VULNERABILITY INTELLIGENCE

CVE-2020-26891

CVSS score6.1 MEDIUM

EPSS probability1.92%

CISA KEVNot currently listed

Published2020-10-19

Last modified2026-06-17

CVSS vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

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.

Weakness classification

nvd@nist.gov: CWE-79

Primary references

cve@mitre.org — Issue Tracking, Patch, Third Party Advisory
cve@mitre.org — Release Notes, Third Party Advisory
cve@mitre.org — Patch, Third Party Advisory
cve@mitre.org — Release Notes, Vendor Advisory
af854a3a-2127-422b-91ae-364da2661108 — Issue Tracking, Patch, Third Party Advisory
af854a3a-2127-422b-91ae-364da2661108 — Release Notes, Third Party Advisory
af854a3a-2127-422b-91ae-364da2661108 — Patch, Third Party Advisory
af854a3a-2127-422b-91ae-364da2661108 — Release Notes, Vendor Advisory