CVE-2020-26891: Matrix Synapse

Medium severity, CVSS 6.1. EPSS: 1.9% chance of exploitation in the next 30 days.

AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth/*/fallback/web or /_matrix/client/unstable/auth/*/fallback/web Synapse endpoints.

Affected products

• Matrix Synapse: before 1.21.0 (fixed in 1.21.0)

Published 2020-10-19. Last modified 2026-06-17.