Details for this vulnerability have not been published by NIST at this point. Descriptions from software vendor advisories for this issue are provided below.
From VID-E4D93D07-297A-11ED-95F8-901B0E9408DC:
Matrix developers report:
The vulnerabilities give an adversary who you share a
room with the ability to carry out a denial-of-service
attack against the affected clients, making it not show all
of a user's rooms or spaces and/or causing minor temporary
corruption.
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