We have discovered a new malware written in Go, which we are calling HabitsRAT, targeting both Windows and Linux machines. The Windows version of the malware was first reported on by Brian Krebs and The Shadowserver Foundation in attacks against Microsoft Exchange servers. In addition to this version, we have identified a newer Windows variant and a variant targeting Linux environments. As of this writing, the Linux version is undetected by all Antivirus engines on VirusTotal.
We assess that the Linux version is used to target Linux servers in an adjacent campaign to the one reported by The Shadowserver Foundation. The malware allows the attacker to control the compromised machine remotely. To protect themself from being taken over by others, the attacker’s commands are signed by a private key that only the attacker has access to. The malware does not execute commands that are not signed by the correct key, suggesting that the malware has been developed by a sophisticated programmer.
Intro
On March 28th, Brian Krebs published a blog post about attacks against Microsoft Exchange servers. In one of those attacks, a webshell called “Babydraco” was deployed. The webshell was used to deploy a new malware. The binary had the filename “krebsonsecurity.exe” and used a Command and Control (C2) server located at “brian[.]krebsonsecurity[.]top”. This malware turns out to be a remote access trojan (RAT) that has been written to target both Windows and Linux machines. Based on strings found in the malware, we have named it HabitsRAT.
While the Windows version of the RAT has been documented being installed on compromised Microsoft Exchange servers, it is not known what type of servers the Linux version is used against. Still, in the last couple of months, numerous remote code execution (RCE) vulnerabilities have been disclosed in hardware and services running on top of Linux. About a month ago, CISA released an advisory urging users of F5 BIG-IP to apply patches to address RCE vulnerabilities.
Technical Analysis
The HabitsRAT is a simple backdoor that allows the malware operator to execute arbitrary code on the infected machine. While the backdoor is simple in design, the malware has functionality making the attack more complex than what is normally seen. The malware is written in Go and targets at least both Windows and Linux machines. The structure for the Windows version of the malware, generated by redress, is shown in the code snippet below. Most of the code is shared between the Windows version and the Linux version. The operating system-specific code has been placed in the files “commandplatform_windows.go”, “keyplatform_windows.go” and “persistencehandler_windows.go”. The rest of the files are shared with the Linux version.
Package main: C:/Users/user/habits/habits-client
File: commandhandler.go
RunSignedCommand Lines: 17 to 35 (18)
File: commandplatform_windows.go
RunCommand Lines: 8 to 13 (5)
File: keyhandler.go
GetOrGenerateKey Lines: 13 to 23 (10)
GenerateKey Lines: 23 to 42 (19)
GetKeyStore Lines: 42 to 50 (8)
SetKey Lines: 50 to 68 (18)
GetKey Lines: 68 to 77 (9)
File: keyplatform_windows.go
GetRootKeyStore Lines: 11 to 19 (8)
GetUserKeyStore Lines: 19 to 27 (8)
IsRoot Lines: 27 to 49 (22)
File: main.go
main Lines: 17 to 34 (17)
File: persistencehandler.go
InstallPersistence Lines: 9 to 17 (8)
CopyBinary Lines: 17 to 22 (5)
File: persistencehandler_windows.go
CheckPersistence Lines: 11 to 21 (10)
GetBinStoreRoot Lines: 21 to 29 (8)
GetBinStoreUser Lines: 29 to 37 (8)
InstallPersistRoot Lines: 37 to 98 (61)
The Linux source code structure is shown in the code snippet below. The Linux specific code has been placed in the files “commandplatform_linux.go”, “keyplatform_linux.go” and “persistencehandler_systemd_linux.go”.
Package main: C:/Users/user/habits/habits-client
File: commandhandler.go
RunSignedCommand Lines: 17 to 35 (18)
File: commandplatform_linux.go
RunCommand Lines: 8 to 13 (5)
File: keyhandler.go
GetOrGenerateKey Lines: 13 to 23 (10)
GenerateKey Lines: 23 to 46 (23)
GetKeyStore Lines: 46 to 54 (8)
SetKey Lines: 54 to 72 (18)
GetKey Lines: 72 to 84 (12)
IsRootAsString Lines: 84 to 86 (2)
File: keyplatform_linux.go
GetRootKeyStore Lines: 9 to 16 (7)
GetUserKeyStore Lines: 16 to 17 (1)
File: main.go
main Lines: 17 to 34 (17)
File: persistencehandler.go
InstallPersistence Lines: 9 to 17 (8)
CopyBinary Lines: 17 to 20 (3)
File: persistencehandler_systemd_linux.go
Systemd_CheckPersistence Lines: 11 to 25 (14)
Systemd_GetBinStoreUser Lines: 25 to 33 (8)
Systemd_InstallPersistRoot Lines: 33 to 64 (31)
Installation
When the binary is run, it installs itself into a folder. The Windows version’s location is “%SystemDrive%WindowsDefenderMsMpEng.exe” while the Linux version is “$HOME/.config/polkitd/polkitd”. This will result in the malware being installed under “/root” if it’s being run with root privileges.
After the malware has installed itself, it checks if the persistence method has been set up. If it hasn’t, it goes ahead and sets it up. On Linux, it uses a “systemd” unit file. The malware checks if it’s already configured by executing the command “systemctl status polkitd”, as shown in Figure 1.
Figure 1: Linux version of the malware checks if persistence has been configured already.
The systemd unit file is created at “/etc/systemd/system/polkitd.service” and its content is shown in the code snippet below.
[Unit]
Description=Authorization Manager
After=network.target
[Service]
GuessMainPID=no
ExecStart=”/path/to/binary”
Restart=always
[Install]
WantedBy=multi-user.target
The Windows version of HabitsRAT uses scheduled tasks for persistence. First, it writes the scheduled task “xml” to a file located at “%TEMP%krebsonsecurity.xml”. The content of the file is shown in the snippet below. The task is added by executing the shell command: “sCHtAsks.exe /create /xml %TEMP%krebsonsecurity.xml /tn WindowsDefenderScan”
<?xml version=”1.0″ encoding=”UTF-16″?>
<Task version=”1.2″
xmlns=”http://schemas.microsoft.com/windows/2004/02/mit/task”>
<RegistrationInfo>
<Date>2020-12-18T09:56:46.3915265</Date>
<Author>Microsoft Corporation</Author>
<URI>\\Microsoft\\MicrosoftUpdater</URI>
</RegistrationInfo>
<Triggers>
<BootTrigger>
<Enabled>true</Enabled>
<Delay>PT1M</Delay>
</BootTrigger>
</Triggers>
<Principals>
<Principal id=”Author”>
<UserId>S-1-5-18</UserId>
<RunLevel>HighestAvailable</RunLevel>
</Principal>
</Principals>
<Settings>
<MultipleInstancesPolicy>IgnoreNew</MultipleInstancesPolicy>
<DisallowStartIfOnBatteries>false</DisallowStartIfOnBatteries>
<StopIfGoingOnBatteries>false</StopIfGoingOnBatteries>
<AllowHardTerminate>false</AllowHardTerminate>
<StartWhenAvailable>true</StartWhenAvailable>
<RunOnlyIfNetworkAvailable>false</RunOnlyIfNetworkAvailable>
<IdleSettings>
<StopOnIdleEnd>true</StopOnIdleEnd>
<RestartOnIdle>false</RestartOnIdle>
</IdleSettings>
<AllowStartOnDemand>true</AllowStartOnDemand>
<Enabled>true</Enabled>
<Hidden>false</Hidden>
<RunOnlyIfIdle>false</RunOnlyIfIdle>
<WakeToRun>false</WakeToRun>
<ExecutionTimeLimit>PT0S</ExecutionTimeLimit>
<Priority>7</Priority>
</Settings>
<Actions Context=”Author”>
<Exec>
<Command>
path\to\binary
</Command>
</Exec>
</Actions>
</Task>
Command and Control Communication
The RAT uses public-key cryptography to both encrypt and authenticate the commands from the C2 server. The malware generates a public-private key pair using an open-source library provided by Proton Mail. Figure 2 shows the call to the GenerateKey function and its arguments. The malware uses the machine’s hostname as the name and an email address of “a@a.a”. No password is provided and it’s requesting a 2048-bit RSA key to be used.
The key is stored and written to disk. The Linux version of HabitsRAT writes to “$HOME/.config/.accounts-daemon/accounts-daemon.login.conf” if it is running as a normal user or to “/usr/share/accounts-daemon/accounts-daemon.so”. The Windows version uses “%SystemDrive%WindowsDefenderMsMpEng.dll” or “%APPDATA%Windows NTDefenderMsMpEng.dll” instead.
Figure 2: Generation of public-private key pair using the open-source library from Proton Mail.
HabitsRAT sends a “check-in” POST request to the C2 server to see if it should execute a command. As part of the request, it sends some data about the infected machine. The form data of the request is shown below. The data includes the “no_replay” field that holds the sha256 hash of some random data. This acts like a nonce to prevent executing the same request multiple times. The request also includes the public key for the malware instance. This is to allow the C2 server to encrypt the commands to it. It also has a version value that is hardcoded to 11.
no_replay: [sha256 hash of random data]
public_key: public key in ascii armour
hostname: [machine hostname]
goos: [linux or window]
goarch: amd64
shell: [$SHELL expanded]
root: [true or false]
version: 11
The data is sent to “https://brian.krebsonsecurity[.]top/checkin”. If no command is returned, the malware sleeps for 10 seconds and sends the request again. If the C2 responds with data, the malware checks that the threat actor’s key has signed it. A hardcoded public key is included in the binary. Extracted information from the key shows that it was generated in December 2020 and includes a name and a Gmail address.
pub rsa3072 2020-12-03 [SC] [expires: 2022-12-03]
uid [REDACTED] <[REDACTED]@gmail.com>
sub rsa3072 2020-12-03 [E] [expires: 2022-12-03]
If the correct key has signed the response, HabitsRAT uses its private key to decrypt the payload. The data has been serialized to JSON and the malware unmarshals it to the data structure shown below.
type main.CommandList struct {
No_replay string
Commands []string
}
The Commands field is passed as arguments to either “bash -c” for the Linux version or “cmd /c” for the Windows version.
HabitsRAT Version 12
A newer Windows version of HabitsRAT has also been found. Much of the functionality is the same as version 11. The main difference is that it’s using a different C2 public key and supports multiple C2 addresses. As can be seen from the snippet below, this key was generated on the 2nd of April.
pub rsa3072 2021-04-02 [SC] [expires: 2023-04-02]
uid Brian Krebs <krebsonsecurity@gmail.com>
sub rsa3072 2021-04-02 [E] [expires: 2023-04-02]
The malware uses four different C2 addresses and picks one out of random. The addresses are as follows, which includes a domain of Brian Krebs’s leaked social security number:
- https://brian-krebs-erectile-dysfunction[.]com
- https://krebsonfellatio[.]net
- http://XXX-XX-XXXX.com (Redacted)
- hxxp://185.193.126.198
The addresses are stored at:
- %SystemDrive%WindowsDefenderDefender.dll
- %APPDATA%Windows NTDefenderDefender.dll
Conclusion
The HabitsRAT is a multi-operating system malware targeting both Windows and Linux environments. There is a lot of code reuse between the two variants. It provides the attacker with the capability to execute arbitrary code on the infected machine. To protect its C2 communication, the data is encrypted and signed using PGP. Ensure internet facing servers are patched to prevent being infected by HabitsRAT. Indicators of Compromise (IoCs) below can be used to detect if a server has been compromised.
Go malware has been hard to detect by Antivirus products so it’s likely this trend will continue. We have seen threat actors pivot and target different operating systems with the same codebase for the malware, resulting in low or undetected malware samples, especially for Linux—which has a large presence in the cloud. Since the malware is derived from the same codebase, detection based on code reuse has proven to be very effective.
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To read the original article:
https://www.intezer.com/blog/malware-analysis/habitsrat-used-to-target-linux-and-windows-servers/