Microsoft revealed that a year-long spear-phishing campaign has targeted Office 365 customers in multiple attacks starting with July 2020.
The attackers used invoice-themed XLS.HTML attachments, Microsoft reported that they changed obfuscation and encryption mechanisms every 37 days on average, a circumstance that demonstrates high motivation and the threat actors’ abilities to constantly evade detection.
Attackers split the HTML attachment in individual segments that appear harmless and bypass security solutions.
Experts believe the attackers aims at harvesting usernames, passwords, and recent waves of attacks, other information such as IP address and location, that could be used in later attacks.
These attacks are very effective because of efforts to obfuscate their phishing emails to bypass email security solutions.
The HTML attachment is divided into multiple segments, which are encoded using various encoding mechanisms.
The segments composing the attachment include:
Threat actors used to change the encoding mechanisms to evade detection, they used different methods for each segment and alternate the use of plaintext HTML code, escaping, Base64, ASCII chars, and even Morse code.
Upon tricking the recipient into launching the attachment, it will display a fake Office 365 login form over a blurred Excel document. The login dialog box is designed using the logo of the recipient’s organization and in some cases the “username” field is filled with the targets’ email addresses and requests the victim to provide its password to view the Office document.
If the recipient provides its password, a script will display an alert informing the victim that the password is incorrect and send the password to the attacker.
Microsoft shared Indicators of Compromise (IoCs) for this campaign.
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(SecurityAffairs – hacking, cybercrime)