Researchers built a WeChat worm that spreads through incoming calls without user action. Tencent has blocked the exploit.Researchers at Calif created a WeChat worm that can take over an account through an incoming call, even if the victim never answers or touches the phone. The attack works only when the caller already appears in the victim’s WeChat contacts. Calif reported the flaw to Tencent in July, and Tencent has blocked the exploit for all users. The good news is that researchers found no evidence that attackers used the flaw in real-world attacks, but the case shows how dangerous zero-click vulnerabilities can be.“Simply by calling a victim, WeWorm can hijack their account and call their friends, spreading from phone to phone.” reads the report at Calif. “If exploited, actors can compromise over a billion phones (or accounts), upending livelihoods and breaking communities worldwide.”Researchers built a demo WeChat worm that spread between three test phones without requiring any action from the victims. They started with a Pixel 10a and called an iPhone 17e, exploiting the flaw while the phone was still ringing. After taking control of the iPhone, they used it to call another Pixel 10a and compromise that device too. In other words, one compromised account can become the starting point for the next attack.The exploit works within seconds and gives the attacker full control of the victim’s WeChat account. They can read and send messages, make calls and use the account as if they were the victim. The victim does not need to answer or touch the phone. Even answering the call does not stop the attack. Declining the call blocks that attempt, but the attacker can simply try again later.The main limitation is that the attacker must already be a WeChat contact. However, compromising one of the victim’s friends could provide a way around that restriction.“The victim does not need to answer the call, or interact with their phone at all. Even if they do answer, they hear nothing, and the exploit still succeeds. Declining the call stops that attempt, but the attacker can simply try again later, for example, while the victim is asleep.” the researchers explain. “This exploit requires the attacker to be on the victim’s friend list. But that’s not much of a barrier: an attacker can compromise one of your friends first and use their account to reach you.”Calif published both Android and iOS RCE demos. The firm did not disclose technical details about the issue and will present the full analysis at a conference. For now, defenders have no clear indicators to search for, and users cannot tell if attackers targeted them with a call.Combined with other Android and iOS flaws, the attack could also lead to full device control.Researchers say AI helped them find the flaw and build an RCE exploit in about two days. They then built the worm in another week. They argue that AI is making advanced attack capabilities available to less-skilled attackers, increasing the risk of zero-click threats.Researchers say the goal of publishing the findings is to raise awareness and encourage governments and technology companies to work together on AI security.The WeChat flaw comes from a memory corruption bug in the app’s VoIP system. Researchers are keeping the technical details private for now and plan to present their full analysis at a future security conference.The team believes this bug is just one example of the unusual attack surfaces found in messaging apps. They are researching similar weaknesses in other apps and working with developers to reduce these risks. Some changes may require cooperation from platform owners. Once the work progresses, they plan to publish more details about the WeChat flaw.In August, Tencent released versions Android 8.0.77 and iOS 8.0.76 that addressed the issue.WeChat is Tencent’s messaging and social platform, launched in 2011 and now much more than a chat app. It lets users send messages, make voice and video calls, create group chats and share content through Moments. It also includes official accounts, video channels, games, search and Mini Programs, which provide services such as shopping, bookings and deliveries without requiring separate apps. In China, its local version, Weixin, also integrates Weixin Pay for mobile payments. Tencent reported 1.418 billion combined monthly active users for Weixin and WeChat at the end of 2025, making the platform one of the world’s largest messaging ecosystems.Follow me on Twitter: @securityaffairs and Facebook and MastodonPierluigi Paganini(SecurityAffairs – hacking, mobile)