Craft CMS is a content management system (CMS). In versions on the 4.x branch through 4.17.8 and the 5.x branch through 5.9.14, 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). Versions 4.17.9 and 5.9.15 patch the issue.

Affected: Packagist: craftcms/cms

### Summary

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).

### Details

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:

1. Attacker sends request with controlled `Host` header. 2. Application derives `baseUrl` from the malicious Host. 3. `url` parameter is required to start with this `baseUrl`. 4. Validation passes. 5. Guzzle performs a server-side HTTP request to the attacker-controlled host. 6. SSRF occurs.

This does not rely on string parsing bypass. It relies on Host header trust.

### PoC (safe reproduction steps)

Environment: - Craft CMS 5.9.12 - Default configuration (no trustedHosts restriction) - Docker deployment

1. Start a listener inside the container: python3 -m http.server 9999

2. Send a request to resource-js with a controlled Host he…