Outdoor Malware Andariel Targets Korean Companies

AhnLab SEcurity intelligence Center (ASEC) recently discovered a case where an unidentified threat actor exploited a Korean ERP solution to carry out an attack. After infiltrating the system, the threat actor is believed to have attacked the update server of a specific Korean ERP solution to take control of systems within the company. In another attack case, a vulnerable web server was attacked to distribute malware. The targets of these attacks have been identified as the Korean defense and manufacturing industries.

Among the identified malware, there is a form where a malicious routine is inserted into the update program of an existing ERP solution. This method is similar to a case in 2017 when the Andariel group used it to install the HotCroissant backdoor. The creator used the string “Xct” during the development process of the malware, and the backdoor ultimately used here is classified as Xctdoor.

1. Past Attack Cases of Andariel

Rifdoor is a backdoor used by Andariel, a subgroup known to be part of the Lazarus group. It was first discovered in November 2015 and its activity was confirmed until early 2016. [1] (This report supports Korean only for now.) Starting in 2017, a variant of Rifdoor was used in attacks, which was identified as identical to Lazarus group’s HotCroissant, a backdoor disclosed by the US CISA [2] and VMware’s Carbon Black [3] in 2020. Carbon Black detailed the similarities and differences between Rifdoor and HotCroissant, and the Rifdoor variant will be classified as HotCroissant here.

Among the attack cases using HotCroissant, there was an incident in 2017 where a Korean ERP solution was exploited to distribute malware. The threat actor inserted a malicious routine into the update program “ClientUpdater.exe”. It is presumed that the threat actor exploited this method to attack the ERP’s update server after breaching a specific organization, with the purpose of propagating internally.

The routine inserted into the update program is responsible for downloading and executing additional payloads from an external source, as shown below. The malware downloaded from this URL was the HotCroissant backdoor, which had been used in attacks since 2017.

2. Recent Attack Case – ERP

A similar attack case was identified in May 2024. Unlike the past incident where a downloader routine was inserted into “ClientUpdater.exe”, this time, a routine was simply inserted to execute a DLL from a specific path using the Regsvr32.exe process.

Although the initial installation process is not confirmed, the identified DLL was found to be malware capable of stealing system information and executing commands from the threat actor. This suggests that, similar to the past incident, the update server of a specific ERP was attacked.

Based on keywords like “XctMain” used by the threat actor during the development process, the final installed DLL malware is classified as Xctdoor here. Xctdoor is in DLL format and developed in the Go language. It is designed to be executed via the Regsvr32.exe process.

When executed by the Regsvr32.exe process, Xctdoor injects itself into processes such as “taskhost.exe,” “taskhostex.exe”, “taskhostw.exe”, and “explorer.exe”. Subsequently, it copies itself to the path “%LOCALAPPDATA%PackagesMicrosoft.MicrosoftEdge.Current_8wekyb3d8bbweSettingsroaming.dat” and creates a shortcut file in the startup folder to ensure it runs after a reboot. The shortcut file “MicrosoftEdge.lnk” does not directly execute “roaming.dat”, but instead uses Regsvr32.exe to execute the “settings.lock” file located in the same path.

“settings.lock” is injector malware classified as XcLoader based on the name used by the threat actor during its creation. XcLoader’s function is simply to inject the “roaming.dat” file into the explorer.exe process.

Additionally, the Go language version of XcLoader was identified for the first time in this attack, whereas previously, the C language version of XcLoader had been used in attacks. In this attack case, both Go language and C language versions of XcLoader were found. The details about these types will be summarized in the next section.

The ultimately executed Xctdoor is a backdoor that transmits basic information such as the username, computer name, and the malware’s PID to the C&C server and can execute commands received from it. Furthermore, it supports information theft functions such as screenshot capture, keylogging, clipboard logging, and transmitting drive information.

Xctdoor communicates with the C&C server using the HTTP protocol, while the packet encryption employs the Mersenne Twister (mt19937) algorithm and the Base64 algorithm.

3. Recent Attack Case – Web Server

In March 2024, instances were confirmed where web servers were attacked to install XcLoader. Considering the targets were Windows IIS web servers running version 8.5, which was developed in 2013, it is presumed that the malware was propagated by exploiting poor configurations or a vulnerability.

Upon examining the commands executed on the IIS server, besides actions related to malware installation, behaviors such as querying system information were observed. This is similar to cases where web shells are installed on web servers to execute commands, suggesting that this system might also have a web shell installed.

The XcLoader used in the attack functions similarly to the type developed in the Go language, reading and decrypting the “roaming.dat” file located in the same directory, and injecting it into processes. The difference is that in the May 2024 case, the “roaming.dat” file is in PE format, whereas in this case, it is encrypted. XcLoader primarily targets the explorer.exe process for injection, but in some cases, it also selects the “sihost.exe” process.

A characteristic of the XcLoader used in this attack is its logging behavior to a specific path, as shown below. This path appears to be a detailed path related to the web server, indicating that the web server has already been compromised by the threat actor.

Additionally, among the systems affected by this attack, logs from Ngrok were also identified. Ngrok is a tunneling program that exposes systems within NAT environments to external access. It is often installed by threat actors to establish remote control via RDP on infected systems and is frequently observed in attacks attributed to the Kimsuky group.

4. Conclusion

ASEC monitors advanced persistent threats (APT) and has recently confirmed cases of attacks exploiting a Korean ERP solution. Similar to methods previously used by the Andariel group, the threat actor exploited the ERP solution to propagate malware within companies.

The recent attacks were confirmed in May 2024, targeting the defense sector, but similar attacks have been occurring since earlier times. In March 2024, instances were confirmed where Korean manufacturing sector web servers were attacked to install XcLoader. XcLoader serves as an injector malware responsible for injecting Xcdoor into normal processes. Xcdoor, in turn, is a backdoor capable of capturing system information such as screenshots, keylogs, clipboard data, and drive information, as well as executing commands issued by threat actors. Threat actors can control infected systems and exfiltrate information through this malware.

Users must be particularly cautious against attachments in emails from unknown sources and executable files downloaded from web pages. Security administrators in companies must enhance monitoring of asset management programs and apply patches for any security vulnerabilities in the programs. Users should also apply the latest patch for OS and programs such as Internet browsers, and update V3 to the latest version to prevent malware infection in advance.

File Detection
– Trojan/Win.XcLoader.C5642132 (2024.06.19.00)
– Trojan/Win.XcLoader.C5641779 (2024.06.17.02)
– Trojan/Win.XcLoader.C5641780 (2024.06.17.02)
– Backdoor/Win.Xctdoor.C5626572 (2024.05.27.03)
– Trojan/Win.Launcher.C5626571 (2024.05.29.00)
– Trojan/Win.Injector.R642750 (2024.04.03.01)
– Backdoor/Win.Xctdoor.C5622753 (2024.05.18.00)
– Trojan/Win.Injector.C5622750 (2024.05.17.02)
– Trojan/Win.Agent.C5622754 (2024.05.29.00)
– Trojan/Win.Injector.C5607331 (2024.04.03.01)
– Trojan/Win32.Rifdoor.R214775 (2017.12.06.00)
– Trojan/Win32.Andaridown.R216669 (2017.12.27.09)

Behavior Detection
– Execution/MDP.Ngrok.M4615

IoCs
MD5
– 235e02eba12286e74e886b6c99e46fb7: Modified ERP update program – past case (ClientUpdater.exe)
– 396bee51c7485c3a0d3b044a9ceb6487: HotCroissant – Past Case (***Kor.exe)
– ab8675b4943bc25a51da66565cfc8ac8: Modified ERP update program – latest case (ClientUpdater.exe)
– f24627f46ec64cae7a6fa9ee312c43d7: Modified ERP update program – latest case (ClientUpdater.exe)
– 6928fab25ac1255fbd8d6c1046653919: XcLoader (XcExecutor.exe)
– 9a580aaaa3e79b6f19a2c70e89b016e3: XcLoader (icsvcext.dll)
– a42ae44761ce3294ce0775fe384d97b6: XcLoader (icsvcext.dll)
– d852c3d06ef63ea6c6a21b0d1cdf14d4: XcLoader (icsvcext.dll)
– 2e325935b2d1d0a82e63ff2876482956: XcLoader (settings.lock)
– 4f5e5a392b8a3e0cb32320ed1e8d0604: XcLoader (test.exe)
– 54d5be3a4eb0e31c0ba7cb88f0a8e720: XcLoader (test.exe)
– b43a7dcfe53a981831ae763a9a5450fd: XcLoader (test.exe)
– e554b1be8bab11e979c75e2c2453bc6a: XcLoader (test.exe)
– 41d5d25de0ca0fdc54c24c484f9f8f55: XcLoader (settings.lock)
– b96b98dede8a64373b539f94042bdb41: XcLoader (settings.lock)
– 375f1cc32b6493662a78720c7d905bc3: XcLoader (settings.lock)
– d938201644aac3421df7a3128aa88a53: XcLoader (onedrive.dll)
– d787a33d76552019becfef0a4af78a11: XcLoader (onedrive.dll)
– 09a5069c9cc87af39bbb6356af2c1a36: XcLoader (onedrive.dll)
– ad96a8f22faab8b9c361cfccc381cd28: Xctdoor (******.***.Common.RegEx.dll)
– 9bbde4484821335d98b41b44f93276e8: Xctdoor (******.***.Common.RegEx.dll)
– 11465d02b0d7231730f3c4202b0400b8: Xctdoor (******.***.Common.RegEx.dll)

C&C Server Addresses
– 195.50.242[.]110:8080: HotCroissant
– hxxp://beebeep[.]info/index.php: Xctdoor

Download URL
– hxxp://www.jikji.pe[.]kr/xe/files/attach/binaries/102/663/image.gif: HotCroissant

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