fd

There are 2 things that we need to achieve in this challenge to get the flag.

• Make read function get input from stdin (Standard input)

• Successfully pass the if condition with LETMEWIN string in buf

read function in line 3 is dependent upon the variable fd (Line 1) which is user-controlled by the value of argv[1]

Our target is fd = 0 (0 - Standard Input File Descriptor [stuff we type using our keyboards]). If we are able to get fd = 0, we will be able to store the value "LETMEWIN" into the buf and successfully pass the strcmp function and get the flag.

int fd = atoi( argv[1] ) - 0x1234;
int len = 0;
len = read(fd, buf, 32);
if(!strcmp("LETMEWIN\n", buf)){
    printf("good job :)\n");
    system("/bin/cat flag");
    exit(0);
}

Converting the hex value of 0x1234 to integer we get 4660.

# To get integer value from a hex value
python -c 'print int("1234", 16)'

Now we have the integer value that if passed to the program, will make it wait for user input, which will be stored into the buf variable for strcmp.