Introduction:
In the long list of complex threats that we see daily, it is interesting to see malware that is rather simple but effective in terms of the payload that it carries. At Zscaler ThreatLabZ, we recently
came across one such innocent looking Bot, which targeted our customers. The file arrived as an attachment to a spam email message. The malware was written in VBScript.
Analysis:
Virustotal scan results show 14 out of 50 vendors detecting the malware.
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| Figure 1: Virus-total Result |
The image below shows the malware opened in notepad. We can see that the file is obfuscated. That’s because the file has a “.vbe” extension (a “.vbe” is an encoded VBscript file), which would otherwise have a “.vbs” extension. The encoding support is provided to prevent people from reading the script.
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| Figure 2: Obfuscated VBScript |
To be persistent, the malware copies itself into the startup folder in Windows (Figure 3)
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| Figure 3: Copy of Malware in the startup folder |
Registry entry created by the malware to run itself at the system startup.(Figure 4)
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| Figure 4: Windows Registry (run entry) |
It also adds a copy of itself to the Windows temporary folder (Figure 5).
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| Figure 5: Copy of Malware in temp folder |
Next, the malware attempts to establish a connection to it's server (here wscript.exe is the Script Engine which executes VBScript).
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Figure 6: Network communication
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To extract more information we need to decode the file and obtain the original malware code in a readable form. Let's have a look at the decoded file.
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| Figure 7: Malware install code |
The image above shows the code that is responsible for adding entry in the registry which
allows the malware to execute every time the system starts , Also create it's own copy in the startup, temporary folder.
Another interesting part of this malware is it's ability to communicate over the network. The malware can actually receive a set of commands and execute them in an infected machine
At the time of analysis, the server to which the malware communicates seems to be down. Therefore, in order to fully understand how the malware communicates and also to demonstrate how effective and damaging this Bot is, I have decided to create an HTTP server and issue commands to the bot directly.
We can see from the code below, the wide range of commands that can be executed by this Bot. The commands are simple and self explanatory.
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| Figure 8: Remote Commands |
Let's see the effects of a few of these commands in detail. The “execute” command is capable of executing additional VBScript statements in the infected machine. The “update” command is issued to update the Bot , while “uninstall” removes the Bot entry from the Windows registry and startup folder.
There are also commands such as “send”, ”recv” and “site-send”. Interesting commands include “enum-driver”, “enum-faf”, “enum-process”, “cmd-shell”, “delete”, “exitprocess”.
Let us execute our server and wait for the Bot to connect to us and send information so we can then issue commands.
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| Figure 9: Malware callback |
As seen in the image above, a “POST” request is made with it's path as “/is-ready”, indicating
that the Bot is up and ready. We can also observe information about the infected machine such as “volumeserialnumber”, “computername”, “username”,“operating system type”, installed
“anti-virus name” etc. To retrieve such information, the malware relies on Windows Management Instrumentation (WMI) queries.
Let us issue the command “enum-driver”. This command fetches the drive name and drive type of the infected machine as seen in the image below.
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| Figure 10: enum-drive |
The next command “enum-faf” enumerates and fetches the content of an input directory or drive of the infected machine.
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| Figure 11: enum files |
The “enum-process” command fetches a list of processes that are running on the infected system.
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| Figure 12: enum-process |
The “cmd-shell” command will allow the attacker to execute all DOS commands on the infected
system.
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| Figure 13: Execute Dos Command |
Those are just a few of the powerful commands that the malware can execute in the infected system. This gives the malware near limitless power to control and steal data from the infected machine. The Bot and all it's communication are blocked by Zscaler.
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