An Open Redirect vulnerability exists in the public form submission endpoint (/form/submit) of Mautic. After a form is submitted, the application redirects the visitor to a return address supplied with the submission without verifying that it points to the Mautic instance or another trusted host. An unauthenticated attacker can exploit this to send visitors from a legitimate Mautic domain to an arbitrary external website, for example to phishing pages that imitate the original landing page or to sites that deliver malware. Any Mautic instance with a published form is affected. Exploitation requires the victim to submit a form prepared by the attacker.
There are no official workarounds. To mitigate this vulnerability without upgrading, configure a reverse proxy or web application firewall rule that blocks redirect responses from /form/submit to hosts other than your Mautic instance and your own websites.