The resource-js endpoint in Craft CMS allows unauthenticated requests to proxy remote JavaScript resources.
When trustedHosts is not explicitly restricted (default configuration), the application trusts the client-supplied Host header.
This allows an attacker to control the derived baseUrl, which is used in prefix validation inside actionResourceJs().
By supplying a malicious Host header, the attacker can make the server issue arbitrary HTTP requests, leading to Server-Side Request Forgery (SSRF).
The vulnerability exists in AppController::actionResourceJs().
The function validates that the url parameter starts with assetManager->baseUrl. However, baseUrl is derived from the current request host. If trustedHosts is not configured, the Host header is fully attacker-controlled.
Attack chain:
Host header.baseUrl from the malicious Host.url parameter is required to start with this baseUrl.This does not rely on string parsing bypass. It relies on Host header trust.
Environment: - Craft CMS 5.9.12 - Default configuration (no trustedHosts restriction) - Docker deployment
Start a listener inside the container: python3 -m http.server 9999
Send a request to resource-js with a controlled Host header.
Observe that the internal listener receives a request (OOB confirmation).
{
"nvd_published_at": "2026-04-22T00:16:28Z",
"cwe_ids": [
"CWE-918"
],
"severity": "MODERATE",
"github_reviewed": true,
"github_reviewed_at": "2026-04-14T23:36:09Z"
}