Forcepoint Security Labs™ recently investigated a trojanized RTF document which we tied to the Carbank criminal gang. The document contains an encoded Visual Basic Script (VBScript) typical of previous Carbanak malware. Recent samples of the malware have now included the ability to use Google services for command-and-control (C&C) communication. We have notified Google of the abuse and are working with them to share additional information.
Carbanak (also known as Anunak) are a group of financially motivated criminals first exposed in 2015. The actors typically steal from financial institutions using targeted malware. Recently a new Carbanak attack campaign dubbed "Digital Plagiarist" was exposed where the group used weaponized office documents hosted on mirrored domains, in order to distribute malware.
The RTF document we analyzed (SHA1 1ec48e5c0b88f4f850facc718bbdec9200e4bd2d) has an embedded OLE object which contains a VBScript file. When the document is opened the targeted user is lured into double-clicking on the embedded OLE object which is disguised as an image:
Double clicking on the image results in a file open dialog for "unprotected.vbe". If the user executes this file then the VBScript malware will begin to execute.
Encoded VBScript Malware
The VBScript malware inside the RTF document (SHA1 cd75662751c59951717b4704ea2cdb6fb7ec19bc) is an encoded VBScript file. We decoded the script and found hallmarks typical of the Carbanak group's VBScript malware, however we also found the addition of a new "ggldr" script module.
The module is base64 encoded inside the main VBScript file along with various other VBScript modules used by the malware. When we analyzed the script we noticed that it is capable of using Google services as a C&C channel.
Abusing Google for C&C Communication
The "ggldr" script will send and receive commands to and from Google Apps Script, Google Sheets, and Google Forms services. For each infected user a unique Google Sheets spreadsheet is dynamically created in order to manage each victim. The use of a legitimate third party service like this one gives the attacker the ability to hide in plain sight. It is unlikely that these hosted Google services are blocked by default in an organization, so it is more likely that the attacker will establish a C&C channel successfully.
The C&C procedure is outlined in the diagram below.
Upon the first attempt to contact the hard-coded Google Apps Script URL with the user's unique infection ID, the C&C will state that no spreadsheet currently exists for the user. The malware will then send two requests to another hard-coded Google Forms URL which will result in the creation of unique Google Sheets spreadsheet and Google Form IDs for the victim.
The second time the Google Apps Script is requested, the C&C will return the unique Google Sheet and Google Form ID values:
The "entry" value is also a unique ID which is sent with each subsequent Google Forms C&C request.
Forcepoint™ customers are protected against this threat via TRITON® ACE at the following stages of attack:
Stage 5 (Dropper File) - The malware components are prevented from being downloaded and/or executed.
Stage 6 (Call Home) - The HTTP-based Carbanak C&C traffic is blocked.
The Carbanak actors continue to look for stealth techniques to evade detection. Using Google as an independent C&C channel is likely to be more successful than using newly created domains or domains with no reputation. Forcepoint will continue to monitor this group's activities and share data with trusted partners.
Indicators of Compromise
1ec48e5c0b88f4f850facc718bbdec9200e4bd2d (3-ThompsonDan.rtf) 400f02249ba29a19ad261373e6ff3488646e95fb (order.docx) 88f9bf3d6e767f1d324632b998051f4730f011c3 (claim.rtf)
Carbanak Google Apps Script C&Cs
hxxps://script.google[.]com/macros/s/AKfycbzuykcvX7j3TlBNyQfxtB1mqii31b4VTON640yiRJT0t6rS4s4/exec hxxps://script.google[.]com/macros/s/AKfycbxxx5DHr0F8AYhLuDjnp7kGNELq6g27J4c_JWWx1p1nDfZh6InO/exec hxxps://script.google[.]com/macros/s/AKfycbwZHCgg5EsCiPup_mNxDbSX7k7yBMeXWenOVN1BWXHmyBpb8ng/exec
Carbanak Google Forms C&Cs
hxxps://docs.google[.]com/forms/d/e/1FAIpQLScx9gwNadC7Vjo11mXLbU3aBQRrqVpoWjmNJ1ZneqpjaYLE3g/formResponse hxxps://docs.google[.]com/forms/d/e/1FAIpQLSfE9kshYBFSDAfRclW8m9rAdajqoYhzhEYmEAgZexE3LQ-17A/formResponse hxxps://docs.google[.]com/forms/d/e/1FAIpQLSdcdE7lTEiqV5MW3Up8Hgcy5NGkIKnLKoe0YPFriD4_9qYq9A/formResponse
Carbanak Cobalt Strike / Meterpreter DNS Beacon C&Cs