4gaBoards is a boards system for realtime project management. Prior to 3.3.9, 4gaBoards allows an authenticated user to modify ssoGoogleId, ssoGoogleEmail, ssoGithubId, ssoGithubUsername, ssoGithubEmail, ssoMicrosoftId, ssoMicrosoftEmail, ssoOidcId, and ssoOidcEmail through PATCH /api/users/:id. The whitelist in server/api/controllers/users/update.js mass assigns these backend-managed identity attributes from user input. An attacker can place a victim's provider identifier on an attacker-controlled account, causing the default lookup in helpers such as server/api/helpers/users/get-create-one-for-github-sso.js to match the victim's first SSO login to the attacker's account before the email-linkage flow runs. The victim is logged into the attacker-controlled account, and projects, boards,…