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|
|League of Legends Restricted Account Notice (Riot Games).chm|
|Written Act for the 2023 1st Provisional General Meeting.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|
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 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 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 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.
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
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.
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.
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
The downloaded BAT file (pung03.bat) decompresses the CAB file (qung03.cab), then runs temprr03.bat. The CAB file contains a total of 6 scripts. The features of each script are outlined in Table 2.
|loyes03.bat||Registers to RunKey (mnasrt.vbs)
|loyestemp03.bat||Collects user information
|uwpp.vbs||Uploads user information|
The final malicious behaviors performed by this script are exfiltrating user information and downloading additional malicious files.
|File Name||Saved Information|
|cudk.txt||List of files on the Desktop (including subfolders)|
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”.
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.
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