Lots of research that concentrates on living off the land techniques focuses on finding legitimate OS binaries (both EXE and DLLs) that allow to:

load unsigned DLLs by the signed EXE files,

load code in an unexpected, unconventional way using phantom DLLs, sideloaded DLLs,

use native OS tools and abuse their functionality to download and upload files, maintain persistence, convert data, etc.,

break the process tree, and

many more.

Here, I propose to take it to a possible next, or at least parallel level.

If you are familiar with the ROP gadgets you know that it relies on re-using pieces of code belonging to loaded libraries already present in memory. These are used to chain together code blocks that may execute code of attackers’ choice. By its sole nature ROP is a complex beast and while it can be, and is now fully automated, it most of the time relies on the fact that the final piece of code is just a regular payload that the ROP chain transfer the control to…

I was wondering if it would be possible to re-use the ROP idea on a file-system level and build a library of high-level gadget-like signed executables and DLLs that could deliver the payload-like functionality, or at least its core building blocks. That would not only reduce the need to write the actual payload code – it would basically transfer the responsibility for the core functionality required by the attackers to signed libraries!

If it sounds weird, or complicated, let’s think for a second about existing installers. They implement and provide functionality that every single piece of malware needs: spawn processes, read file, write files, copying, downloading, uploading, same with the Registry operations, and more.

The installers have been abused by malware for a very long time, so it’s just a trivial example. I was thinking of something a little bit more refined and stealthy. Consider an example like this: a malicious document executes a macro; the macro drops a clean, signed executable produced by a well-known company – a file that not a single security solution can detect as malicious. It then instruments that signed executable to do all the dirty work. The chances are high that it would possibly bypass antivirus solutions, EDR, sandboxes and who knows, maybe even the Holy Grail – the whitelisting solutions.

Now that I formulated the idea in my head it was time to do some legwork…

I kicked off a number of searches within my files repository. After some poking around, eyeballing some code, a number of failed attempts I finally got lucky and hit the jackpot. To my surprise, I found a number of really interesting potentials!

The first interesting reusigned binary I came across is described below.

The nvuhda.exe and nvuhda6.exe are NVIDIA Uninstallers for 32- and 64-bit. When you execute them from a command line you will see the following screen:

The list of commands is shown below:

AddUninstall, Call, CheckPath, CheckRAID, ClassSweep, Copy, CopyV, CreateDevice, CreateShortcut, Del, DelBoot, DelBootQuiet, DelIniIfMatched, DelOemInfs, DelReg, DelRegE, DirAndApply, Echo, EnumDevices, EnumRegCmd, EnumRegNamesCmd, Eval, FindOEMInf, GetDrivePort, GetFolderPath, GetInfGUID, GetReg, Help, If, InstallDriver, InstallDriverEx, KillApp, RemoveDevice, Run, RunOnce, SendMessage, Set, SetEnv, SetReg, Sleep, Splash, StartLogging, StopLogging, SysCallAndWait, System, UnifyUninst, Uninstall, UnInstallEx, UninstallGUI, UninstallService, WaitOnRegDel

Hmm some of them look really interesting.

Using the ‘Help’ command we can retrieve more information about the commands:

We can run a few tests:

nvuhda6.exe System calc.exe spawns the Calculator

nvuhda6.exe Copy test.txt,test-2.txt copies ‘test.txt’ to ‘test-2.txt’

nvuhda6.exe SetReg HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run\malware=malware.exe adds a persistence key

nvuhda6.exe CreateShortcut test.lnk,"Test","c:\windows\system32\calc.exe","","c:\windows\system32" creates a shortcut file ‘test.lnk’ pointing to calculator

nvuhda6.exe KillApp calculator.exe kills the instances of ‘calculator.exe’ process

and so on and and so forth, and finally

nvuhda6.exe Run foo will run commands from the file ‘foo’ (where commands are from the list above; it’s basically an install script)



It’s pretty much a Swiss-Army tool for doing a lot of legitimate operations on the system, but it could be certainly abused. The only issue is the UAC, because the file required Admin privileges in its manifest.

And… last, but not least – here’s the full list of commands:

Help

Command Name

If <Command Name> is provided displayes syntax and description of that command otherwise displays all the avilable commands

Command Name If <Command Name> is provided displayes syntax and description of that command otherwise displays all the avilable commands System

Command

Executes the given system command (returns immidiately)

Command Executes the given system command (returns immidiately) SysCallAndWait

Exact path to App + arguments

call and wait untill app is done.

Exact path to App + arguments call and wait untill app is done. Set

Name[=Value]

Defines the variable if not defined and Sets the its value to the given value.

Name[=Value] Defines the variable if not defined and Sets the its value to the given value. Call

Section Name

runs all the commands in that section and returnd and continues executing commands after that line

Section Name runs all the commands in that section and returnd and continues executing commands after that line Run

Path to an NVU File

Executes all the commands in that file and returns

Path to an NVU File Executes all the commands in that file and returns RunOnce

Path to an NVU File

Run all the commands in this file and delete the file

Path to an NVU File Run all the commands in this file and delete the file DirAndApply

DirPath,Command

A dir will be applied to the given path and for each file found it sets variable {current file} to that file and calls the given command.

DirPath,Command A dir will be applied to the given path and for each file found it sets variable {current file} to that file and calls the given command. If

[!]Exp1}[=%]{Exp2} then {Command}

Executes command if Exp1 is the same as Exp2 for = or Exp1 has Exp2 for % if ! is added to the beging of Exp1 it will negate the result

[!]Exp1}[=%]{Exp2} then {Command} Executes command if Exp1 is the same as Exp2 for = or Exp1 has Exp2 for % if ! is added to the beging of Exp1 it will negate the result Eval

Command

reEvaluates the given command and executes it

Command reEvaluates the given command and executes it StartLogging

Filename

Opens the given log files ad Logs all the commands in the given file

Filename Opens the given log files ad Logs all the commands in the given file StopLogging

N/A

Stops logging close the file

N/A Stops logging close the file Sleep

milisconds

Waits for a number of miliseconds passed before executing the next command

milisconds Waits for a number of miliseconds passed before executing the next command Splash

{milisconds}, {BitmapPath}

Shows the given bitmap for the given time number on the screen

{milisconds}, {BitmapPath} Shows the given bitmap for the given time number on the screen WaitOnRegDel

{Key,Value,LoopDelay,MaxLoopCount,TimeoutCommand}

Allows waiting for a specific registry key or value to be deleted

{Key,Value,LoopDelay,MaxLoopCount,TimeoutCommand} Allows waiting for a specific registry key or value to be deleted FindOEMInf

{Variable} , INF name

Looks under the windows INF directory for additional NVIDIA display driver infs (oem*.inf). Returns TRUE if found.

{Variable} , INF name Looks under the windows INF directory for additional NVIDIA display driver infs (oem*.inf). Returns TRUE if found. CheckPath

{Variable} = Local Path

Returns TRUE if path exists.

{Variable} = Local Path Returns TRUE if path exists. Uninstall

{NVU File}, {Uninstall Reg key}

Runs the given file and deletes it. It also removes the given key from Add/Remove Program list

{NVU File}, {Uninstall Reg key} Runs the given file and deletes it. It also removes the given key from Add/Remove Program list UnInstallEx

Uninstall Display Reg key

Uninstalls the given product from add remove programs. If it detects uninstall as its nvu it uses internal uninstall command otherwise calls the appropriate uninstall command.

Uninstall Display Reg key Uninstalls the given product from add remove programs. If it detects uninstall as its nvu it uses internal uninstall command otherwise calls the appropriate uninstall command. AddUninstall

{Uninstall File} , {Display Name}

Adds Display name as the title to in Add/Remove programs list and copies uninstall binary and script file to windows system directory

{Uninstall File} , {Display Name} Adds Display name as the title to in Add/Remove programs list and copies uninstall binary and script file to windows system directory DelBoot

Path to files to be deleted on reboot

Put the given file for delete in the next reboot. If the file exists it will ask user for the reboot when program ends.

Path to files to be deleted on reboot Put the given file for delete in the next reboot. If the file exists it will ask user for the reboot when program ends. Del

Path to file to be deleted

Deletes the given file if it exists, It also deletes a directory if it is empty. If the file is locked it will set the system to delete the file upon reboot

Path to file to be deleted Deletes the given file if it exists, It also deletes a directory if it is empty. If the file is locked it will set the system to delete the file upon reboot DelBootQuiet

Path to files to be deleted on reboot without reboot request

Put the given file for delete in the next reboot. If the file exists it will ask user for the reboot when program ends.

Path to files to be deleted on reboot without reboot request Put the given file for delete in the next reboot. If the file exists it will ask user for the reboot when program ends. Copy

Srcfile,DstFile

Copies a file from the given source to given dest

Srcfile,DstFile Copies a file from the given source to given dest CopyV

Srcfile,DstFile

Copies a file from the given source to given dest only if source is a higher version.

Srcfile,DstFile Copies a file from the given source to given dest only if source is a higher version. DelReg

Registry pattern matching string

Deletes regkey(s) matching the given registry key

Registry pattern matching string Deletes regkey(s) matching the given registry key DelRegE

Registry pattern matching string

Only deletes regkey(s) matching the given registry key if they have no subkeys or values

Registry pattern matching string Only deletes regkey(s) matching the given registry key if they have no subkeys or values SetReg

Registry pattern matching string\ame[=value]

For all the matching registry paths, sets values if [=value] is present, creates subkeys otherwise

Registry pattern matching string\ame[=value] For all the matching registry paths, sets values if [=value] is present, creates subkeys otherwise GetReg

{Variable} = {regkeypath

ame}

If it finds name under regkeypath copies its value to the given variable, othewise does nothing.

{Variable} = {regkeypath

ame} If it finds name under regkeypath copies its value to the given variable, othewise does nothing. UninstallService

Service Name

uninstalls the given service name

Service Name uninstalls the given service name RemoveDevice

{Enum Type} , {Hardware ID}, {Device type}

Remove any device matched with the given description from the system using setupdi calls.

Enum can be (PCI, EISA, etc), HWID usually is VEN_10DE and device type can be DISPLAY,HDC,MEDIA,NET,SYSTEM

{Enum Type} , {Hardware ID}, {Device type} Remove any device matched with the given description from the system using setupdi calls. Enum can be (PCI, EISA, etc), HWID usually is VEN_10DE and device type can be DISPLAY,HDC,MEDIA,NET,SYSTEM InstallDriverEx

{Hardware ID}, {InfFullPath}

Installs the given driver for any device matched with the given device ID from the system using setupdi calls.

{Hardware ID}, {InfFullPath} Installs the given driver for any device matched with the given device ID from the system using setupdi calls. InstallDriver

{Hardware ID}, {InfFullPath}

Installs the given driver for any device matched using UpdateDriverForPlugAndPlayDevices for Win200 and above and InstallDriverEx for Win95 and Win98

{Hardware ID}, {InfFullPath} Installs the given driver for any device matched using UpdateDriverForPlugAndPlayDevices for Win200 and above and InstallDriverEx for Win95 and Win98 CreateDevice

{Hardware ID}, {InfFullPath}

This function first creates a device Installs the driver for this device using given inf. This function should work for all versions of Windows

{Hardware ID}, {InfFullPath} This function first creates a device Installs the driver for this device using given inf. This function should work for all versions of Windows GetInfGUID

Variable Name = Full Path to Inf

Sets the given variable to a string representing the inf’s GUID

Variable Name = Full Path to Inf Sets the given variable to a string representing the inf’s GUID DelOemInfs

{WildCard} , {Section} , {Name} , {Value} , {[…]

it searches in all inf files under sysdir\\inf\\[Wildcard and OEM*.inf] and if it finds a match with given parameters it deletes it on reboot

{WildCard} , {Section} , {Name} , {Value} , {[…] it searches in all inf files under sysdir\\inf\\[Wildcard and OEM*.inf] and if it finds a match with given parameters it deletes it on reboot DelIniIfMatched

Inffile} , {Section} , {Name} , {Value} , {[,…]

it searches in all inf files under sysdir\\inf\\OEM*.inf and if it finds a match with given parameters it deletes it on reboot

Inffile} , {Section} , {Name} , {Value} , {[,…] it searches in all inf files under sysdir\\inf\\OEM*.inf and if it finds a match with given parameters it deletes it on reboot ClassSweep

GUID,{StrPattern1};{StrPattern2};….

Pass in the GUID of the inf and the string patterns of the reg names to be deleted. It will remove all the reg names corresponding to the str pattern under the GUID 0001….. except in Properties)

GUID,{StrPattern1};{StrPattern2};…. Pass in the GUID of the inf and the string patterns of the reg names to be deleted. It will remove all the reg names corresponding to the str pattern under the GUID 0001….. except in Properties) EnumRegNamesCmd

RegPathToEnumatrate} , {Command

Enumerates all keys matching the given regpath and when matched enumarates all names under that key then calls the given command {Current Reg name} and {Current Reg value} are set when the given command is executed

RegPathToEnumatrate} , {Command Enumerates all keys matching the given regpath and when matched enumarates all names under that key then calls the given command {Current Reg name} and {Current Reg value} are set when the given command is executed EnumRegCmd

{RegPathToEnumatrate} , {Command}

Enumerates all keys matching the given regpath and when matched calls the given command {Current Reg Key} is set then the given command is executed

{RegPathToEnumatrate} , {Command} Enumerates all keys matching the given regpath and when matched calls the given command {Current Reg Key} is set then the given command is executed UnifyUninst

N/A

This is a legacy command to maintain backward compatibility.

N/A This is a legacy command to maintain backward compatibility. SetEnv

Name=Value

Adds the given name to environment variables and sets its value

Name=Value Adds the given name to environment variables and sets its value UninstallGUI

WindowName

Displays the GUI to uninstall whatever user chooses

WindowName Displays the GUI to uninstall whatever user chooses KillApp

AppName

Given an appname enumerates all the running app and kills its process if it is running

AppName Given an appname enumerates all the running app and kills its process if it is running SendMessage

{AppName}, {Message}

Given an appname, sends its window a message

{AppName}, {Message} Given an appname, sends its window a message CreateShortcut

LnkFile,Title,ProgFile,ProgArgs,ProgWorkingDir

Creates a shell link (shortcut)

LnkFile,Title,ProgFile,ProgArgs,ProgWorkingDir Creates a shell link (shortcut) GetDrivePort

{Variable} = {path}

It uses the first char of the path as drive letter and assigns the port number of that to the given variable name.

{Variable} = {path} It uses the first char of the path as drive letter and assigns the port number of that to the given variable name. EnumDevices

{DeviceId},{Class},{Cmd}

It enumerats all the devices present on the system if any device has {DeviceID} and is from the given {Class} then variable {Current Device} is set to that device ID and the given command gets executed.

{DeviceId},{Class},{Cmd} It enumerats all the devices present on the system if any device has {DeviceID} and is from the given {Class} then variable {Current Device} is set to that device ID and the given command gets executed. GetFolderPath

Variable Name = FolderCode

Sets the given variable to the path of a special folder, identified by its CSIDL (check MSDN SHGetSpecialFolderPath Function)

Variable Name = FolderCode Sets the given variable to the path of a special folder, identified by its CSIDL (check MSDN SHGetSpecialFolderPath Function) CheckRAID

N/A

Returns TRUE if RAID exists.