Cobalt Strike Malware (W32/Cobalt) - What is It and How to Remove It
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Cobalt Strike Malware (W32/Cobalt) – What is It and How to Remove It

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This article aims to explain what is Cobalt Strike malware, how does it infect computers and how to remove it effectively from your computer.

New form of malware has been reported to cause infections via a vulnerability for Windows Machines that is 17 years old. The virus, related to the pen-testing (penetration testing) tool Cobalt Strike is also dubbed Cobalt Strike malware and it’s primary goal after infecting your computer is to turn it into a slave of the hackers who are behind the infection. In the event that you have seen W32/Cobalt or other similar detection on your computer, we recommend that you read this article and learn how to fully remove the Cobalt Strike malware from your PC and protect it against future infections and intrusions.

Threat Summary

NameCobalt Strike Malware
TypeJavaScript Malware Infection
Short DescriptionAims to make your computer remotely controlled via shell commands by the hackers behind the virus.
SymptomsNo symptoms so far as the malware is file-less, but your computer may behave strange and can contain other malware in it.
Distribution MethodVia malicious URLs or compromised websites.
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Cobalt Strike Malware – Infection Analysis

In order to cause a successful infection, the malicious file of this virus is actually a semi-legitimate Microsoft Word document, which pretends to be a legitimate file sent by a bank or a big company. The file itself infects via malicious macros which contain the exploit CVE-2017-11882 that is over 17 years old at the time of writing this. So far, what is known about this vulnerability is that it was only patched once, two weeks after it was discovered by Microsoft.

The way the cyber-criminals who are believed to be the Cobalt hacker group have decided to use malicious macro infections for their attacks in e-mails that they pretend are from Visa. One sample was detected by malware researchers Jasper Manual and Joie Salvio at Fortinet and it uses the following e-mail:

The message (In Russian) pretends to come from Visa’s payWave system and it aims to get the victim to open what appears to be a .zip and .doc files that pose as “New security system changes in Visa payWave”. Once this document is opened, the victim sees a legitimate Microsoft Word file that only says the words “Enable Editing”:

In reality however, the file triggers a script for PowerShell which initiates the download of the infection file of Cobalt Strike, while obfuscating it with the CVE-2017-11882 vulnerability. The whole process of initiating the download is done by taking control of the Microsoft HTML Application Host also known as the process mshta.exe, that has a purpose to execute HTML apps. The result of this is that the mshta.exe service connects to the IP address 104.254.99.77 and downloads the JavaScript infection payload of Cobalt Strike on the victim’s computer. But what is downloaded is not the final JavaScript payload, but instead this JavaScript triggers another download from the host 104.254.99.77 which downloads a file with a random name and the .ps1 suffix on the victim’s computer. The file is located in the %AppData% directory and may appear like the following:

→ %AppData%\28dh32d9233.ps1

When this .ps1 file is executed, the actual Cobalt Strike infection takes place on the victim’s computer.

Cobalt Strike Malware – Malicious Activity

Once downloaded, the .ps1 file, belonging to Cobalt Strike is automatically activated. It triggers yet another PowerShell script which has the client DLL files of Cobalt Strike – a rather outdated, but still very effective tool, previously used for penetration testing of the defenses in Windows Operating Systems. Once it detects whether your Windows is running based on 32-bit or 63-bit architecture, the appropriate Cobalt Strike DLL is executed directly in your Windows PowerShell memory and this results in the malware not being actually present on your drive, while remaining embedded in PowerShell and even if you delete the .ps1 file, the virus is still active on your PC. This also helps any antivirus programs that are conventional to detect the virus.

Once the DLL of Cobalt Strike is triggered, the cyber-criminals have obtained complete control of your computer system – it belongs to them. They can perform all of the activities which can be performed using the Cobalt Strike shell commands themselves. Here is only a small part of commands that may be triggered, using Cobalt malware:

spawnto – spawns sessions into processes.
inject – injects various malicious scripts in legitimate Windows processes.
dllinject – same as inject but for DLLs
download – can download other malware or files on your computer.
upload – can upload files to the hacker’s server, i.e. it can steal your files.
timestomp – to assist with blending files, obfuscation and updating itself.
ls – similar to Linux, it helps you to list different files.
mkdir – to create folders on your computer.
keylogger pid – to log your keystrokes.
screenshot pid – to take a screenshot of your desktop.
jobkill – to kill a job.
socks 8080 – to set a Proxy server on a selected port (in this case 8080).

These are only the main commands that are likely to be used on your computer and you have no way of knowing that they are triggered as it all happens in the background of your computer. The full list of commands can be seen on Cobalt Strike’s web page.

How to Remove Cobalt Strike Malware from Your PC

In order to remove this malware, you will need to have some experience in Working with Windows PowerShell and definitely some malware removal experience. So if manual removal is not your type of thing to do, security experts strongly recommend to use an advanced malware removal tool instead of conventional antivirus program. Such software will not only automatically scan for and remove Cobalt Strike malware from your computer, but will also make sure that your computer stays protected against other JavaScript and PowerShell infections in the future.

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