FreshRSS is vulnerable to RCE attacks by authenticated admin

FreshRSS is a free, self-hostable RSS aggregator. In versions 1.26.1 and below, an authenticated administrator user can execute arbitrary code on the FreshRSS server by modifying the update URL to one they control, and gain code execution after running an update. After successfully executing code, user data including hashed passwords can be exfiltrated, the instance can be defaced when file permissions allow. Malicious code can be inserted into the instance to steal plaintext passwords, among others. This is fixed in version 1.26.2.