Stacked Buffer Overflow

Exploiting stack buffer overflows

The canonical method for exploiting a stack based buffer overflow is to overwrite the function return address with a pointer to attacker-controlled data (usually on the stack itself). This is illustrated in the example below:

An example with strcpy

void foo (char *bar)
char c[12];

strcpy(c, bar); // no bounds checking…

int main (int argc, char **argv)

This code takes an argument from the command line and copies it to a local stack variable c. This works fine for command line arguments smaller than 12 characters . Any arguments larger than 11 characters long will result in corruption of the stack. (The maximum number of characters that is safe is one less than the size of the buffer here because in the C programming language strings are delimited by a zero byte character. A twelve-character input thus requires thirteen bytes to store the input followed by the sentinel zero byte. The zero byte then ends up overwriting a memory location that's one byte beyond the end of the buffer.)

When an argument larger than 11 bytes is supplied on the command line foo() overwrites local stack data, the saved frame pointer, and most importantly, the return address. When foo() returns it pops the return address off the stack and jumps to that address (i.e. starts executing instructions from that address). The attacker has overwritten the return address with a pointer to the stack buffer char c, which now contains attacker supplied data. In an actual stack buffer overflow exploit the string of "A"'s would be replaced with shellcode suitable to the platform and desired function. If this program had special privileges (e.g. the SUID bit set to run as the superuser), then the attacker could use this vulnerability to gain superuser privileges on the affected machine.

Platform related differences

A number of platforms have subtle differences in their implementation of the call stack that can affect the way a stack buffer overflow exploit will work. Some machine architectures store the top level return address of the call stack in a register. This means that any overwritten return address will not be used until a later unwinding of the call stack. Another example of a machine specific detail that can affect the choice of exploitation techniques is the fact that most RISC style machine architectures will not allow unaligned access to memory. Combined with a fixed length for machine opcodes this machine limitation can make the jmp to ESP technique almost impossible to implement (with the one exception being when the program actually contains the unlikely code to explicitly jmp to the stack register).

Stacks that grow up

Within the topic of stack buffer overflows an often discussed but rarely seen architecture is one in which the stack grows in the opposite direction. This change in architecture is frequently suggested as a solution to the stack buffer overflow problem because any overflow of a stack buffer that occurs within the same stack frame can not overwrite the return pointer. Further investigation of this claimed protection finds it to be a naïve solution at best. Any overflow that occurs in a buffer from a previous stack frame will still overwrite a return pointer and allow for malicious exploitation of the bug. For instance, in the example above, the return pointer for foo will not be overwritten because the overflow actually occurs within the stack frame for strcpy. However, because the buffer that overflows during the call to strcpy resides in a previous stack frame, the return pointer for strcpy will have a numerically higher memory address than the buffer. This means that instead of the return pointer for foo being overwritten, the return pointer for strcpy will be overwritten. At most this means that growing the stack in the opposite direction will change some details of how stack buffer overflows are exploitable, but it will not reduce significantly in the number of exploitable bugs.

Protection schemes

Main article: Stack-smashing protection

Over the years a number of schemes have been developed to inhibit malicious stack buffer overflow exploitation. These usually have taken one of two forms. The first method is to detect that a stack buffer overflow has occurred and thus prevent redirection of the instruction pointer to malicious code. The second attempts to prevent the execution of malicious code from the stack without directly detecting the stack buffer overflow.

Stack canaries

Stack canaries, so named because they operate as a canary in the coal mine so to speak, are used to detect a stack buffer overflow before execution of malicious code can occur. This method works by placing a small integer, the value of which is randomly chosen at program start, in memory just before the stack return pointer. Most buffer overflows overwrite memory from lower to higher memory addresses, so in order to overwrite the return pointer (and thus take control of the process) the canary value must also be overwritten. This value is checked to make sure it has not changed before a routine uses the return pointer on the stack. This technique can greatly increase the difficulty of exploiting a stack buffer overflow because it forces the attacker to gain control of the instruction pointer by some nontraditional means such as corrupting other important variables on the stack.

Nonexecutable stack

Main article: Data Execution Prevention

Another approach to preventing stack buffer overflow exploitation is to enforce memory policy on stack memory region to disallow execution from the stack. This means that in order to execute shellcode from the stack an attacker must either find a way to disable the execution protection from memory, or find a way to put his shellcode payload in a non-protected region of memory. This method is becoming more popular now that hardware support for the no-execute flag is available in most desktop processors. While this method definitely makes the canonical approach to stack buffer overflow exploitation fail it is not without its problems. First it is common to find ways to store shellcode in unprotected memory regions like the heap, and so very little need change in the way of exploitation. Even if this were not so there are other ways. The most damning is the so called return to libc method for shellcode creation. In this attack the malicious payload will load the stack not with shellcode, but with a proper call stack so that execution is vectored to a chain of standard library calls, usually with the effect of disabling memory execute protections and allowing shellcode to run as normal. This works because the execution never actually vectors to the stack itself. Still if used in conjunction with techniques like ASLR a nonexecutable stack can be somewhat resistant to return to libc attacks and thus can greatly improve the security of an application.

Notable examples

* The Morris worm spread in part by exploiting a stack buffer overflow in the Unix finger server.
* The Witty worm spread by exploiting a stack buffer overflow in the Internet Security Systems BlackICE Desktop Agent.
* The Slammer worm spread by exploiting a stack buffer overflow in Microsoft's SQL server.

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