GIFShell Attack Through Microsoft Teams: What Is It and How You Can Protect Yourself from It

Another day, another attack method. 

The Short Story

GIFShell attack technique enables bad actors to exploit several Microsoft Teams features to act as a C&C for malware, and exfiltrate data using GIFs without being detected by EDR and other network monitoring tools. The technique assumes an already-compromised target.

GifShell Attack Architecture & Process

Discovered by Bobby Rauch, the main component of this attack allows an attacker to create a reverse shell that delivers malicious commands via base64 encoded GIFs in Teams, and exfiltrates the output through GIFs retrieved by Microsoft's own infrastructure. 

How does it work?

• To create this reverse shell, an attacker must first compromise a computer to plant the malware — which means the bad actor needs to convince the user to install a malicious stager, like with phishing, that executes commands and uploads command output via a GIF url to a Microsoft Teams web hook. 

• Once the stager is in place, the threat actor creates their own Microsoft Teams tenant and contacts other Microsoft Teams users outside of the organization. 
• The threat actor can then use a GIFShell Python script to send a message to a Microsoft Teams user that contains a specially crafted GIF. This legitimate GIF image has been modified to include commands to execute on a target's machine.
• When the target receives the message, the message and the GIF will be stored in Microsoft Team’s logs. Important to note: Microsoft Teams runs as a background process, so the GIF does not even need to be opened by the user to receive the attacker's commands to execute.
• The stager monitors the Teams logs and when it finds a GIF, it extracts and runs the commands.
• Microsoft's servers will connect back to the attacker's server URL to retrieve the GIF, which is named using the base64 encoded output of the executed command.
• The GIFShell server running on the attacker's server will receive this request and automatically decode the data allowing the attackers to see the output of the command run on the victim's device.

Microsoft’s response

As reported by Lawrence Abrams in BleepingComputer, Microsoft agrees that this attack method is a problem, however, it “does not meet the bar for an urgent security fix.” They “may take action in a future release to help mitigate this technique.” Microsoft is acknowledging this research but asserting that no security boundaries have been bypassed. 

While Rauch claims that indeed “two additional vulnerabilities discovered in Microsoft Teams, a lack of permission enforcement and attachment spoofing”, Microsoft argues, "For this case… these all are post exploitation and rely on a target already being compromised.” Microsoft is asserting that this technique is using legitimate features from the Teams platform and not something they can mitigate currently. 

In accordance with Microsoft's assertions, indeed this is the challenge many organizations face — there are configurations and features that threat actors can exploit if not hardened. A few changes to your tenant’s configurations can prevent these inbound attacks from unknown Teams tenants.

How to Protect Yourself from the GifShell Attack Method

There are security configurations within Microsoft that if hardened can help to prevent this type of attack. ‍

1. Disable External Access:

Microsoft Teams by default allows for all external senders to send messages to users within that tenant. Many organization admins likely are not even aware that their organization allows for External Teams collaboration. You can harden these configurations: 

• Disable external domain access — Prevent people in your organization from finding, calling, chatting, and setting up meetings with people external to your organization in any domain.
  While not as seamless of a process as through Teams, this better protects the organization and is worth the extra effort.
• Disable unmanaged external teams start conversation — Block Teams users in your organization from communicating with external Teams users whose accounts are not managed by an organization. 

2. Gain Device Inventory Insight‍

Endpoint security tools are  your first line of defense against suspicious activity such as accessing the device's local teams log folder which is used for data exfiltration in GIFShell. You can ensure your entire organization's devices are fully compliant and secure by using your XDR / EDR / Vulnerability Management solution, like CrowdStrike or Tenable. 

You can even go a step further and integrate an SSPM (SaaS Security Posture Management) solution, like Adaptive Shield, with your endpoint security tools  to gain visibility and context to easily see and manage the risks that stem from these types of configurations, your SaaS users and their associated devices.