Bytebase 0.12.0 before 1.0.1 contains an authentication bypass vulnerability that allows unauthenticated attackers to log in as any user. The flaw stems from trusting client-supplied GitLab provider configuration in the POST /api/auth/login/GITLAB_SELF_HOST endpoint. Attackers can set instanceUrl to a server they control that returns an active user with a chosen email. Bytebase then issues session tokens for that principal, letting attackers impersonate existing users including the Owner or create new members.