How to overflow the result through inputs without editing code

Question

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

char username[64], password[64], hostname[64];

int main(int argc, char **argv) {
        char result[256];

        if (argc != 4 ||
                strlen(argv[1]) > sizeof(username) ||
                strlen(argv[2]) > sizeof(password) ||
                strlen(argv[3]) > sizeof(hostname)) {
                fprintf(stderr, "bad arguments\n");
                return -1;
        }

        strcpy(username, argv[1]);
        strcpy(password, argv[2]);
        strcpy(hostname, argv[3]);

        result[0] = 0;
        strcat(result, "http://");
        strcat(result, username);
        strcat(result, ":");
        strcat(result, password);
        strcat(result, "@");
        strcat(result, hostname);
        strcat(result, "/");

        printf("%s\n", result);
        return 0;
}

Answer 1

Note that your size comparisons are off-by-one.

strlen(argv[1]) > sizeof(username) ||
strlen(argv[2]) > sizeof(password) ||
strlen(argv[3]) > sizeof(hostname)

In the event a string length is 64, there won't be room for the null-terminating byte, thus overflowing the buffer(s).

The fix is the >= operator.

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As an aside: the copies are meaningless; lengths can be asserted, and the contents of argv used directly instead.