The notification in the accompanying photo is exactly the type of thing you don't want popping up on your phone unexpectedly. But if you have an iPhone and you haven't paid attention to the settings for Apple's AirDrop file-sharing feature, anyone could "cyberflash" you at any moment.
As the website Naked Security (aptly named) explains, stories about people being cyberflashed in public spaces like trains are starting to crop up. And the AirDrop preview images are what make it possible. "Apple's workflow of sending you an image to see if you want the sender to send it to you may not be quite the right way to do it," Paul Ducklin wrote.
If you have AirDrop set to "Make yourself discoverable to: Everyone," both Bluetooth and Wi-Fi will be automatically turned on, and anyone with an updated Apple device who is in close range can detect and attempt to AirDrop files to you. You're asked to accept or reject, but not before seeing a preview of what the person is trying to send.
The way to protect yourself from being cyberflashed is to turn off AirDrop whenever you're not using it. Naked Security also recommends taking the same precaution with Bluetooth and Wi-Fi. And if you want to be less of a target when you are using all of these services, change your iDevice's name from the personalized default (Lily's iPhone) to something generic like myiPhone or mac1234. Women might also consider something like "John Smith's MacBook." But keeping AirDrop off most of the time is the best safeguard.
Usually people end up making their devices discoverable to everyone because it doesn't seem like a big deal, and they want a friend or colleague to be able to quickly share something topical. But tell everyone you know about how to protect themselves. Friends don't let friends get cyberflashed.
Postscript: Apple does give you the option to be discoverable by "Contacts Only," so if the person you want to AirDrop with is in your contacts you can choose that instead of "Everyone" when you turn the service on. But still you might as well turn AirDrop off when you're done.