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A group of friends is being stalked by an unknown person (or bot?).

Everything started with person A. A was sent authentic screenshots of several of his WhatsApp chats. Screenshots were sometimes sent from an unknown number in WhatsApp, sometimes using other Social Media.

Later, A met B and they started a conversation on WhatsApp. Soon B also received screenshots of several of her chats.

B just clarified neither A or she receives actual screenshots. The messages are authentic, they also contain authentic pictures sometimes. But the look is different, as if a screenshot was made in another app. It's quite different from WhatsApp, for example, the background of every message is plain white. Also, the pictures only show the messages, nothing else. B also told me that some messages directly address her. These are not copied from WhatsApp. For example, she might receive a message like "(Some insult here), look what I've found: (Some message copied from a WhatsApp chat here)"**

A and B are located in different towns. A is using an iPhone, B had an iPhone at first, but changed to an Android phone later and changed his number at the same time (which didn't help). Both are using recent versions of WhatsApp with end-to-end encryption. B (at least) was also using some web client for WhatsApp.

It's highly improbable that someone had physical access to either phone or to local WiFis.

Is there some common explanation on how the stalker might have gained access to my friend's chats? Is there anything I could recommend to them other that switching to a different app?

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    Are you sure that we can rule out thet fact that A is sending these screenshots ? Either intentionnally to mess with B, either unintentionnally by mistaking different accounts or different applications. – Quentin Aug 22 '17 at 8:39
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    sounds like A might have an infected phone. End-to-end encryption does not help here, because the intrude is probably just taking screenshots of A's phone. – Theolodis Aug 22 '17 at 8:42
  • @Quentin: Yes, I'm sure. Both A and B receive screenshots of several chats: B is being sent a screenshot of a chat between B and C and so on. – Pida Aug 22 '17 at 8:53
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    @Theolodis: Because of what I replied to Quentin, I think all three phones would have to be infected. – Pida Aug 22 '17 at 8:53
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    First thing I would do is look very critically at all the download our app now and look what a great app I have; you must download it too apps, and remove them from the phone if not strictly necessary. – user13695 Aug 22 '17 at 9:36
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Edit for compression:

Possibilities:

There are too many possible ways for intrusion in this non-detailed situation, therefore the question is not really answerable.

The possibilities range from firmware-os level infection, till disguised apps, or vulnerabilities in the app or encryption processes. Also wifi hacking is possible even if not probable. This needs a lot of time and testing to find it out, what very likely noone will do do for free for a stranger.

Explanation:

Many hardware come nowadays with preinstalled backdoor firm-software, which are exploitable by anyone the day you start it up first time. If it was not the case, there are many other possibilities to infect it later, even imposing the malware as part of the system/app. There are other ways to work around encryption too.

Tips:

Average users definitely need help from professionals if they wish to have a secure/private digital communication or they need personalized help. Not even an IT professional may help, since they might be are motivated by money or limited by local law. The best way is to understand, that if you are not a security professional, your digital life is definitely not secure. Noone should live in false sense of security.

Achieving high level of security needs a lot of personal work and knowledge, if you do not trust others, and you should start being interested in open source hard-firm- and software, encryption, hidden and decentralized services. Not even those are going to protect you from organized criminals for sure.

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