Threat Level: green Handler on Duty: Russell Eubanks

SANS ISC: Decoding Custom Substitution Encodings with - SANS Internet Storm Center SANS ISC InfoSec Forums

Sign Up for Free!   Forgot Password?
Log In or Sign Up for Free!
Decoding Custom Substitution Encodings with

Reader Jan Hugo submitted a malicious spreadsheet (MD5 942e941ed7344ffc691f569083949a31).

It has some aspects that I want to highlight in this diary entry. can be used to analyze it:

The obfuscated command is a single string:

Function BOOL decodes the string, by calling function Check to do the decoding character per character. Unlike similar functions in malicious documents that use a For or While loop to iterate over each character of an encoded string, function Check uses recursion (RED):

The decoding is done by shifting each character 9 positions to the left (BLUE), not using the ASCII table, but using a custom table hidden as form property "without" (BLUE and GREEN):

It's a substitution cipher. This encoding can also be decoded with, albeit not with a single expression, but with a small function:

def Substitute(number):
    key = [ord(char) for char in 'qwertyuiopasdfghjklzxcvbnm/"\'()[]${}.,\\;-%_|: 1234567890']
    if not number in key:
        return None
    return key[(key.index(number) - 9) % len(key)]

After extraction of the encoded string with and replacing 2 double-quotes with a single double-quote using sed (that's how VBA encodes a double-quote inside a string), it can be simply decoded with by passing the script with option -s and calling the decoding function Substitute:

You can use this translate script if you encounter similar encodings: just replace the offset (9) and the custom table ("qwerty...") with your own.


Didier Stevens
Senior handler
Microsoft MVP


316 Posts
ISC Handler
This is great for learning, thanks for the video.

Sign Up for Free or Log In to start participating in the conversation!