Hackers Using New Obfuscation Mechanisms to Evade Detection Of Phishing Campaign

Recently, Microsoft has unveiled details of a deceptive social engineering campaign, in which the operators proceeded to change their obfuscation and encryption devices every 37 days on average.

The campaign also includes relying on Morse code, cover their routes, and reap secretly the credentials of users. The research team of Microsoft stated in this kind of phishing attack, the threat actors encourage the victims to assign their Office 365 credentials using XLS.HTML attachments.

Not only this but to make the campaign legitimate criminals hide the letters as statements and use different information about possible victims, like:- 

  • Email addresses
  • Company logos

From plaintext to Morse code 

However, this kind of phishing attack is quite unique in nature and the lengths attackers exert to encode the HTML file so that they can easily bypass security controls.

As previously the HTML attachments are generally divided into different segments, and all of them were encoded using different encoding mechanisms. 

Moreover, this type of phishing attack’s has segments that are generally deconstructed in the following diagram:-

As we said that there is a different segment that deals with a different category, that’s why here we have mentioned them below:-

  • Segment 1 – Email address of the victim
  • Segment 2 – Logo of the attacked user’s business from logo[.]clearbit[.]com, i[.]gyazo[.]com, or api[.]statvoo[.]com; in case the logo is not ready, this segment stores the Microsoft Office 365 logo.
  • Segment 3 – A script that packs an image of a blurred document, symbolizing that sign-in has probably timed out.
  • Segment 4 – A script that assists the user to enter their password, and submits the listed password to a remote phishing kit, and later displays a fake page with an error message to the user.

Modified file extensions

Here’s the list of all modified file extensions and variations used by the threat actors:-

  • xls.HTML
  • xslx.HTML
  • Xls.html
  • .XLS.html
  • xls.htML
  • xls.HtMl
  • xls.htM
  • xsl_x.h_T_M_L
  • .xls.html
  • ._xslx.hTML
  • ._xsl_x.hTML

Detecting Dynamically Changing Email Obfuscation Techniques

However, the Microsoft Defender for Office 365 discovered the malicious emails from this phishing campaign through different, multi-layered, and cloud-based machine that has learning models and dynamic interpretation. 

Not only this but the Microsoft Defender for Office 365 also has a built-in sandbox where different portfolios and URLs are exploded and tested for maliciousness like it has particular file characteristics, methods, and other behavior.

Moreover, the Microsoft Defender for Office 365 is also withdrawn by Microsoft experts that generally monitor the threat panorama for new attacker tools and methods.


The security analysts have suggested some mitigation, that is to be followed by the victims as well as the users, and that’s why here we have mentioned them below:-

  • Initially, victims must use Office 365 mail flow rules or Group Policy for Outlook to strip .html or .htm or other file types that are not needed for business.
  • Users must turn on Safe Attachments methods to check attachments to inbound emails.
  • Always avoid passwords that are reuse among accounts and also use in multi-factor authentication (MFA), like Windows Hello, within high-value systems. 
  • Train end-users on approval regarding the phishing tactics as part of protection or phishing awareness education.
  • Assist each and every user to use Microsoft Edge and other web browsers that generally support Microsoft Defender SmartScreen, and not only this but it also identifies and blocks malicious websites, that include phishing sites, scam sites, and other sites that carry exploits and host malware.

The cybersecurity researchers also asserted that all the tactics and methods used by the threat actors are simply allowing them to enhance their security mechanisms against emerging security threats.

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BALAJI is an Ex-Security Researcher (Threat Research Labs) at Comodo Cybersecurity. Editor-in-Chief & Co-Founder - Cyber Security News & GBHackers On Security.


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