CVE-2021-21273

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.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-21273
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-21273
https://github.com/matrix-org/synapse/commit/30fba6210834a4ecd91badf0c8f3eb278b72e746
https://github.com/matrix-org/synapse/pull/8821
https://github.com/matrix-org/synapse/releases/tag/v1.25.0
https://github.com/matrix-org/synapse/security/advisories/GHSA-v936-j8gp-9q3p