosed
- 2026-03-23: patched: Fixed in version 2.0.1-rc.18

## References

- [https://github.com/h3js/h3/security/advisories/GHSA-fp4x-ggrf-wmc6](https://github.com/h3js/h3/security/advisories/GHSA-fp4x-ggrf-wmc6)
- [https://github.com/h3js/h3/commit/459a1c6593365b0810e9c502df7c3e82837321d7](https://github.com/h3js/h3/commit/459a1c6593365b0810e9c502df7c3e82837321d7)

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- [h3 SSE event injection via unsanitized carriage return](https://junglewise.ai/threats/cve-2026-86252-h3-sse-event-injection-via-unsanitized-carriage-return)
- [h3 serveStatic path traversal via double-encoded dot segments](https://junglewise.ai/threats/cve-2026-86251-h3-servestatic-path-traversal-via-double-encoded-dot-segments)
- [H3: Unbounded Chunked Cookie Count in Session Cleanup Loop may Lead to Denial of Service](https://junglewise.ai/threats/cve-2026-86250-h3-unbounded-chunked-cookie-denial-of-service)