Indico is an event management system that uses Flask-Multipass, a multi-backend authentication system for Flask. In Indico prior to version 3.3.4, corresponding to Flask-Multipass prior to version 0.5.5, there is a Cross-Site-Scripting vulnerability during account creation when redirecting to the "next" URL. Exploitation requires initiating the account creation process with a maliciously crafted link, and then finalizing the signup process. Because of this, it can only target newly created (and thus unprivileged) Indico users. Indico 3.3.4 upgrades the dependency on Flask-Multipass to version 0.5.5, which fixes the issue. Those who build the Indico package themselves and cannot upgrade can update the "flask-multipass" dependency to ">=0.5.5" which fixes the vulnerability. Otherwise one could configure one's web server to disallow requests containing a query string with a "next" parameter that starts with "javascript:".