Buffer Overflows

Buffer Overflows have been around since the very beginnings of the Von-Neuman architecture. They first gained widespread notoriety in 1988 with the Morris Internet worm. Unfortunately, the same basic attack remains effective today. By far the most common type of buffer overflow attack is based on corrupting the stack.
Most modern computer systems use a stack to pass arguments to procedures and to store local variables. A stack is a last in first out (LIFO) buffer in the high memory area of a process image. When a program invokes a function a new “stack frame” is created. This stack frame consists of the arguments passed to the function as well as a dynamic amount of local variable space. The “stack pointer” is a register that holds the current location of the top of the stack. Since this value is constantly changing as new values are pushed onto the top of the stack, many implementations also provide a “frame pointer” that is located near the beginning of a stack frame so that local variables can more easily be addressed relative to this value. The return address for function calls is also stored on the stack, and this is the cause of stack-overflow exploits since overflowing a local variable in a function can overwrite the return address of that function, potentially allowing a malicious user to execute any code he or she wants.
Although stack-based attacks are by far the most common, it would also be possible to overrun the stack with a heap-based (malloc/free) attack.
The C programming language does not perform automatic bounds checking on arrays or pointers as many other languages do. In addition, the standard C library is filled with a handful of very dangerous functions.

strcpy(char *dest, const char *src)    May overflow the dest buffer
strcat(char *dest, const char *src)    May overflow the dest buffer
getwd(char *buf)    May overflow the buf buffer
gets(char *s)    May overflow the s buffer
[vf]scanf(const char *format, …)    May overflow its arguments.
realpath(char *path, char resolved_path[])    May overflow the path buffer
[v]sprintf(char *str, const char *format, …)    May overflow the str buffer.

Example Buffer Overflow
The following example code contains a buffer overflow designed to overwrite the return address and skip the instruction immediately following the function call.

#include <stdio.h>

void manipulate(char *buffer) {
char newbuffer[80];
strcpy(newbuffer,buffer);
}

int main() {
char ch,buffer[4096];
int i=0;

while ((buffer[i++] = getchar()) != ‘\n’) {};

i=1;
manipulate(buffer);
i=2;
printf(“The value of i is : %d\n”,i);
return 0;
}

Let us examine what the memory image of this process would look like if we were to input 160 spaces into our little program before hitting return.
[XXX figure here!]
Obviously more malicious input can be devised to execute actual compiled instructions (such as exec(/bin/sh)).

Avoiding Buffer Overflows
The most straightforward solution to the problem of stack-overflows is to always use length restricted memory and string copy functions. strncpy and strncat are part of the standard C library. These functions accept a length value as a parameter which should be no larger than the size of the destination buffer. These functions will then copy up to `length’ bytes from the source to the destination. However there are a number of problems with these functions. Neither function guarantees NUL termination if the size of the input buffer is as large as the destination. The length parameter is also used inconsistently between strncpy and strncat so it is easy for programmers to get confused as to their proper usage. There is also a significant performance loss compared to strcpy when copying a short string into a large buffer since strncpy NUL fills up the size specified.