Synapse is a Matrix reference homeserver written in python. Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker.