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Introduction to Shellcoding - How to Exploit Buffer Overflows
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SUMMARY
The paper provides a tutorial on how to write shellcodes for the Windows
and Unix environments, with a focus on Linux shellcodes.
The paper starts from a simple buffer overflow scenario, and explains how
to detect the vulnerability and how to write an appropriate shellcode for
it.
DETAILS
What Is a Shellcode?
Shellcode is a piece of machine-readable code, or script code that has
just one mission; to open up a command interpreter (shell) on the target
system so that an attacker can type in commands in the same fashion as a
regular authorized user or system administrator of that system can do
(with a few not-so-important exceptions of course). However, in order to
get remote access to the shell, you're going to need some kind of
networking support in that shellcode too. There's more to shellcoding than
just having a program execute /bin/sh or cmd.exe. This white paper will
introduce you to shellcodes, how they're used in practice, and how they
are used with buffer overflow vulnerabilities.
Since it's important that the shellcode is very small, the shellcode
hacker usually writes the code in the assembly programming language. In
this white paper I will be using x86 Intel syntax assembly under Linux.
The GNU compiler (gcc) uses AT&T syntax, which is somewhat different from
Intel syntax. All assembly examples can be compiled with Netwide Assembler
<http://nasm.sourceforge.net> nasm a portable Intel syntax assembler
available for a wide variety of operating systems. nasm is readily
available in most GNU/Linux distributions.
What About the Code in Shellcode?
Shellcode is primarily used to exploit buffer overflows (including heap
overflows) or format string bugs in binary, machine-readable software. In
these software, the shellcode has to be machine-readable too, and to make
things more complicated it can't contain any null bytes (0x00). Null (0)
is a string delimiter which instructs all C string functions (and other
implementations) to, once found, stop processing the string (thus, a
null-terminated string). There are other delimiters like linefeed (0x0A),
carriage return (0x0D), 0xFF, and others. Some depend on how the
programmer wrote the program (or the vulnerable function that handles
input) and other implementations depend on underlying C library functions
or 3rd party libraries, etc.
ADDITIONAL INFORMATION
The original article can be found at:
<http://tigerteam.se/dl/papers/intro_to_shellcoding.pdf>
http://tigerteam.se/dl/papers/intro_to_shellcoding.pdf
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