Can't find the vulnerability in this C program
Solution 1:
email is declared with char email[USR_LEN]; and USR_LEN is 16, but it is read with fscanf(stdin, "%31s", email);.
Also, the results of fscanf are not tested. The user can use Control-D (on Unix) to cause no input, leaving the buffers uninitialized, and then strcmp can overrun a buffer.