{"id":"CVE-2020-26891","aliases":["GHSA-3x8c-fmpc-5rmq","PYSEC-2020-238"],"url":"https://o3.security/vulnerability/CVE-2020-26891","summary":"Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint","details":"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.","published":"2020-10-19T17:15:13.220Z","modified":"2026-07-09T10:21:01.967957Z","cvss":{"score":6.1,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"PyPI","name":"matrix-synapse","fixedVersion":"1.21.0"}],"fix":{"url":"https://github.com/matrix-org/synapse/pull/8444","label":"matrix-org/synapse#8444"},"references":[{"type":"ADVISORY","url":"https://github.com/matrix-org/synapse/releases/tag/v1.21.2"},{"type":"ADVISORY","url":"https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory"},{"type":"FIX","url":"https://github.com/matrix-org/synapse/pull/8444"},{"type":"FIX","url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmq"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-09T10:21:01.967957Z"}}