Summary
A server-side request forgery vulnerability in GetSimple CMS allows an attacker who can submit the update form to make the server request attacker-selected URLs. When the response is saved in a web-accessible file, the attacker may retrieve data from internal services or cloud metadata endpoints.
Affected context
The vulnerability occurs in GetSimple CMS and its community edition before version 1.5 when the update handler processes a user-supplied URL. URL format validation is performed, but the destination is not checked against internal, loopback, link-local, or otherwise restricted networks. The response is written to a web-accessible temporary file and may remain available when the fetched content is not a valid update archive.
Exploit preconditions
- Attack surface: The update-handling form and its server-side URL fetch operation - Required access: Ability to submit the relevant form, subject to the deployment's
authentication and authorization controls
- Trigger point: The server requests the supplied URL and stores the response body - Why exploitation is possible: Format validation confirms that input resembles a
URL but does not restrict its destination, while the resulting response can be
exposed through a web-accessible file
Why this happens
The underlying bug is a failure to enforce destination validation for a server-side HTTP-like request. Treating FILTER_VALIDATE_URL as sufficient validation checks syntax but does not establish that the destination is trustworthy or externally reachable. This maps to CWE-918 because the application retrieves attacker-selected resources from the server's network context and exposes the response through a reachable file.
Potential impact
- An attacker may read responses from internal-only services.
- Cloud instance metadata or other sensitive configuration data could be disclosed.
- Internal network services may be probed from the CMS host.
- The exposed response file could increase the persistence or reach of information disclosure.
General mitigation guidance
Mitigation generally focuses on reducing trust in user-supplied destinations and preventing server-side fetches from reaching sensitive networks. Common defensive patterns include using an explicit allowlist of permitted schemes, hosts, and ports; resolving and validating destinations before connecting; rechecking destinations across redirects; blocking loopback, private, link-local, multicast, and metadata address ranges; and avoiding exposure of fetched content through web-accessible paths. Temporary files should use restrictive permissions and lifecycle controls, while the CMS process should run with least-privilege network and filesystem access. Where feasible, secure-by-default designs should replace arbitrary URL fetching with controlled artifact retrieval.
How teams detect issues like this
Detection approaches include static analysis for server-side request functions that consume request parameters without destination allowlisting, and code review for URL validation that checks syntax but not resolved addresses, redirects, schemes, or ports. Runtime monitoring can identify unexpected connections from the CMS process to internal, link-local, or metadata ranges, as well as unusual access to temporary response files. Network egress policies and application security testing can enforce restrictions on outbound requests.
