Software engineer Rob Heaton has identified a vulnerability in WhatsApp that could allow a stalker to work out when two contacts are communicating via the service.

If you go offline and then come back online to read and reply to a message, that fact can be logged.

Correlating times when you come back online with times when other people do the same can allow patterns to be seen that effectively identify two people messaging each other.

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His blog post begins by using the vulnerability to see when an avid WhatsApp user is going to bed and waking again, in a delightfully whimsical scenario about spying on the sleep patterns of a friend supposedly in training for a charity walk.

And while WhatsApp allows you to hide your ‘last seen’ times, it doesn’t allow you to hide when you are and aren’t online – that is, actively using the service.