Plane is an open-source project management tool. Prior to 1.4.0, ProjectJoinEndpoint at GET /api/workspaces/{slug}/projects/{project_id}/join/{pk}/ uses permission_classes = [AllowAny] and returns the full ProjectMemberInvite record, including its email, token, and role, to unauthenticated callers. The corresponding POST endpoint checks only whether the submitted email matches project_invite.email and does not validate the invitation token. An attacker who knows the invitation UUID can discover the invited email, register an account with that email, and accept the invitation without receiving the original invite. This issue is fixed in 1.4.0.