CVE-2021-29431: Matrix Sydent

Medium severity, CVSS 6.5. EPSS: 1.2% chance of exploitation in the next 30 days.

Sydent is a reference Matrix identity server. Sydent can be induced to send HTTP GET requests to internal systems, due to lack of parameter validation or IP address blacklisting. It is not possible to exfiltrate data or control request headers, but it might be possible to use the attack to perform an internal port enumeration. This issue has been addressed in in 9e57334, 8936925, 3d531ed, 0f00412. A potential workaround would be to use a firewall to ensure that Sydent cannot reach internal HTTP resources.

Affected products

• Matrix Sydent: before 2.3.0 (fixed in 2.3.0)

Published 2021-04-15. Last modified 2026-06-17.