Outline is a service that allows for collaborative documentation. Prior to 1.7.1, the Slack integration callback for GET /auth/slack.post accepts an unsigned, session-independent OAuth state value. A third party who can obtain a Slack OAuth code for the same Outline Slack client can make a logged-in Outline user complete the callback and link that user's Outline account to the attacker's Slack team_id and user_id. The linked Slack identity can then use the Slack /outline search command as the victim Outline user. This vulnerability is fixed in 1.7.1.