changedetection.io is a free open source web page change detection tool. In versions prior to 0.54.1, changedetection.io is vulnerable to Server-Side Request Forgery (SSRF) because the URL validation function `is_safe_valid_url()` does not validate the resolved IP address of watch URLs against private, loopback, or link-local address ranges. An authenticated user (or any user when no password is configured, which is the default) can add a watch for internal network URLs. The application fetches these URLs server-side, stores the response content, and makes it viewable through the web UI — enabling full data exfiltration from internal services. Version 0.54.1 contains a fix for the issue.