Urgent warning for millions of Gmail users over security flaw that could allow your Facebook to be hacked

According to the researcher, he was able to hijack the accounts of Facebook users who signed up through their Gmail credentials.

He said that he accomplished this by using a Google OAuth id_token/code to log in to the site.

This is called "open authorization." It is a standard used by Amazon, Microsoft, Twitter, and other big names that allows users to link accounts to third-party sites by using log in information from one of these bigger sites, said.

That being said, this technique can apparently be used to hack other accounts not just Facebook, Sammouda said.

However, the tech vigilante may have prevented many users from being hacked.

Sammouda revealed that he was paid a $44,625 'bounty' by Facebook after he shared his discovery.

He said that Facebook responded and implemented measures against the open authorization hack.

Forbes shared a statement from security provider regarding accounts that are linked through open authorization.

"Linked accounts were invented to make logging in easier," said malware intelligence researcher Pieter Arntz.

"You can use one account to log in to other apps, sites and services... All you need to do to access the account is confirm that the account is yours."

"We wouldn't recommend it because if anyone gets hold of the one password that controls them all, you're in even bigger trouble than you would be if only one site's password is compromised," he said.

This comes as Google recently issued an urgent security warning to billions of phone owners.

And Android users should check their phones right now for a huge security danger.