{"id":"UBUNTU-CVE-2020-26891","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.","modified":"2025-10-24T04:48:49Z","published":"2020-10-19T17:15:00Z","upstream":["CVE-2020-26891"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2020-26891"},{"type":"REPORT","url":"https://github.com/matrix-org/synapse/pull/8444"},{"type":"REPORT","url":"https://github.com/matrix-org/synapse/releases"},{"type":"REPORT","url":"https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2020-26891"}],"affected":[{"package":{"name":"matrix-synapse","ecosystem":"Ubuntu:Pro:18.04:LTS","purl":"pkg:deb/ubuntu/matrix-synapse@0.24.0+dfsg-1ubuntu0.1~esm4?arch=source&distro=esm-apps/bionic"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["0.19.2+dfsg-6","0.24.0+dfsg-1","0.24.0+dfsg-1ubuntu0.1~esm1","0.24.0+dfsg-1ubuntu0.1~esm4"],"ecosystem_specific":{"binaries":[{"binary_version":"0.24.0+dfsg-1ubuntu0.1~esm4","binary_name":"matrix-synapse"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2020/UBUNTU-CVE-2020-26891.json"}},{"package":{"name":"matrix-synapse","ecosystem":"Ubuntu:Pro:20.04:LTS","purl":"pkg:deb/ubuntu/matrix-synapse@1.11.0-1ubuntu0.1~esm2?arch=source&distro=esm-apps/focal"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-1","1.4.0-1","1.5.0-1","1.5.1-1","1.6.0-1","1.6.1-1","1.7.0-2","1.7.1-1","1.7.2-1","1.7.3-1","1.8.0-1","1.9.0-1","1.9.1-1","1.10.0-1","1.10.0-2","1.11.0-1","1.11.0-1ubuntu0.1~esm2"],"ecosystem_specific":{"binaries":[{"binary_version":"1.11.0-1ubuntu0.1~esm2","binary_name":"matrix-synapse"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2020/UBUNTU-CVE-2020-26891.json"}},{"package":{"name":"matrix-synapse","ecosystem":"Ubuntu:Pro:22.04:LTS","purl":"pkg:deb/ubuntu/matrix-synapse@1.53.0-1ubuntu0.1~esm2?arch=source&distro=esm-apps/jammy"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.39.0-1","1.47.0-2","1.47.1-1","1.48.0-1","1.49.0-1","1.49.2-1","1.50.1-1","1.50.2-1","1.51.0-1","1.52.0-1","1.53.0-1","1.53.0-1ubuntu0.1~esm2"],"ecosystem_specific":{"binaries":[{"binary_version":"1.53.0-1ubuntu0.1~esm2","binary_name":"matrix-synapse"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2020/UBUNTU-CVE-2020-26891.json"}}],"schema_version":"1.7.3","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"},{"type":"Ubuntu","score":"medium"}]}