Cybercriminals take over user accounts and send fake emails to the users' colleagues and contacts. The emails sent contain fake links, including a fake OneDrive share link used to steal credentials and take over more accounts.

In the first example, criminals took over an account of a finance employee. “The employee most likely followed a phishing link from the attackers, which prompted them to enter their credentials into a fake Outlook sign-in page,” the research noted. Once they did that, the criminals had their credentials, and could use them to access the email account. The criminals then sent out emails to over a dozen members of the finance team from the compromised account. The goal of the compromised emails was to steal additional credentials.

The message itself seems like an innocuous paid invoice notification. However, if the other employees click on the link, they move to a fake Office 365 sign-in page where they receive a request to enter their credentials. If they fall for the bait and submit their authorizations, the takeover of their accounts begins.

“On their own, stolen credentials of a reputable organization are worth a handsome sum in the dark web,” the researchers relayed.

In addition, hackers can use the stolen credentials to conduct spear phishing, or CEO fraud attacks. In these attacks, the hackers send an email from the compromised account with the goal of tricking the recipient (who is usually in the finance department) to send a wire transfer to a financial account owned by the attacker.

Like the first example, hackers took over a user's email account after criminals included a OneDrive share link that when clicked led lead to a fake sign-in page used to steal credentials. In this attack, the criminals logged in multiple times to the user's account, gathered targets from the user's address book, and sent out hundreds of emails to both employees and external contacts.

Once criminals steal user credentials, these attacks can snowball quickly. “And what's really scary, is that standard email security solutions won't detect these types of attacks because they originate from internal emails,” the researchers explained.

So, how can users stay out of harm's way? Real-time spear phishing and cyber fraud defense; and user training and awareness. Barracuda Sentinel said it can automatically prevent email account takeover utilizing artificial intelligence to learn an organization's communications history and prevent future spear-phishing attacks. The firm said it also provides comprehensive user training and testing as well as phishing simulation for emails, voicemail, and SMS along with other tools to train users to identify cyberattacks.