Threat Level: green Handler on Duty: Xavier Mertens

SANS ISC: Dealing with obfuscated RTF files - SANS Internet Storm Center SANS ISC InfoSec Forums

Sign Up for Free!   Forgot Password?
Log In or Sign Up for Free!
Dealing with obfuscated RTF files

I see a lot of malicious RTF files that are heavily obfuscated. Last, I received a sample that rtfobj or rtfdump could not handle properly to correctly identify OLE objects ("Not a well-formed OLE object"). But my rtfdump tool has an option that can help decode objects that are not well-formed. Let's take a closer look.

rtfdump does not identify OLE objects in this sample, however, the h= indicator tells us that there are a lot of hexadecimal characters.

Let's take a closer look at the first sequence with hexadecimal strings (sequence 4 is the first, inner most nested sequence with 6989 hexadecimal characters and an hexadecimal string of 252 characters):

It looks like an embedded object, however the control word is \.\objdata while we expect \*\objdata. And it contains an obfuscated hexadecimal string that indicates the presence of an OLE file: d0cf11e0a1b11ae1.

So let's convert this hexadecimal data to binary with option -H:

First we see the string "Equation.3", so this could be an exploit for CVE 2017-11882 (the equation editor vulnerability).

Next we see that was not able to deobfuscate the hexadecimal string correctly: da 0c f1 1e 0a 1b 11 ae 10. There is one extra nibble (a), which shifts all subsequent bytes by 4 bits.

Using option -S, we can try to fix this, by shifting the byte sequence by 4 extra bits, making the total shift by 8 bits, e.g. one byte:

This time, when we scroll down, we find indeed the exploit command:

This decoded sequence can be dumped (option -d) to extract strings:

Conclusion: when rtfdump is not able to correctly deobfuscate hexadecimal strings, try option -S.

Didier Stevens
Microsoft MVP Consumer Security


213 Posts
ISC Handler
Any suggested repository for download of this sample?

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