{"cve":"CVE-2020-26891","nvd":{"published":"2020-10-19T17:15:13.220","modified":"2026-06-17T03:08:28.523","status":"Modified","description":"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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N","score":6.1},"cwes":["CWE-79"],"references":["https://github.com/matrix-org/synapse/pull/8444","https://github.com/matrix-org/synapse/releases/tag/v1.21.2","https://github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmq","https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory","https://github.com/matrix-org/synapse/pull/8444"],"products":["matrix:synapse"],"source":"nvd","synced_at":"2026-10-19T21:09:53.489947993+00:00"},"epss":{"cve":"CVE-2020-26891","score":0.01923,"percentile":0.79927,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-3x8c-fmpc-5rmq","url":"https://osv.dev/vulnerability/GHSA-3x8c-fmpc-5rmq","summary":"Cross-site scripting (XSS) vulnerability in the fallback authentication endpoint","modified":"2026-07-08T06:30:00.309784796Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.21.0"],"fixed":["1.21.0"]}]},{"id":"PYSEC-2020-238","url":"https://osv.dev/vulnerability/PYSEC-2020-238","summary":"","modified":"2023-11-08T04:03:20.342815Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.21.0"],"fixed":["1.21.0"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}