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Let’s take some common C and C++ code constructs and see what they look like in assembly. If you have a firm grasp of these examples, you should be ready to move forward with the rest of the book. Let’s look at declaring an integer in C++, then using that same integer for counting: int number; . . more code . . qxd:WileyRed 8 Part I 7/11/07 ■ 7:24 AM Page 8 Introduction to Exploitation: Linux on x86 This could be translated to, in assembly: number dw 0 . more code . . mov eax,number inc eax mov number,eax We use the Define Word (DW) instruction to define a value for our integer, number.

Determine the address of system(). 2. Determine the address of /bin/sh. 3. Find the address of exit(), so we can close the exploited program cleanly. qxd:WileyRed 7/11/07 7:25 AM Page 37 Chapter 2 ■ Stack Overflows The address of system() can be found within libc by simply disassembling any C or C++ program. gcc will include libc by default when compiling, so we can use the following simple program to find the address of system(): int main() { } Now, let’s find the address of system() with gdb: [root@0day local]# gdb file (gdb) break main Breakpoint 1 at 0x804832e (gdb) run Starting program: /usr/local/book/file Breakpoint 1, 0x0804832e in main () (gdb) p system $1 = {} 0x4203f2c0 (gdb) We see the address of system() is at 0x4203f2c0.

Finally, the prolog creates enough space on the stack for our local variable, array, at . “array” is 5 * 4 bytes in size (20 bytes), but the stack allocates 0x20 or 30 bytes of stack space for our locals. Overflowing Buffers on the Stack You should now have a solid understanding of what happens when a function is called and how it interacts with the stack. In this section, we are going to see what happens when we stuff too much data into a buffer. Once you have developed an understanding of what happens when a buffer is overflowed, we can move into more exciting material, namely exploiting a buffer overflow and taking control of execution.

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