This vulnerability occurs when AppFlowy-Cloud fails to verify that an authenticated user is a member of the workspace during the establishment of WebSocket connections handled by establish_ws_connection_v2. As a result, a user can bind a WebSocket session to any workspace, even those they are not a member of, and then issue sync Manifest messages containing object identifiers from other workspaces. The attacker can read the full document or database state from those other workspaces without any victim interaction, thereby exposing sensitive collaboration data. This defect aligns with CWE-863, which denotes user‑controlled access paths.

The flaw affects AppFlowy-IO's AppFlowy-Cloud product in all releases up through version 0.9.64. Any installation that has not obtained a later version containing the WebSocket membership validation logic is vulnerable. The issue does not affect earlier or non‑Cloud releases of AppFlowy.

The vulnerability has a medium CVSS score of 6, and no EPSS score is publicly available. It is not listed in the CISA KEV catalog, and there is no known exploitation in the wild yet. Attackers would need to authenticate to the target system, then merge a WebSocket connection to a non‑member workspace, after which they can request data via sync Manifest payloads. Because the flaw allows data exfiltration without required victim participation, the potential impact is significant if an attacker has any valid user credential, but the medium severity reflects the need for authentication and the mitigation measures already in place.