CVE-2021-21273: Open redirects on some federation and push requests

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.8% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-02-26 · updated 2026-06-17
Weakness: CWE-601

Description

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.

Affected products

Fedoraproject Fedora: 34
matrix-org synapse: before 1.25.0 — fixed in 1.25.0

How to fix

Upgrade to a fixed version: matrix-org synapse 1.25.0. Check the vendor advisory for mitigations.

References

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