Description

Security researcher and pentester Bobby Rauch has identified threat actors leveraging insecure design elements/vulnerabilities in Microsoft Teams to create a reverse shell between an attacker and victim, where no communication is directly exchanged between a hacker and a victim, but is entirely piped through malicious GIFs sent in Teams messages. As per the researcher, the main component of the attack is 'GIFShell,' which uses base64 encoded GIFs to deliver malicious commands as well as exfiltrate output through retrieved GIFs. An attacker creates the reverse shell by convincing a user to install a malicious GIFShell stager which executes commands and uploads command output via a GIF URL to a Microsoft Teams webhook. However, as we know, phishing attacks are effective at infecting devices. Threat actors can use Rauch's GIFShell Python script to send a message containing a GIF that contains commands designed to execute on the target's computer. As soon as the stager detects a GIF, it extracts the base64-encoded commands and executes them on the device. When Microsoft attempts to retrieve the GIFs, the threat actors can continue sending GIFs with additional embedded commands to execute. As these requests are made via Microsoft's website 'urlp.asm.skype.com', which is regularly used by Microsoft Teams, security software will not detect them. Even worse, because Microsoft Teams runs in the background, the user does not even need to open it in order for the attacker's commands to be executed. The Microsoft Teams logs folder has also been found to be accessed by other programs, including malware and business monitoring software like Veriato. Microsoft acknowledged the investigation but stated that since no security restrictions were violated, it would not be fixed. However, Microsoft does not permit external users to transmit attachments to members of another tenant in order to stop attackers from utilizing Microsoft Teams in malware phishing assaults. Rauch discovered that when someone sends a file to another user in the same tenant, Microsoft creates a Sharepoint link that is included in a JSON POST request to the Teams endpoint. Then, an attacker can change this JSON message to include whatever download link they want. Moreover, Microsoft Teams did not fix any flaws yet, as per researchers.