Facebook's head of security has a big concern: Android

Facebook's head of security, Alex Stamos, has a seriously high-pressure job: Keeping 1.5 billion Facebook users safe and secure every day.

The Greek software engineer works at Facebook's global head of security, where he is ultimately responsible for protecting its billions of users - whether that's from opportunistic teenagers hackers in their parents' basements, or sophisticated nation-state backed attacks.

So what worries him the most? "Mobile devices."

Speaking at the Irish tech conference Web Summit on Wednesday afternoon, Stamos told CNN's Laurie Siegel that the proliferation of cheap smartphones across the globe has made hundreds of millions of people vulnerable to hacking.

Google's mobile operating system Android is given away freely to hardware manufacturers to put on their devices. This has given Google unparalleled reach, and put smartphones in the hands on billions of people. But this also means Google isn't responsible for getting updates to users - that is down to the handset makers themselves.

But this doesn't always happen: A smartphone goes obsolete and stops being updated, or a manufacturer goes under altogether. Either way, it means the necessary updates to protect users can't get through, even if the security issue is well-documented, and easily fixable.

"Android is a huge success," Stamos said. "But there are hundreds of million of Android devices that will never be patched," with vulnerabilities that are "very easy to attack."

This puts a user's Facebook account - as well as the rest of their online life - at risk of identify theft, fraud, or defacement.