This vulnerability was disclosed with proof-of-concept exploit code demonstrating how to leverage the 'remote_url' field in conjunction with valid authentication cookies and a CSRF token. By bypassing input validation, attackers can probe internal services or interact with locally hosted applications that are otherwise inaccessible from the public internet. Organizations using Librarian should audit their access logs for unusual POST requests to the PDF save endpoint, particularly those containing non-standard or internal URLs in the 'remote_url' field.

Attack Chain

1. Attacker performs credential harvesting or utilizes valid account access to gain an authenticated session for the Librarian application.
2. Attacker obtains a valid session cookie (e.g., IL=[COOKIE]) and current CSRF token from the application's authenticated session.
3. Attacker targets the /librarian/index.php/pdf/save endpoint to initiate a PDF generation request.
4. Attacker crafts a malicious HTTP POST request, supplying an internal IP address or internal hostname within the 'remote_url' parameter.
5. The Librarian server receives the POST request and fails to sanitize the 'remote_url' input.
6. The server-side service initiates an outbound request to the target URI specified by the attacker, effectively performing SSRF.
7. Attacker receives information or state changes from the internal resource, facilitating further lateral movement or data exfiltration.

Impact

Successful exploitation of this SSRF vulnerability grants an authenticated attacker the ability to bypass network segmentation and interact with internal-only services or APIs. This could lead to the exposure of sensitive internal data, exploitation of secondary internal vulnerabilities, or administrative access to other internal systems.

Recommendation

1. Monitor web server logs for HTTP POST requests to /librarian/index.php/pdf/save that exhibit suspicious 'remote_url' parameters, such as internal IP addresses (e.g., 127.0.0.1, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
2. Deploy the provided Sigma rule to detect attempts at exploiting the endpoint.
3. Implement strict allow-listing for the 'remote_url' parameter on the server side to ensure only trusted, external domains are reachable.
4. Ensure that the web server running Librarian is appropriately firewalled to minimize the impact of SSRF if this vulnerability is present.