Vulnerability CVE-2020-26891
Published: 2020-10-19

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.

Type:

CWE-79

 (Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'))

CVSS2 => (AV:N/AC:M/Au:N/C:N/I:P/A:N)

CVSS Base Score
Impact Subscore
Exploitability Subscore
4.3/10
2.9/10
8.6/10
Exploit range
Attack complexity
Authentication
Remote
Medium
No required
Confidentiality impact
Integrity impact
Availability impact
None
Partial
None
Affected software
Matrix -> Synapse 

 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
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