A new, large-scale phishing campaign has been observed using adversary-in-the-middle (AitM) techniques to get around security protections and compromise enterprise email accounts.

"It uses an adversary-in-the-middle (AitM) attack technique capable of bypassing multi-factor authentication," Zscaler researchers Sudeep Singh and Jagadeeswar Ramanukolanu said in a Tuesday report. "The campaign is specifically designed to reach end users in enterprises that use Microsoft's email services."

Prominent targets include fintech, lending, insurance, energy, manufacturing, and federal credit union verticals located in the U.S., U.K., New Zealand, and Australia.

This is not the first time such a phishing attack has come to light. Last month, Microsoft disclosed that over 10,000 organizations had been targeted since September 2021 by means of AitM techniques to breach accounts secured with multi-factor authentication (MFA).

The ongoing campaign, effective June 2022, commences with an invoice-themed email sent to targets containing an HTML attachment, which includes a phishing URL embedded within it.

Opening the attachment via a web browser redirects the email recipient to the phishing page that masquerades as a login page for Microsoft Office, but not before fingerprinting the compromised machine to determine whether the victim is actually the intended target.

AitM phishing attacks go beyond the traditional phishing approaches designed to plunder credentials from unwitting users, particularly in scenarios where MFA is enabled – a security barrier that prevents the attacker from logging into the account with only the stolen credentials.

To circumvent this, the rogue landing page developed using a phishing kit functions as a proxy that captures and relays all the communication between the client (i.e., victim) and the email server.

"The kits intercept the HTML content received from the Microsoft servers, and before relaying it back to the victim, the content is manipulated by the kit in various ways as needed, to make sure the phishing process works," the researchers said.

This also entails replacing all the links to the Microsoft domains with equivalent links to the phishing domain so as to ensure that the back-and-forth remains intact with the fraudulent website throughout the session.

Zscaler said it observed the attacker manually logging into the account eight minutes after the credential theft, following it up by reading emails and checking the user's profile information.

What's more, in some instances, the hacked email inboxes are subsequently used to send additional phishing emails as part of the same campaign to conduct business email compromise (BEC) scams.

"Even though security features such as multi-factor authentication (MFA) add an extra layer of security, they should not be considered as a silver bullet to protect against phishing attacks," the researchers noted.

"With the use of advanced phishing kits (AiTM) and clever evasion techniques, threat actors can bypass both traditional as well as advanced security solutions."