Tautulli vulnerable to Unauthenticated Path Traversal in `real_pms_image_proxy`

Summary

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.