Pierluigi Paganini February 17, 2022

Attackers compromise Microsoft Teams accounts to attach malicious executables to chat and spread them to participants in the conversation.

While the popularity of Microsoft Teams continues to grow, with roughly 270 million monthly active users, threat actors started using it as an attack vector.

Starting in January 2022, security researchers from Avanan observed attackers compromising Microsoft Teams accounts attach malicious executables to chat and infect participants in the conversation.

In the attacks observed by the experts, threat actors inserted a .exe file called “User Centric” into a chat in an attempt to trick participants into opening it. Upon opening the executable, the malicious code will install DLL files and create shortcut links to self-administer.

“Starting in January 2022, Avanan observed how hackers are dropping malicious executable files in Teams conversations. The file writes data to the Windows registry, installs DLL files and creates shortcut links that allow the program to self-administer. Avanan has seen thousands of these attacks per month.” reads the analysis published by Avanan. “In this attack brief, Avanan will analyze how these .exe files are being used by hackers in Microsoft Teams.”

Experts believe that attackers can launch the attack by compromising a partner organization and listening in on inter-organizational chats. In another attack scenario, threat actors can compromise an email address and use it to access Teams. Once an attacker obtained Microsoft 365 credentials, for example from a previous phishing campaign or data breach, that can access Teams and other Office applications.

Once the attackers have gained access to an organization, they can determine defense solutions that are installed and use the proper malware to bypass existing protections. 

“Compounding this problem is the fact that default Teams protections are lacking, as scanning for malicious links and files is limited. Further, many email security solutions do not offer robust protection for Teams.” continues the analysis. “Hackers, who can access Teams accounts via East-West attacks, or by leveraging the credentials they harvest in other phishing attacks, have carte blanche to launch attacks against millions of unsuspecting users.” 

Researchers also explained that in some specific contests, users have no perception of the threat that could use Teams as a vector.

Avanan experts analyzed the use of Microsoft Teams in hospitals where the internal staff uses this technology to share patient medical information ignoring the risks associated with opening files when they come through Teams.

“Most employees have been trained to second-guess identities in email, but few know how to make sure that the name and photo they see in a Teams conversation are real. It is simple to edit a profile and become most anyone you like. So when someone attaches a file to a Teams chat, particularly with the innocuous-sounding file name of “User Centric”, many users won’t think twice and will click on it.” concludes the report. “This attack demonstrates that hackers are beginning to understand and better utilize Teams as a potential attack vector As Teams usage continues to increase, Avanan expects a significant increase in these sorts of attacks.”

Below is a list of recommendations provided by Avanan:

    • Implement protection that downloads all files in a sandbox and inspects them for malicious content
    • Deploy robust, full-suite security that secures all lines of business communication, including Teams
    • Encourage end-users to reach out to IT when seeing an unfamiliar file

Follow me on Twitter: @securityaffairs and Facebook

Pierluigi Paganini

(SecurityAffairs – hacking, Microsoft Teams)

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