CVE-2021-21274: Fedoraproject Fedora

Medium severity, CVSS 6.5. EPSS: 2.2% chance of exploitation in the next 30 days.

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.

Affected products

• Fedoraproject Fedora: version 34 only
• Matrix Synapse: from 0.99.0, before 1.25.0 (fixed in 1.25.0)

Published 2021-02-26. Last modified 2026-06-17.