CVE-2021-21392

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 1.28.0 requests to user
provided domains were not restricted to external IP addresses when
transitional IPv6 addresses were used. Outbound requests to federation,
identity servers, when calculating the key validity for third-party invite
events, sending push notifications, and generating URL previews are
affected. This could cause Synapse to make requests to internal
infrastructure on dual-stack networks. See referenced GitHub security
advisory for details and workarounds.