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:
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).
| Software | From | Fixed in |
|---|---|---|
craftcms / cms
|
5.0.0-RC1 | 5.9.15 |
craftcms / cms
|
4.0.0-RC1 | 4.17.9 |
A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.
A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.
Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.
Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.
SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.