Advice below from Neighbourhood Watch Sussex:

There’s a WhatsApp scam going round whereby you receive a message from someone in your Contacts directory saying, “Can I ask a favour?” That’s all it says. Of course the temptation is to message back asking what they want but their phone has been hacked so it’s not them you’re contacting but some fraudster who’s controlling their phone and their WhatsApp account. Very easy to reveal information you shouldn’t, so if you receive a message from a friend or relative just saying “Can I ask a favour?” ignore it or, if you can’t do that, contact them in some other way NOT via WhatsApp or their mobile phone because either or both could have been hacked.