In short
Synapse Matrix homeserver could be redirected to make requests to internal network addresses when processing federation requests and push notifications, potentially exposing internal infrastructure. This happens because user-provided domains weren't properly validated to prevent access to private IP ranges.
Technical detail
Open redirect vulnerability in Synapse's federation and push notification handlers (CWE-601) where requests to user-controlled domains lack IP address validation, allowing attackers to trigger connections to internal infrastructure (RFC 1918 addresses, loopback). Impact is limited by restricted request type control and body modification constraints, but could facilitate SSRF attacks against internal services.
Summary generated and translated by AI from the official description.
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, requests to user provided domains were not restricted to external IP addresses when calculating the key validity for third-party invite events and sending push notifications. This could cause Synapse to make requests to internal infrastructure. The type of request was not controlled by the user, although limited modification of request bodies was possible. For the most thorough protection server administrators should remove the deprecated `federation_ip_range_blacklist` from their settings after upgrading to Synapse v1.25.0 which will result in Synapse using the improved default IP address restrictions. See the new `ip_range_blacklist` and `ip_range_whitelist` settings if more specific control is necessary.