Kimsuky Distributing CHM Malware Under Various Subjects

AhnLab Security Emergency response Center (ASEC) has continuously been tracking the Kimsuky group’s APT attacks. This post will cover the details confirmed during the past month of May. While the Kimsuky group often used document files for malware distribution, there have been many recent cases where CHM files were used in distribution. Also, unlike in the past when the document files contained North Korea-related topics, the group is now attempting to attack using a variety of subjects.

(1) Cases of Distribution

The names of the distributed files found during May are as follows. They show a variety of subjects such as cryptocurrency, tax accounting, and contracts, and it seems the personal data of a certain individual is being used.

File Names Used in Distribution
(Coinone)Client Transaction Confirmation.chm
202305050017 Order Sheet (1).chm
BITWAK Application Form.chm
20230412_Tax Investigation Return Guidelines.chm
2023 Annual Membership Fee Payment-related Materials(****).chm
Revised Lease Contract.chm
Payment Slip.chm
League of Legends Restricted Account Notice (Riot Games).chm
Written Act for the 2023 1st Provisional General Meeting.chm
Tuition Receipt.chm
CTP Lockup Cancellation Notice(***).chm
Materials for Publication Fees for Volume 23 Issue 5(***).chm
Rental(Renewal) Application Materials for Gumi General Business Support Center (***).chm
Listing Deliberation Materials.chm
*** Proof of Social Insurance Subscription.chm
Table 1. File names used in distribution

The CHM malware in distribution generates a normal help window upon execution and performs malicious behaviors through the malicious script inside. It is not easy for users to notice the malicious behaviors, having been deceived with the help window disguised as a normal file. The help window generated in the user’s PC has a different topic according to which particular field the target works in. Below are some of the common examples.

Figure 1. Disguised as a tax investigation return form

Figure 1 shows the type that was disguised as a National Tax Service tax investigation return guide for users that must file tax returns. The global income tax return season in Korea falls in May, and the threat actor seems to have taken advantage of this fact.

Figure 2. Disguised as financial transaction data

Figure 2 shows the type disguised as financial transaction data between certain users. The actual account number and transaction histories can be seen, and this may have been created using stolen personal data.

Figure 3. Disguised as cryptocurrency transaction data

Figure 3 shows the type disguised as cryptocurrency transaction data. Like the second case, it contains personal data such as an actual user’s email and phone number.

Figure 4. Disguised as documents such as contracts

There are also other types such as contracts, certificates, and order sheets as shown in Figure 4. These are the major files in distribution, but as there are files disguised as the household register of a certain individual, ticket reservation details, and other topics, users are advised to practice particular caution.

(2) Operation Process

Figure 5. Overall operation process

The overall operation flow of this CHM type is shown in Figure 5. Additional scripts are downloaded to exfiltrate user information and download additional malware. Each step is outlined below.

Figure 6. Malicious script within the CHM

The malicious script in the CHM is shown in Figure 6. Malicious commands are executed through a shortcut object, and this object is called through the Click method.

  • Executed Command
    cmd, /c start /MIN REG ADD HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run /v oeirituttvv /t REG_SZ /d “%USERPROFILE%\Links\oeirituttvv.vbs” /f & echo [Encoded command] > “%USERPROFILE%\Links\oeirituttbb.dat” & echo [Encoded command] > “%USERPROFILE%\Links\oeirituttvv.dat” & start /MIN certutil -decode “%USERPROFILE%\Links\oeirituttvv.dat” “%USERPROFILE%\Links\oeirituttvv.vbs” & start /MIN certutil -decode “%USERPROFILE%\Links\oeirituttbb.dat” “%USERPROFILE%\Links\oeirituttbb.bat” & start /MIN timeout -t 1 /nobreak & start /MIN CScript “%USERPROFILE%\Links\oeirituttvv.vbs” & start /MIN timeout -t 2 /nobreak & start /MIN CScript “%USERPROFILE%\Links\oeirituttvv.vbs

This command saves two encoded commands under “%USERPROFILE%\Links\oeirituttbb.dat” and “%USERPROFILE%\Links\oeirituttvv.dat” and saves the commands decrypted through certutil in the files oeirituttbb.vbs and oeirituttvv.bat. Afterward, it runs oeirituttbb.vbs and registers oeirituttbb.vbs to the RUN key to enable it to run continuously.

Figure 7. oeirituttbb.vbs and oeirituttvv.bat

Oeirituttbb.vbs is a runner that runs the oeirituttvv.bat file created with it. oeirituttvv.bat downloads additional malicious files through curl. Two files are downloaded: a BAT file and a CAB file.

  • Download URL
Figure 8. pung03.bat and

The downloaded BAT file (pung03.bat) decompresses the CAB file (, then runs temprr03.bat. The CAB file contains a total of 6 scripts. The features of each script are outlined in Table 2.

File Name Feature
temprr03.bat Runs loyes03.bat
loyes03.bat Registers to RunKey (mnasrt.vbs)
Runs loyestemp03.bat
Runs dwpp.vbs
mnasrt.vbs Runs loyes03.bat
loyestemp03.bat Collects user information
Runs uwpp.vbs
dwpp.vbs Downloads CAB
uwpp.vbs Uploads user information
Table 2. Features of each script

The final malicious behaviors performed by this script are exfiltrating user information and downloading additional malicious files.

Figure 9. Exfiltrating user information
File Name Saved Information
cudk.txt List of files on the Desktop (including subfolders)
ipif.txt IP information
stif.txt System information
Table 3. Exfiltrated information

The code for the exfiltration of user information is shown in Figure 9, and the pieces of exfiltrated information are shown in Table 3. User information is collected through loyestemp03.bat, and uwpp.vbs sends the collected information along with the PC name to “hxxp://vndjgheruewy1[.]com/uun06/uwpp.php”.

Figure 10. Download feature

The code for file download is shown in Figure 10. It seems that the threat actor checks the stolen user information, and only when the system is a target of attack, uploads additional malicious files to the C2. If the system is a target, the threat actor uploads files with the infected PC’s name. Infected PCs continuously make attempts to download through the script registered to RunKey, and when additional files are uploaded, the files are downloaded. It then decompresses the downloaded files through the expand command before executing them. This allows us to assume that the additional file is also a CAB file.

  • Download URL

As such, more elaborate attacks have become possible because the types of malicious files downloaded may differ according to the attack target. Recently, there has been an increase in malware distribution targeting particular users using personal information. Cases of using CHM files in APT attacks are also commonly found. Users must carefully check the senders of emails and refrain from opening files from unknown sources. They should also perform routine PC checks and always keep their security products updated to the latest version.

[File Detection]
Downloader/CHM.Generic (2023.06.03.00)
Trojan/BAT.Runner (2023.06.17.00)
Trojan/VBS.Runner (2023.06.17.00)
Downloader/BAT.Generic (2023.06.17.00)
Downloader/VBS.Generic (2023.06.17.00)
Infostealer/BAT.Generic (2023.06.17.00)
Infostealer/VBS.Generic (2023.06.17.00)


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