xianyu-auto-reply-fix through 2.0.7 contains a missing authorization vulnerability in the POST /send-message endpoint that allows unauthenticated attackers to send messages from other users' accounts using publicly known default API secrets. Attackers can supply the hard-coded apikey and another user's cookieid, plus chosen chatid, touserid and message values, to send arbitrary Xianyu messages to buyers from that merchant account.