CVE-2021-21394

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 Synapse is missing
input validation of some parameters on the endpoints used to confirm
third-party identifiers could cause excessive use of disk space and memory
leading to resource exhaustion. Note that the groups feature is not part of
the Matrix specification and the chosen maximum lengths are arbitrary. Not
all clients might abide by them. Refer to referenced GitHub security
advisory for additional details including workarounds.