Details for this vulnerability have not been published by NIST at this point. Descriptions from software vendor advisories for this issue are provided below.
Matrix developers reports:
The fallback authentication endpoint served via Synapse were vulnerable
to cross-site scripting (XSS) attacks. The impact depends on the
configuration of the domain that Synapse is deployed on, but may allow
access to cookies and other browser data, CSRF vulnerabilities, and
access to other resources served on the same domain or parent domains.
With Rapid7 live dashboards, I have a clear view of all the assets on my network, which ones can be exploited, and what I need to do in order to reduce the risk in my environment in real-time. No other tool gives us that kind of value and insight.
– Scott Cheney, Manager of Information Security, Sierra View Medical Center