Europol called Emotet “the world’s most dangerous malware” – and it is back in the security news this week – but what is Emotet, and how can you protect your network from Emotet based attacks?

Emotet is the name given to a strain of malware and the cyber-crime organisation that created it.  First detected in 2014, Emotet was originally a Banking Trojan, focused on stealing login credentials for online banking systems so the criminals could drain the funds from the victims accounts.  Today, Emotet malware has been developed into a delivery mechanism for other malware – with a huge botnet of infected computers around the world which are then hired out in a ‘malware-as-a-service’ model to other cyber criminals who use it to deliver their own attacks such as ransomware.

How does Emotet work?

Emotet uses email to deliver malicious documents – usually Microsoft Office files containing hidden code – which have been weaponised to download an initial payload if the victim can be tricked into opening the file.  The payload has typically been malware such as TrickBot or QBot which steal credentials and attempt to spread through the victims network or Ryuk ransomware.  More recently Emotet has been spotted dropping Cobalt Strike beacons directly onto victim’s computers.

When the malicious email arrives, the victim is persuaded to open the attachment by the email text – perhaps suggesting it is an overdue invoice or a ‘protected document’ sent through a fictitious secure file sharing service.  Once opened, the malicious document contains images and text that tries to convince the victim to ‘Enable Editing’ and ‘Enable Content’ which will bypass the security protections built into Microsoft Office and allow the malicious code embedded in the document to run.

The malicious code will then typically run a PowerShell script which downloads the Emotet DLL from a compromised website that is being used for file storage. The DLL is then loaded and run on the victim’s computer where it waits for instructions from Command & Control servers controlled by the cyber criminals.  Emotet is able to obey commands such as searching for credentials or emails to steal, spread laterally through the network or download and install other malware components.

The death and resurrection of Emotet

In January 2021, a co-ordinated global action by law enforcement shut down the Emotet network in an operation co-ordinated by Europol.  The network of hundreds of servers which controlled Emotet where infiltrated and taken over by the law enforcement team and the infected victim machines that made up the botnet were redirected to the benign law enforcement controlled infrastructure. A database was published by the Dutch Police of credentials that had been captured by Emotet and recovered from the infrastructure after the takedown. The database can be checked here.

For ten months the Emotet network was silent but in mid November 2021 another malware network, TrickBot, started dropping the Emotet loader onto already infected devices indicating an attempt to rebuild the Emotet botnet.

Emotet Today

The list of Command and Control servers tracked by abuse.ch involved in the rebuilding Emotet network is growing daily and the tactics and tools used by Emotet are evolving.  Recent reports indicate that Emotet has been observed dropping Cobalt Strike beacons directly onto infected machines as the initial payload.

Cobalt Strike is a penetration testing tool that was originally designed to simulate network attacks and detect network vulnerabilities while probing network defences. However, the power and flexibility of Cobalt Strike were adopted by cyber criminals using it to run malicious command and control servers and to deploy ransomware onto victims networks.  The 2021 ransomware attack against the Irish Health Service was achieved through the use of Cobalt Strike modules.

How to defend against Emotet

Emotet is a sophisticated and developing adversary and so Security Managers need to keep abreast of the changing threat landscape to ensure their mitigations are updated accordingly.

Because Emotet includes ‘worm like’ abilities to spread itself through a network after an initial infection, it is especially important to ensure strong perimeter security limits the opportunity for Emotet to arrive in anyone’s inbox.

Key steps Security Managers can take to harden their network against Emotet and similar threats include:

• Ongoing Security Awareness Training is vital to reduce the likelihood of your team opening a malicious email and being tricked into enabling malicious content
• Block email attachments that include executable code (e.g. dll, exe, javascript or powershell)
• Block email attachments that have been obfuscated to prevent scanning (eg zip files)
• Scan for and remove suspicious email attachments; ensure the scanned attachment is its “true file type” (i.e., the extension matches the file header).
• Block attachments with double extensions (eg invoice.xls.js)
• Block known malicious IP addresses that act a C2 servers at the firewall (subscribe to an information service to keep this list updated daily)
• Segment your network and limit lateral communication to thwart horizontal travel of malware if it does gain access to a device.
• Deploy Endpoint Protect / Anti-malware software and ensure it is always up to date
• Formalise patch management and patch promptly to close the vulnerabilities that malware exploits
• Network penetration testing will identify weaknesses in your network design that could be exploited by malware and malicious actors