Tautulli vulnerable to Unauthenticated Path Traversal in `real_pms_image_proxy`

Tautulli is a Python based monitoring and tracking tool for Plex Media Server. The real_pms_image_proxy endpoint in Tautulli v2.15.3 and prior is vulnerable to path traversal, allowing unauthenticated attackers to read arbitrary files from the application server's filesystem. The real_pms_image_proxy is used to fetch an image directly from the backing Plex Media Server. The image to be fetched is specified through an img URL parameter, which can either be a URL or a file path. There is some validation ensuring that img begins with the prefix interfaces/default/images in order to be served from the local filesystem. However this can be bypassed by passing an img parameter which begins with a valid prefix, and then adjoining path traversal characters in order to reach files outside of intended directories. An attacker can exfiltrate files on the application file system, including the tautulli.db SQLite database containing active JWT tokens, as well as the config.ini file which contains the hashed admin password, the JWT token secret, and the Plex Media Server token and connection details. If the password is cracked, or if a valid JWT token is present in the database, an unauthenticated attacker can escalate their privileges to obtain administrative control over the application. Version 2.16.0 contains a fix for the issue.