Stirling‑PDF, a locally hosted web application that manipulates PDF files, contains a flaw in its /api/v1/security/add‑watermark endpoint. By supplying excessively large values for the fontSize and widthSpacer parameters, an authenticated user can cause the server to consume excessive CPU and memory, eventually crashing. The flaw is a resource exhaustion weakness described by CWE‑770 and results in a denial of service.

The affected product is Stirling‑Tools’ Stirling‑PDF. Versions from 2.1.5 up to but not including 2.5.2 are vulnerable. The issue requires authentication and can be triggered only by users who have valid login credentials to the application.

The CVSS score of 6.5 indicates moderate severity, and the EPSS score below 1 % shows a low likelihood of real-world exploitation. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The attack is limited to authenticated users, so reachability is constrained. An attacker would need to craft a special request to the watermark endpoint, and the effect is limited to exhausting server resources until a crash occurs.