CVE-2026-33490

The record

H3 is a minimal H(TTP) framework. In versions 2.0.0-0 through 2.0.1-rc.16, the `mount()` method in h3 uses a simple `startsWith()` check to determine whether incoming requests fall under a mounted sub-application's path prefix. Because this check does not verify a path segment boundary (i.e., that the next character after the base is `/` or end-of-string), middleware registered on a mount like `/admin` will also execute for unrelated routes such as `/admin-public`, `/administrator`, or `/adminstuff`. This allows an attacker to trigger context-setting middleware on paths it was never intended to cover, potentially polluting request context with unintended privilege flags. Version 2.0.2-rc.17 contains a patch.

Affected packages

Related vulnerabilities

• GHSA-2j6q-whv2-gh6w h3: Missing Path Segment Boundary Check in `mount()` Causes Middleware Execution on… Same vulnerability
• GHSA-fp4x-ggrf-wmc6 H3 has an Open Redirect via Protocol-Relative Path in redirectBack() Referer Validation Also affects h3
• GHSA-q5pr-72pq-83v3 H3: Unbounded Chunked Cookie Count in Session Cleanup Loop may Lead to Denial of Service Also affects h3
• GHSA-4hxc-9384-m385 h3: SSE Event Injection via Unsanitized Carriage Return (`\r`) in EventStream Data and… Also affects h3
• GHSA-72gr-qfp7-vwhw h3: Double Decoding in `serveStatic` Bypasses `resolveDotSegments` Path Traversal… Also affects h3
• CVE-2026-33131 H3 is a minimal H(TTP) framework. Also affects h3
• CVE-2026-33128 H3 is a minimal H(TTP) framework. Also affects h3
• CVE-2026-33129 H3 is a minimal H(TTP) framework. Also affects h3