C Demonstration of Buffer Overflow

buffer.c

/**
 * This Program Only Works in DOS-BOX on Turbo C++ Compiler
 * To Get DOS-BOX emulator with Turbo C : https://www.google.com/search?q=turbo+C
 *
 * @author Tomrock D'souza, St. Francis Institute Of Technology, University of Mumbai, 2017
 * Email: [email protected]
 * No reproduction in whole or part without maintaining this notice
 */
#include<stdio.h>
#include<conio.h>
#include<string.h>
void main()
{
    int i;
    char u[8], p[8], *a;
	// Assigning adress 9th value of char array to a.
	// It is clearly seen the "a" is using a adress space outside the scope of the declared array.
    a = &u[9];
	//data in "a" is the number of attempts a user can retry a wrong User Name Password Combination
    *a = 3;
    printf("User:");
    gets(u);
	// If user enters user name more than 9 characters the address *a will be overwriten
	// This will lead to undesirable results i.e. data in "a" will take the value of the 10th characters ASCII value
	// This will increase the number of attempts
    for (i = 0; i < (*a); i++) {
        printf("pass:");
        gets(p);
        if (strcmp(p, "password")) {
            printf("Attempts remaining= %d\n", (*a) - i - 1);
        }
        else {
            printf("Password Matched");
            break;
        }
    }
    getch();
}