Matrix JavaScript SDK is a Matrix Client-Server SDK for JavaScript and TypeScript. matrix-js-sdk before 38.2.0 has insufficient validation of room predecessor links in MatrixClient::getJoinedRooms, allowing a remote attacker to attempt to replace a tombstoned room with an unrelated attacker-supplied room. The issue has been patched and users should upgrade to 38.2.0. A workaround is to avoid using MatrixClient::getJoinedRooms in favor of getRooms() and filtering upgraded rooms separately.
The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.