Operation Spalax: Targeted malware attacks in Colombia

In 2020 ESET saw several attacks targeting Colombian entities exclusively. These attacks are still ongoing at the time of writing and are focused on both government institutions and private companies. For the latter, the most targeted sectors are energy and metallurgical. The attackers rely on the use of remote access trojans, most likely to spy on their victims. They have a large network infrastructure for command and control: ESET observed at least 24 different IP addresses in use in the second half of 2020. These are probably compromised devices that act as proxies for their C&C servers. This, combined with the use of dynamic DNS services, means that their infrastructure never stays still. We have seen at least 70 domain names active in this timeframe and they register new ones on a regular basis.

The attackers

The attacks we saw in 2020 share some TTPs with previous reports about groups targeting Colombia, but also differ in many ways, thus making attribution difficult.

One of those reports was published in February 2019, by QiAnXin researchers. The operations described in that blogpost are connected to an APT group active since at least April 2018. We have found some similarities between those attacks and the ones that we describe in this article:

• We saw a malicious sample included in IoCs of QiAnXin's report and a sample from the new campaign in the same government organization. These files have fewer than a dozen sightings each.
• Some of the phishing emails from the current campaign were sent from IP addresses corresponding to a range that belongs to Powerhouse Management, a VPN service. The same IP address range was used for emails sent in the earlier campaign.
• The phishing emails have similar topics and pretend to come from some of the same entities - for example, the Office of the Attorney General (Fiscalia General de la Nacion) or the National Directorate of Taxes and Customs (DIAN).
• Some of the C&C servers in Operation Spalax use linkpc.net and publicvm.com subdomains, along with IP addresses that belong to Powerhouse Management. This also happened in the earlier campaign.

However, there are differences in the attachments used for phishing emails, the remote access trojans (RATs) used and in most of the operator’s C&C infrastructure.

There is also this report from Trend Micro, from July 2019. There are similarities between the phishing emails and parts of the network infrastructure in that campaign and the one we describe here. The attacks described in that article were connected to cybercrime, not espionage. While we have not seen any payload delivered by the attackers other than RATs, some of the targets in the current campaign (such as a lottery agency) don’t make much sense for spying activities.

These threat actors show perfect usage of the Spanish language in the emails they send, they only target Colombian entities, and they use premade malware and don’t develop any themselves.

Attack overview

Targets are approached with emails that lead to the download of malicious files. In most cases, these emails have a PDF document attached, which contains a link that the user must click to download the malware. The downloaded files are regular RAR archives that have an executable file inside. These archives are hosted in legitimate file hosting services such as OneDrive or MediaFire. The target has to manually extract the file and execute it for the malware to run.

We’ve found a variety of packers used for these executables, but their purpose is always to have a remote access trojan running on the victimized computer, usually by decrypting the payload and injecting it into legitimate processes. An overview of a typical attack is shown in Figure 1. We have seen the attackers use three different RATs: Remcos, njRAT and AsyncRAT.

Figure 1. Overview of the attack

Phishing emails

The attackers use various topics for their emails, but in most cases they are not specially crafted for their victims. On the contrary, most of these emails have generic topics that could be reused for different targets.

We found phishing emails with these topics:

• A notification about a driving infraction
• A notification to take a mandatory COVID-19 test
• A notification to attend a court hearing
• An open investigation against the recipient for misuse of public funds
• A notification of an embargo of bank accounts

The email shown in Figure 2 pretends to be a notification about a driving infraction for a value of around US$250. There is a PDF file attached that promises a photo of the infraction, as well as information about time and place of the incident. The sender has been spoofed to make the email look like it is coming from SIMIT (a system for paying transit violations in Colombia).

Figure 2. Example of a phishing email

The pdf file only contains an external link that has been shortened with the acortaurl service, as shown in Figure 3. The shortened URL is: https://acortaurl[.]com/httpsbogotagovcohttpsbogotagovcohttpsbogotagovco.

After the shortened link is expanded, a RAR archive is downloaded from: http://www.mediafire[.]com/file/wbqg7dt604uwgza/SIMITcomparendoenlineasimitnumeroreferenciaComparendo2475569.uue/file.

Figure 3. PDF attached to phishing email

Figure 4 shows part of the email’s header. The spoofed sender is notificacionesmultas@simit.org[.]co but we can see that the real sender is IP address 128.90.108[.]177, which is connected with the domain name julian.linkpc[.]net, as found in historic DNS data. It’s not a coincidence that the same domain name is used for contacting the C&C server in the malicious sample contained in the RAR archive. This IP address belongs to Powerhouse Management, a VPN service provider.

Figure 4. Header of a phishing email

In more recent emails, the shortened link in the PDF file resolves to https://bogota.gov[.]co (a legitimate site) when visited from outside of Colombia.

Also, in some cases the GetResponse service has been used to send the email. This is probably done to track whether the victim has clicked on the link. In these cases there is no attachment: a link to the GetResponse platform leads to the download of malware.

You can see the other emails in the following gallery (click to enlarge):

Figures 5 to 13. Various phishing emails and their attached files

Malicious artifacts

Droppers

The executable files contained in compressed archives that are downloaded via the phishing emails are responsible for decrypting and running remote access trojans on a victimized computer. In the following sections, we describe the various droppers we have seen.

NSIS installers

The dropper that is most commonly used by these attackers comes as a file that was compiled with NSIS (Nullsoft Scriptable Install System). To try to evade detection, this installer contains several benign files that are written to disk (they are not part of NSIS binaries and they are not used at all by the installer) and two files that are malicious: an encrypted RAT executable and a DLL file that decrypts and runs the trojan. An NSIS script for one of these installers is shown in Figure 14. The benign files are usually different in different droppers used by the attackers.

Figure 14. NSIS script for one of the droppers; the malicious files are highlighted

The files Bonehead (encrypted RAT) and ShoonCataclysm.dll (dropper DLL) are written in the same folder and the DLL is run with rundll32.exe using Uboats as its argument. The names of these files change between executables. Some more examples are:

• rundll32.exe Blackface,Breathing
• rundll32.exe OximeLied,Hostage
• rundll32.exe Conservatory,Piggins

We used the name of the benign files contained in some of these NSIS installers to find more malicious installers used by the Spalax operators. Table 1 lists details of three different NSIS installers used by the attackers that contained all the same benign files. The only difference among them was the encrypted file, which pointed to different C&C servers.

Table 1. NSIS installers with identical benign files used by this group

However, we also found malicious NSIS installers used by other unrelated groups that had the same benign files as the ones used by this group. Figure 15 lists the files contained in two different NSIS installers. The one on the left (SHA-1: 3AC39B5944019244E7E33999A2816304558FB1E8) is an executable used by this group and the one on the right (SHA-1: 6758741212F7AA2B77C42B2A2DE377D97154F860) is unrelated. The SHA-1 hashes for all the benign files are the same (and also the filenames) and even the malicious DLL is the same. However, the encrypted file Bonehead is different.

Figure 15. Files contained in NSIS droppers from unrelated campaigns

This means that these installers were generated with the same builder, but by different actors. The builder is probably offered in underground forums and includes these benign files. This, along with a complete analysis of the dropper, was described earlier this year by Sophos in their RATicate article. There is also an article by Lab52 describing one of the NSIS installers used in Operation Spalax, which they attribute to APT-C-36.

In the vast majority of cases these NSIS droppers decrypt and run the Remcos RAT, but we have also seen cases where the payload is njRAT. These will be described later in the Payloads section.

Agent Tesla packers

We have seen several droppers that are different variants of a packer that uses steganography and is known to be used in Agent Tesla samples. Interestingly, the attackers use various payloads, but none of them are Agent Tesla. Even though there are differences in all the samples regarding the layers of encryption, obfuscation or anti-analysis used, we can summarize the actions taken by the droppers as follows:

• The dropper reads a string (or binary data) from its resource section and decrypts it. The result is a DLL that will be loaded and called in the same address space.
• The DLL reads pixels from an image contained in the first binary and decrypts another executable. This one is loaded and executed in the same address space.
• This new executable is packed with CyaX. It reads data from its own resource section and decrypts a payload. There are anti-analysis checks; if they pass, the payload can be injected into a new process or loaded in the same process space.

The initial dropper is coded in C#. In all the samples that we have seen, the code for the dropper was hiding in non-malicious code, probably copied from other apps. The benign code is not executed; it’s there to evade detection.

In Figure 16 we see an example of the resources contained in one of these droppers. The text in green (only shown partially) is a string that will be decrypted to generate the next stage to be executed and the image that we see below the green text will be decrypted by the second stage malware. The algorithm used for decryption of the string varies from sample to sample, but sometimes the resource is just an unencrypted binary.

Figure 16. Resources contained in Agent Tesla's packer

The method to be executed in the DLL is always named StartGame or StartUpdate. It reads the image from the first executable, and stores every pixel as three numbers according to its red, green and blue components. Then it decrypts the array by doing a single-byte XOR operation, cycling through the key. After that, the array is gzip-decompressed and executed. Part of the code for the mentioned operations is shown in Figure 17.

Figure 17. Code to decrypt and run the third-stage malware

The third stage is in charge of decrypting and running the payload. The .NET packer known as CyaX is used to perform this task. The version of the packer used by the attackers is v4, although they used v2 in some cases. Figure 18 shows the hardcoded configuration for one of their samples.

Figure 18. Hardcoded configuration in CyaX-Sharp packer

The decryption of the payload is based on XOR operations and is the same as the algorithm previously shown but with an extra step: the payload is XORed with its first 16 bytes as a key. Once it’s decrypted, it can be run in the same address space or injected into a different process, depending on the configuration.

This packer supports various anti-analysis operations such as disabling Windows Defender, checking for security products, and detecting virtual environments and sandboxes.

The majority of the payloads for these droppers are njRAT, but we have also seen AsyncRAT. We saw Remcos in one of these droppers, but the code in the packer was different. Part of the main routine for the injection of the payload is shown in Figure 19.

Figure 19. Code for the last stage of a dropper

We have noticed that the configuration is contained in different variables. Values like #startup_method# or #bind# mean that the configuration was not set for those options. The payload is read from an encrypted resource and XORed with a hardcoded password. The shellcode that performs the injection is contained in an array and is dynamically loaded. There are no anti-analysis checks or protection mechanisms.

AutoIt droppers

For some of their droppers, the attackers have used an AutoIt packer that comes heavily obfuscated. Unlike the cases that were previously described, in this case the first-stage malware performs the injection and execution of the payload. It does so by using two shellcodes contained in the compiled AutoIt script: one to decrypt the payload and another to inject it into some process.

The payload is constructed by concatenating several strings, as shown in Figure 20. By inspecting the last two characters, we can see that the string is in reverse order.

Figure 20. Concatenation of the payload

The routine that decrypts the payload contains a small shellcode that is loaded with VirtualAlloc and executed. The decryption done by the shellcode is based on a single-byte XOR algorithm. The code that loads the shellcode is shown in Figure 21.

Figure 21. Execution of shellcode to decrypt the payload

We can see that the shellcode is stored encrypted. In fact, before deobfuscating the script, all strings were encrypted with this same XOR-based algorithm. The decryption routine used is shown in Figure 22.

Figure 22. Routine to decrypt strings

Once the payload is decrypted, a shellcode with RunPE code is used to perform the injection. The shellcode is concatenated in the same manner as the payload and executed like the previous shellcode.

To achieve persistence, a VBS script is created to execute a copy of the dropper (which is renamed to aadauthhelper.exe). Then an Internet Shortcut (.url) file is created in the Startup folder to execute the script. The code that generates these files is shown in Figure 23.

Figure 23. Code for persistence in AutoIt droppers

The dropper contains code that is not executed. It can:

• Check for VMware and VirtualBox
• Delete the dropper executable
• Run the dropper continuously
• Download and execute files
• Terminate if a “Program Manager” window is found
• Read a binary from its resource section, write it to disk and execute it
• Modify the security descriptor (ACL) for the injected process

For more information see this analysis by Morphisec where similar AutoIt droppers were used with Frenchy shellcode.

Payloads

The payloads used in Operation Spalax are remote access trojans. These provide several capabilities not only for remote control, but also for spying on targets: keylogging, screen capture, clipboard hijacking, exfiltration of files, and the ability to download and execute other malware, to name a few.

These RATs were not developed by the attackers. They are:

• Remcos, sold online
• njRAT, leaked in underground forums
• AsyncRAT, open source

There is not a one-to-one relationship between droppers and payloads, as we have seen different types of droppers running the same payload and also a single type of dropper connected to different payloads. However, we can state that NSIS droppers mostly drop Remcos, while Agent Tesla and AutoIt packers typically drop njRAT.

Remcos is a tool for remote control and surveillance. It can be purchased with a six-month license that includes updates and support. There is also a free version with limited functionalities. While the tool can be used for legitimate purposes, it is also used by criminals to spy on their victims.

Most of the Remcos samples used by this group are v2.5.0 Pro, but we have also seen all versions that were released since September 2019, which may indicate that the attackers bought a license after that month and have been actively using the different updates that they received during their six month license period.

Regarding njRAT, this group mostly uses v0.7.3 (also known as the Lime version). That version includes functionalities such as DDoS or ransomware encryption, but only spy features such as keylogging are used by the attackers. For a more complete description of this version, refer to this 2018 article by Zscaler.

Another njRAT version used by the attackers is v0.7d (the “green edition”) which is a simpler version focused on spying capabilities: keylogging, taking screenshots, access to webcam and microphone, uploading and downloading files, and executing other binaries.

The final type of payload that we will mention is AsyncRAT. In all cases we have observed v0.5.7B, which can be found on GitHub, has been used. The functionalities in this RAT are similar to those in the previously mentioned RATs, which allow attackers to spy on their victims.

Network infrastructure

During our research we saw approximately 70 different domain names used for C&C in the second half of 2020. This amounts to at least 24 IP addresses. By pivoting on passive DNS data for IP addresses and known domain names, we found that the attackers have used at least 160 additional domain names since 2019. This corresponds to at least 40 further IP addresses.

They’ve managed to operate at such scale by using Dynamic DNS services. This means that they have a pool of domain names (and also register new ones on a regular basis) that are dynamically assigned to IP addresses. This way a domain name can be related to several IP addresses over a period of time and IP addresses can be related to many domain names. Most of the domain names we have seen were registered with Duck DNS, but they have also used DNS Exit for publicvm.com and linkpc.net subdomains.

Regarding IP addresses, almost all of them are in Colombia. Most are IP addresses related to Colombian ISPs: 60% of them are Telmex and 30% EPM Telecomunicaciones (Tigo). As it is highly unlikely that the criminals own so many residential IP addresses, it is possible that they use some victims as proxies, or some vulnerable devices to forward communication to their real C&C servers.

Finally, a subset of the IP addresses belongs to Powerhouse Management, a VPN service provider. They are used in conjunction with DNS Exit subdomains. Similar findings can be found in this analysis by Lab52.

Conclusion

Targeted malware attacks against Colombian entities have been scaled up since the campaigns that were described last year. The landscape has changed from a campaign that had a handful of C&C servers and domain names to a campaign with very large and fast-changing infrastructure with hundreds of domain names used since 2019. Even though TTPs have seen changes, not only in how malware is delivered in phishing emails but also in the RATs used, one aspect that remains the same is that the attacks are still targeted and focused on Colombian entities, both in the public and private sectors. It should be expected that these attacks will continue in the region for a long time, so we will keep monitoring these activities.

A comprehensive list of Indicators of Compromise (IoCs) and samples can be found in our GitHub repository.

For any inquiries, or to make sample submissions related to the subject, contact us at threatintel@eset.com.

MITRE ATT&CK techniques

Note: This table was built using version 7 of the MITRE ATT&CK framework.