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Writing Small Shellcode In Windows
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SUMMARY
This paper describes an attempt to write Win32 shellcode that is as small
as possible, to perform a common task subject to reasonable constraints.
The solution presented implements a bindshell in 191 bytes of null-free
code, and outlines some general ideas for writing small shellcode.
DETAILS
Introduction:
Size is important for shellcode because when exploiting vulnerabilities in
compiled software we are often constrained in the amount of data we can
work with. Smaller solutions than ours are certainly possible, but at
this size the amount of work involved increases exponentially as each
additional byte is trimmed from the code.
It is assumed that the reader has some familiarity with x86 assembly
language.
The task to be performed by our code is as follows:
1. Bind a shell to port 6666.
2. Allow one connection to the shell.
3. Exit cleanly.
It must work on Windows NT4, 2000, XP and 2003, and will be launched
using:
void main()
{
unsigned char sc[256] = "";
strncpy(sc,
"shellcode goes here",
256);
__asm
{
lea eax, sc
push eax
ret
}
}
Hence, we can observe the following:
* The shellcode cannot contain any null bytes.
* The shellcode must be run from the stack.
* Winsock has not been initialised.
* We may assume that eax points to the start of our code.
Our full solution is in the Appendix to this paper. First, we present some
notes on the approach taken and some of the details of the solution.
The full whitepaper can be downloaded from:
<http://www.ngssoftware.com/papers/WritingSmallShellcode.pdf>
http://www.ngssoftware.com/papers/WritingSmallShellcode.pdf.
ADDITIONAL INFORMATION
The information has been provided by <mailto:daf@ngssoftware.com> Dafydd
Stuttard.
The original article can be found at:
<http://www.ngssoftware.com/papers/WritingSmallShellcode.pdf>
http://www.ngssoftware.com/papers/WritingSmallShellcode.pdf
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