Relay Server enables real‑time collaboration in Obsidian. Versions 0.9.0 through 0.9.6 contained an authentication bypass in the multi‑document WebSocket endpoints. When authentication is enabled, connections that omit the required token query parameter are treated as fully authorized. As a result, an unauthenticated attacker who knows or can guess a document ID can open the sync WebSocket, read the entire document and modify its contents without possessing a valid token.

The vulnerability affects all installations of No‑Instructions Relay Server version 0.9.0 through 0.9.6. The fix is available in 0.9.7 and later. Users of earlier or pre‑1.0 builds are also at risk if they use the WebSocket sync features.

The CVSS score of 9.1 indicates high severity. The EPSS score is not available, but the lack of a KEV listing suggests no known active exploitation yet. The likely attack vector is a network‑based attacker who can reach the Relay Server’s WebSocket endpoints. Given the requirement to know or guess a document ID, an attacker can simply test common identifiers or rely on leaked IDs, then establish a WebSocket connection without a token and gain full read/write access to that document.