This is a "Guest Diary" submitted by Ismael Valenzuela and Marc Rivero. Interested in writing a guest diary? Let us know via our contact page.
Macro based malware that hides in Microsoft Word or Excel documents is nothing new to Incident Responders and Malware Analysts.
However, something that caught our attention in the last few days was the use of a 'fileless' method to bypass UAC implemented in a malicious Excel file. This method leverages eventvwr.exe and was described in detail by the Enigma0x3 team in this post: https://enigma0x3.net/2016/08/15/fileless-uac-bypass-using-eventvwr-exe-and-registry-hijacking/
Bypassing UAC is nothing new either (see the UACME project created by hfiref0x). In fact, a few days ago we knew of a new Dridex sample that attempts to bypass UAC by using application compatibility databases (http://blog.jpcert.or.jp/2015/02/a-new-uac-bypass-method-that-dridex-uses.html). What is most interesting about the method described by the Enigma0x3's team, however, is that it doesn't require any kind of privileged file copy, code injection, or placing a DLL anywhere on the disk.
This particular Excel file employs this UAC bypass method to download and execute a malicious binary that is part of a well-known data-stealing family called KEYBASE.
SHA256 HASH: e431bc1bacde51fd39a10f418c26487561fe7c3abee15395314d9d4e621cc38e
KEYBASE is a primarily a keylogger with some other additional capabilities that are commonly found in other non-sophisticated Trojans such as password stealing, clipboard copying, etc.
To understand how this sample behaves and have a look at its capabilities we can use a popular free online resource like "Hybrid Analysis" (https://www.hybrid-analysis.com/) from Payload Security.
Looking at the process list details we can observe what specific processes were spawned when opening the Excel file, along with command line arguments:
Image 2: Dynamic analysis shows the execution of eventvwr.exe and pu457.exe
While the output is pretty self-explanatory, let's dive a bit deeper and explain what's going on there:
The sequence of events described above will ultimately result in code execution in a high integrity process, effectively bypassing UAC!
As expected, there is an HTTP connection to ridart.ru to download an additional binary (mi.exe):
Image 3: Powershell initiates an HTTP GET request to ridart.ru to download mi.exe
The static analysis performed on pu457.exe helps us to confirm the capabilities of this Portable Executable:
Finally, using these IOCs found during our investigation, we can leverage Virustotal (https://www.virustotal.com) to check the reputation of this site and pivot to associated URLs, domains, other related samples. If you check the IP's on the network traffic on Hybrid Analysis, you can extract more malicious information related:
Image 4: Associated artifacts for 188.8.131.52 (ridart.ru)
As the Enigma0x3 team reminds us in their post, this method to bypass UAC is expected to work on all versions of Windows that implement UAC, including Windows 10, but can be prevented by removing the current user from the Local Administrators group, which is something that you should do anyways!
From a monitoring perspective, it's recommended to monitor and alert on any new registry entries in HKCU\Software\Classes, something that can be easily implemented with the latest version of Microsoft's Sysmon, v5 (https://technet.microsoft.com/en-us/sysinternals/sysmon).
Full report in Hybrid Analysis:
Ismael Valenzuela, GSE #132 (@aboutsecurity)
Marc Rivero @seifreed
Jan 31st 2017
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Jan 31st 2017
4 years ago
Quote:From a monitoring perspective, it's recommended to monitor and alert on any new registry entries in HKCU\Software\Classes, something that can be easily implemented with the latest version of Microsoft's Sysmon, v5 (https://technet.microsoft.com/en-us/sysinternals/sysmon).
You don't need any 3rd party tools to monitor creation and/or modification of registry entries!
Enable auditing and add the appropriate SACL on [HKCU\Software\Classes]
See https://blogs.msdn.microsoft.com/cobold/2011/11/29/monitoring-when-registry-keys-are-modified/ for example.
Feb 1st 2017
4 years ago