A VBScript with Obfuscated Base64 Data, (Sat, Jul 8th)

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, Im using width:800px” />

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 padding:5px 10px”>
H=TVqQAAMAAAAEAAAA//8AALgAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAgAAAAA4fug4AtAnNIb
gBTM~*hVGhpcyBwcm9ncmFtIGNhbm5vdCBiZSBydW4gaW4gRE9TIG1vZGUuDQ~*KJAAAAAAAAABQRQAATAEDAGGnBFkAAAAAAAAAA
OAAAgELAQsAALIAAAAIAAAAAAAAjtEAAAAgAAAA4AAAAABAAAAgAAAAAgAABAAAAAAAAAAEAAAAAAAAAAAgAQAAAgAAAAAAAAIAQI
UAABAAABAAAAAAEAAAEAAAAAAAABAAAAAAAAAAAAAAADTRAABXAAAAAOAAAPgEAAAAAAAAAAAAAAAAAAAAAAAAAAABAAwAAAD8zwA
AHAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAIAAACAAAAAAAAAAAAAAACCAAAEgAAAAAAAAAAAAA
AC5~*ZXh~*AAAAlLEAAAAgAAAAsgAAAAIAAAAAAAAAAAAAAAAAACAAAGAucnNyYwAAAPgEAAAA4AAAAAYAAAC~*AAAAAAAAAAAAAA
AAAABAAABAL…(redacted)

If you check in the VBScript code, youll indeed see an instruction padding:5px 10px”>
$_b=$_b.replace(~*,0

When just replace the string by 0 padding:5px 10px”>
$ sed s/~*/0/g base64.txt | base64 -d padding:5px 10px”>
O.regwrite D,H,REG_SZ
O.Run C chrw(34) $_b = (get-itemproperty -path HKCU:SOFTWAREMicrosoft -name KeyName
$_b=$_b.replace(~*,0 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

(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.

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