GHSA-95wr-3f2v-v2wh Craft CMS has a host header injection leading to SSRF via resource-js endpoint

• Packagist craftcms/cms: >= 5.0.0-RC1, < 5.9.15 → fixed in 5.9.15 • Packagist craftcms/cms: >= 4.0.0-RC1, < 4.17.9 → fixed in 4.17.9

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 header. 3. Observe that the internal listener receives a request (OOB confirmation).