Matrix developers report:
matrix-react-sdk is a react-based SDK for inserting a Matrix chat/VoIP
client into a web page. Prior to version 3.71.0, plain text messages
containing HTML tags are rendered as HTML in the search results.
To exploit this, an attacker needs to trick a user into searching
for a specific message containing an HTML injection payload. No
cross-site scripting attack is possible due to the hardcoded content
security policy. Version 3.71.0 of the SDK patches over the issue.
As a workaround, restarting the client will clear the HTML injection.