Cyber criminals are stealing the login credentials of Microsoft Office 365 users using a phishing campaign that tricks victims into believing they’ve been left voicemail messages.

In the last few weeks, there’s been a surge in the number of employees being sent malicious emails that allege they have a missed call and voicemail message, along with a request to login to their Microsoft accounts.

The phishing emails also contain an HTML file, which varies slightly from victim to victim, but the most recent messages observed include a genuine audio recording, researchers with McAfee Labs have discovered.

Users are sent fake emails that inform them of a missed call and a voicemail message

When loaded, this HTML file redirects victims to a phishing website that appears to be virtually identical to the Microsoft login prompt, where details are requested and ultimately stolen.

“What sets this phishing campaign apart from others is the fact that it incorporates audio to create a sense of urgency which, in turn, prompts victims to access the malicious link,” said McAfee’s senior security researcher Oliver Devane.

“This gives the attacker the upper hand in the social engineering side of this campaign.

This Office 365 campaign has made great efforts to appear legitimate, such as through designing the phishing site to resemble the Microsoft login page. Another trick the cyber scammers use to look real is by prepopulating victims’ email addresses into the phishing site and requesting just the password.

The phishing site appears virtually identical to the actual Microsoft login prompt and preloads victims’ emails

Users are presented with a successful login message once the password is provided, and are then redirected to the office.com login page.

Researchers found three different phishing kits being used to generate malicious websites, Voicemail Scmpage 2019, Office 365 Information Hollar, and a third unbranded kit without attribution.

The first two kits aim to gather users’ email addresses, passwords, their IP addresses and location data. The third kit uses code from a previous malicious kit targeting Adobe users in 2017, the researchers said, and it’s likely the old code has been reused by a new group.

A wide range of employees across several industries, from middle management to executive level, have been targeted, although the predominate victims are in the financial and IT services fields. There’s also evidence to suggest several high-profile companies have been targeted.

McAfee has recommended as a matter of urgency that all Office 365 users implement two-factor authentication (2FA). Moreover, enterprise users have been urged to block .html and .htm attachments at the email gateway level so this kind of attack doesn’t reach the final user.

“We urge all our readers to be vigilant when opening emails and to never open attachments from unknown senders,” the researchers added. “We also strongly advise against using the same password for different services and, if a user believes that his/her password is compromised, it is recommended to change it as soon as possible.”

The use of audio in this campaign points to a greater tenacity among cyber fraudsters, who are adopting more sophisticated social engineering techniques. For example, earlier this year artificial intelligence (AI) combined with voice technology was used to impersonate a business owner and fool his subordinate into wiring £200,000 to a hacker’s bank account.