IPFire versions prior to 2.29 (Core Update 198) contain a stored cross-site scripting (XSS) vulnerability that allows an authenticated attacker to inject arbitrary JavaScript code through the QUOTA_USERS parameter when creating a user quota rule. When a user adds a new user quota rule the application issues an HTTP POST request to /cgi-bin/urlfilter.cgi with the MODE parameter set to USERQUOTA and the assigned user(s) provided in the QUOTA_USERS parameter. The value of this parameter is stored and later rendered in the web interface without proper sanitation or encoding, allowing injected scripts to execute in the context of other users who view the affected quota entry.