CVE-2026-33490 h3: Missing Path Segment Boundary Check in `mount()` Causes Middleware Execution on Unrelated Prefix-Matching Routes
Attackers can trigger unintended middleware on prefix-matching routes, potentially escalating privileges or bypassing security checks in h3 applications.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-33490?
CVE-2026-33490 is a low-severity missing authorization vulnerability in h3js h3, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-706
Which versions are affected by CVE-2026-33490?
2 open source packages ship an affected version, including h3. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
h3 npm | From 2.0.1-alpha.0 | 2.0.1-rc.17 |
h3 npm | From 2.0.1-alpha.0 | 2.0.1-rc.17 |
Products include H3 H3.
How do I fix CVE-2026-33490?
Upgrade h3 to 2.0.1-rc.17 or later.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-33490 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33490 exists.
Other vulnerabilities in h3js h3
Sources
Also known as CLSA-2026-1791201400, GHSA-2j6q-whv2-gh6w.