buffer-low/malicious

ASM Malicious code - PE infector

Malicious

ASM Malicious Code - Let's play a game
Create the most undetectable ASM virus that we can using http://www.virustotal.com (55 AntiVirus)

Preview

Infecting the calculator which will infect other .exe and download a payload to execute

Capacities

• Reproduction by infecting near PE files
• Basic polymorphism and encrypt/decrypt on the fly
• Trying some AV anti-detection techniques
• Download payload from an http server and execute it

Building (tried on Windows 7 & 10)

• Download MASM32 and install it
• Launch ./build.bat which will generate malicious.exe (first virus)
• Run it in a folder where there are some executable files (in test/ for exemple)
• If you want the virus to download the payload : cd website & npm install & node server.js

Workflow logs

Starting

We start by testing on a HelloWorld sample using g++. Why ? Because an empty main with gcc will result into 5 flags. HelloWorld with gcc 2 flags and only 1 with g++. Yeah... amazing start. Good job AVs !

Version #1

• creating new section
• changing entry point (EP)
• infect *.exe in current directory
• get back to old EP

> 13 flags (out of 54!)

Version #2

• polymorphism (xoring by random value)

We encrypt the part of the program that is our virus by xoring it with a random value and put at the beginning of the program a decryption process. We still expect a lot of flags since it doesn't change our behaviour (in a sandbox) but we should be more resistant to static analysis.

> 6 flags. Nice.

That means half of the flags from version #1 was pattern recognition.

Strangely, all the flags are from unknown AVs (from the general public). Avast, Avira, AVG are all bypassed.

Let's do some testing

• We note that we only get 1 flag if we don't change the EP in our infected file
• When we create a new section, empty with an exit, only changing the EP to our section will get us 3 flags
• If we add our virus, without going back to the old EP, we get 13 flags !
• We get the same flags if the virus is not infecting files, but the code is still present. Interesting.
• We get different result by using the same infected file : the encryption randomness is sometimes broken

What does this mean

• Most of the AVs do not check the section if it is not executed.
• The action to change the EP to the last section will alert half of the one I have in version #2.
• Even if we do not infect files, a code showing characteristics of doing it will be flagged.

What we can do

• Find an other way to execute our section
• A better encryption
• Blur the way of infecting files

Few days later

Our virus is now flagged by 12 AVs. We've got 6 more who joined the fight. Strangely, they all have the same result flag : "Gen:Variant.Graftor.158431". They share informations. It makes sense. Variant Grafter seems to be a "Generic Detection of a file that appears to have trojan-like features or behavior".

Version #3

• Inserting JMP in entry point to our section
• Encrypt it and decrypt it on the fly
• Download virus via HTTP and launch it
• Make the real program run in background to avoid http get lag
• Hook ExitProcess to avoid being killed by thread

> 3 flags. Magic.

Looks like doing a JMP instead of changing the entry point is the best way to avoid detection. Other features are just for fun, for example, the HTTP download process could be used to update our virus any time we want.

Going deeper

We know have a good version of the virus. It is mostly undetectable, can reproduce and communicate. But that's only the beginning. When an AV will detect it (and it will), with our sort of 'polymorphism' we will not be able to hold against a detection based on our signature. Our decrypter can be easily marked, same as our behavior. We could improves our encryption, add junk code or even sandboxes detection. But it's just not interesting anymore.