full-disclosure - Re: Alphanumeric Shellcode Encoding and Detection

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Date: Tue, 5 Aug 2008 19:00:16 +0300
From: "Avraham Moshe Schneider" <Avraham.Schneider@...ddin.com>
To: <full-disclosure@...ts.grok.org.uk>
Subject: Re: Alphanumeric Shellcode Encoding and Detection

I fixed a couple of bugs -

1. The srand() function was called after calls to rand() - causing a fixed string in the decoder which an IDS could signature on
2. Case of ESP register pointing to the head of the decoder was not handled, it is fixed now, but needs to be randomized. Right now, in the case of ESP pointing to the shellcode, the following fixed string would exist at the head of the decoder routine: "TX4640"
This translates to:
_asm
{
	push esp;
	pop eax;
	xor al, 0x36;
	xor al, 0x30;
}

The '6' and the '0' can be any alphanumeric byte where the first is the second+6 or vice versa.

You may add alphanumeric NOP instructions in between and change the diff between the bytes accordingly.
The diff between the two XOR values should be the length of the resulting string.

I used the EAX register, as XOR'ing it with an immediate value is alphanumeric.

Regards,
Avri
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