FreshRSS is a free, self-hostable RSS aggregator. Versions 1.26.3 and below do not sanitize certain event handler attributes in feed content, so by finding a page that renders feed entries without CSP, it is possible to execute an XSS payload. The Allow API access authentication setting needs to be enabled by the instance administrator beforehand for the attack to work as it relies on api/query.php. An account takeover is possible by sending a change password request via the XSS payload / setting UserJS for persistence / stealing the autofill password / displaying a phishing page with a spoofed URL using history.replaceState() If the victim is an administrator, the attacker can also perform administrative actions. This issue is fixed in version 1.27.0.