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.
Source: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-26891
Reference (s):
- https://github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmq
- https://github.com/matrix-org/synapse/pull/8444
- https://github.com/matrix-org/synapse/releases/tag/v1.21.2
- https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory