CVE-2021-21274

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, a malicious
homeserver could redirect requests to their .well-known file to a large
file. This can lead to a denial of service attack where homeservers will
consume significantly more resources when requesting the .well-known file
of a malicious homeserver. This affects any server which accepts federation
requests from untrusted servers. Issue is resolved in version 1.25.0. As a
workaround the federation_domain_whitelist setting can be used to
restrict the homeservers communicated with over federation.