ZeroFont Phishing: Hackers Manipulating Font Size to Bypass Office 365 Security

A new but ancient technique for Phishing emails has been recently identified called ZeroFont Phishing. Threat actors have followed several tactics for sending phishing emails, bypassing all the security mechanisms.

However, using this technique, threat actors could bypass Microsoft’s Natural Language Processing, which was acting as a Phishing email protection for Office users. 

Office 365 – Natural Language Processing

Microsoft has been working towards their way of securing its customers in all aspects. One of the major areas they focus on is phishing (Business Email Compromise) attacks, which have been the most used technique by threat actors for infiltrating organizations.

To prevent these phishing emails, Microsoft has been relying on Natural Language Processing, which scans the contents of an email for signs of impersonation or fraud. If an email content includes text like “© 2018 Microsoft Corporation. All rights reserved” and the email is not from Microsoft.com, Microsoft immediately flags this email as fraudulent.

This technique was also used to interpret email contents like banking information, user accounts, password resets, and financial requests and are checked for authenticity. However, threat actors bypassed this technique using the ZeroFont Phishing attack.

ZeroFont Phishing

The threat actor sends an email to the victim impersonating an Office 365 quota limit notification, which looks like an administrative service email. However, this phishing email bypassed the protection due to the use of the ZeroFont attack.

Threat actors inserted random text inside the email, which had <span style=”FONT-SIZE: 0px”> for a zero font size, and broke up the text strings to bypass Microsoft’s natural language processing. 

A complete report has been published by Avanan, which provides detailed information about this attack and bypass scenarios used by threat actors. 

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