An insecure deserialization (PHP object injection) vulnerability exists in Mautic's handling of segment filters and custom field properties. Several code paths that read these stored values from the database deserialize them without restricting which PHP classes may be instantiated, instead of using Mautic's safe deserialization helper. An attacker who is able to store a crafted serialized value in these segment filter or custom field properties can cause arbitrary PHP objects to be instantiated when Mautic later processes the data, for example during segment evaluation, contact export, or custom field API requests. Combined with gadget chains available in Mautic's dependencies, this could allow an attacker to execute arbitrary code on the server, read, modify, or delete data stored in the Mautic instance, including contact data, and affect its availability. Exploitation requires high privileges or an existing ability to write to the affected stored data.
There are no official workarounds. To reduce the risk without upgrading, restrict administrative access (including permissions to manage segments and custom fields, and API access) to trusted users, and make sure no untrusted party has write access to the Mautic database.