CVE-2020-26891

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/m.login.recaptcha or /_matrix/client/r0/auth/m.login.terms Synapse 974923.

Source: CVE-2020-26891