Malware Devil

Friday, December 4, 2020

The chronicles of Emotet

More than six years have passed since the banking Trojan Emotet was first detected. During this time it has repeatedly mutated, changed direction, acquired partners, picked up modules, and generally been the cause of high-profile incidents and multimillion-dollar losses. The malware is still in fine fettle, and remains one of the most potent cybersecurity threats out there. The Trojan is distributed through spam, which it sends itself, and can spread over local networks and download other malware.

All its “accomplishments” have been described thoroughly in various publications and reports from companies and independent researchers. This being the case, we decided to summarize and collect in one place everything that is currently known about Emotet.

2014

June

Emotet was first discovered in late June 2014 by TrendMicro. The malware hijacked user banking credentials using the man-in-the-browser technique. Even in those early days, the malware was multicomponent: browser traffic was intercepted by a separate module downloaded from the C&C server. Its configuration file with web injections was also loaded from there. The banker’s main targets were clients of German and Austrian banks, and its main distribution vector was spam disguised as bank emails with malicious attachments or links to a ZIP archive containing an executable file.

Examples of malicious emails with link and attachment

November

In the fall of 2014, we discovered a modification of Emotet with the following components:

Module for modifying HTTP(S) traffic
Module for collecting email addresses in Outlook
Module for stealing accounts in Mail PassView (a password recovery tool)
Spam module (downloaded additionally as an independent executable file from addresses not linked to C&C)
Module for organizing DDoS attacks

We came across the latter bundled with other malware, and assume that it was added to Emotet with a cryptor (presumably back then Emotet’s authors did not have their own and so used a third-party one, possibly hacked or stolen). It is entirely possible that the developers were unaware of its presence in their malware. In any event, this module’s C&C centers were not responsive, and it itself was no longer updated (compilation date: October 19, 2014).

In addition, the new modification had begun to employ techniques to steal funds from victims’ bank accounts automatically, using the so-called Automatic Transfer System (ATS). You can read more about this modification in our report.

December

The C&C servers stopped responding and the Trojan’s activity dropped off significantly.

2015

January

In early 2015, a new Emotet modification was released, not all that different from the previous one. Among the changes were: new built-in public RSA key, most strings encrypted, ATS scripts for web injection cleared of comments, targets included clients of Swiss banks.

June

The C&C servers again became unavailable, this time for 18 months. Judging by the configuration file with web injects, the Trojan’s most recent victims were clients of Austrian, German and Polish banks.

2016

December

Emotet redux: for the first time in a long while, a new modification was discovered. This version infected web-surfing victims using the RIG-E and RIG-V exploit kits. This distribution method was not previously used by the Trojan, and, fast-forwarding ahead, would not be employed again. We believe that this was a trial attempt at a new distribution mechanism, which did not pass muster with Emotet’s authors.

The C&C communication protocol in this modification was also changed: for amounts of data less than 4 KB, a GET request was used, and the data itself was transmitted in the Cookie field of the HTTP header. For larger amounts, a POST request was used. The RC4 encryption algorithm had been replaced by AES, with the protocol itself based on a slightly modified Google Protocol Buffer. In response to the request, the C&C servers returned a header with a 404 Not Found error, which did not prevent them from transmitting the encrypted payload in the body of the reply.

Examples of GET and POST requests used by Emotet

The set of modules sent to the Trojan from C&C was different too:

Out was the module for intercepting and modifying HTTP(S) traffic
In was a module for harvesting accounts and passwords from browsers (WebBrowserPassView)

2017

February

Up until now, we had no confirmation that Emotet could send spam independently. A couple of months after the C&C servers kicked back into life, we got proof when a spam module was downloaded from there.

April

In early April, a large amount of spam was seen targeting users in Poland. Emails sent in the name of logistics company DHL asked recipients to download and open a “report” file in JavaScript format. Interestingly, the attackers did not try the further trick of hiding the executable JavaScript as a PDF. The calculation seemed to be that many users would simply not know that JavaScript is not at all a document or report file format.

Example of JS file names used:
dhl__numer__zlecenia___4787769589_____kwi___12___2017.js (MD5:7360d52b67d9fbb41458b3bd21c7f4de)

In April, a similar attack involving fake invoices targeted British-German users.

invoice__924__apr___24___2017___lang___gb___gb924.js (MD5:e91c6653ca434c55d6ebf313a20f12b1)
telekom_2017_04rechnung_60030039794.js (MD5:bcecf036e318d7d844448e4359928b56)

Then in late April, the tactics changed slightly when the spam emails were supplemented with a PDF attachment which, when opened, informed the user that the report in JavaScript format was available for download via the given link.

Document_11861097_NI_NSO___11861097.pdf (MD5: 2735A006F816F4582DACAA4090538F40)

Example of PDF document contents

Document_43571963_NI_NSO___43571963.pdf (MD5: 42d6d07c757cf42c0b180831ef5989cb)

Example of PDF document contents

As for the JavaScript file itself, it was a typical Trojan-Downloader that downloaded and ran Emotet. Having successfully infected the system, the script showed the user a pretty error window.

Error message displayed by the malicious JavaScript file

May

In May, the scheme for distributing Emotet via spam changed slightly. This time, the attachment contained an Office document (or link to it) with an image disguised as an MS Word message saying something about the version of the document being outdated. To open the document, the user was prompted to enable macros. If the victim did so, a malicious macro was executed that launched a PowerShell script that downloaded and ran Emotet.

Screenshot of the opened malicious document ab-58829278.dokument.doc (MD5: 21542133A586782E7C2FA4286D98FD73)

Also in May, it was reported that Emotet was downloading and installing the banking Trojan Qbot (or QakBot). However, we cannot confirm this information: among the more than 1.2 million users attacked by Emotet, Qbot was detected in only a few dozen cases.

June

Starting June 1, a tool for spreading malicious code over a local network (Network Spreader), which would later become one of the malware modules, began being distributed from Emotet C&C servers. The malicious app comprised a self-extracting RAR archive containing the files bypass.exe (MD5: 341ce9aaf77030db9a1a5cc8d0382ee1) and service.exe (MD5: ffb1f5c3455b471e870328fd399ae6b8).

Self-extracting RAR archive with bypass.exe and service.exe

bypass.exe:

Searches network resources by brute-forcing passwords using a built-in dictionary
Copies service.exe to a suitable resource
Creates a service on the remote system to autorun service.exe

Screenshot of the function for creating the service (bypass.exe)

Screenshot with a list of brute-force passwords (bypass.exe)

In terms of functionality, service.exe is extremely limited and only sends the name of the computer to the cybercriminals’ server.

Function for generating data to be sent to C&C

Function for sending data to C&C

The mailing was obviously a test version, and the very next day we detected an updated version of the file. The self-extracting archive had been furnished with a script for autorunning bypass.exe (MD5: 5d75bbc6109dddba0c3989d25e41851f), which had not undergone changes, while service.exe (MD5: acc9ba224136fc129a3622d2143f10fb) had grown in size by several dozen times.

Self-extracting RAR archive with bypass.exe and service.exe

The updated service.exe was larger because its body now contained a copy of Emotet. A function was added to save Emotet to disk and run it before sending data about the infected machine to C&C.

New functions in service.exe for saving Emotet to disk and running it

July

An update to the Emotet load module was distributed over the botnet. One notable change: Emotet had dropped GET requests with data transfer in the Cookie field of the HTTP header. Henceforth, all C&C communication used POST (MD5: 643e1f4c5cbaeebc003faee56152f9cb).

August

Network Spreader is included in the Emotet “distribution kit” as a DLL (MD5: 9c5c9c4f019c330aadcefbb781caac41), the compilation date of the new module is July 24, 2017, but it was obtained only in August. Recall that it used to be a self-extracting RAR archive with two files: bypass.exe and service.exe. The distribution mechanism did not change much, but the list of brute-force passwords was expanded significantly to exactly 1,000.

Screenshot of the decrypted password list