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Backdoor Wipes MBR, Locks Screen


German users are at risk of having their systems rendered unusable by a malware that we’re seeing being sent via spam messages. This particular malware, on top of its ability to remotely control an affected system, is able to wipe out the Master Boot Record – a routine that had previously caused a great crisis in South Korea.

We recently uncovered this noteworthy backdoor as an attached file in certain spam variants. The spam sample we found is in German and forces recipients to pay for a certain debt, the details of which are contained in the attachment. Those who open the attachment are actually tricked into executing the malware, in this instance, a backdoor.


Figure 1. Email attachment detected as BKDR_MATSNU.EJ

Like any backdoor, BKDR_MATSNU.MCB performs certain malicious commands, which include gathering machine-related information and send it to its command-and-control (C&C) server. However, the backdoor’s most noteworthy feature is its capability to wipe the Master Boot Record (MBR). The wiping of the MBR was recently used in the high-profile (but different) attack against certain South Korean institutions. What makes this routine problematic is that once done, infected systems won’t reboot normally and will leave users with unusable machines.

Another command is the backdoor’s capability to lock and unlock a screen. This locking of screen is definitely a direct copy from ransomware’s playbook, in which the system remains completely or partially inaccessible unless the victim pays for the “ransom”. Ransomware is a malware that locks an infected system’s screen and display a message, which instructs users to pay for a “ransom” thru certain payment methods.

Ransomware was initially found and limited in Russia. But by 2012, ransomware had another lease in life via Police Trojan/REVETON variants. They perform the locking of screen like any ransomware. The only difference, however, is that it shows a message purportedly from the victim’s local law enforcement agency to scare users into paying the ransom.

During our testing, BKDR_MATSNU.MCB readily performed the MBR wiping routine. The remote malicious (via server) only needs to communicate this command to the backdoor and it can execute this routine immediately. However, this is not the case with the screen locking. BKDR_MATSNU.MCB is likely to download a different module onto the system, which will then lock the screen. As to what routines will be first executed or not is dependent on the remote malicious user. Attackers may opt to lock the screen first then initiate the MBR overwriting or just initiate any of the two.

Another possible scenario is that another version of BKDR_MATSNU is integrated with the screen blocking routine, which will make the screen locking command easier to execute. Currently, we are on the lookout for this version and we will update our readers should we find new developments.

For better protection, users should always be cautious be the email they receive and must not readily open any attachments. If your system is already infected, it is a safer bet to not pay for the “ransom”, as paying does not guarantee anything. For those with system already infected with this backdoor, you may refer to BKDR_MATSNU.MCB Threat Encyclopedia page for manual cleanup procedure.

To know more about ransomware, you can check out our Threat Encyclopedia page about ransomware and how it has evolved.

With additional insights from Threat engineer Anthony Melgarejo.

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