[fd]
After you SSH into the machine you see 3 files: the executable, source code, and the flag file. I took a look into the source code first:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char buf[32];
int main(int argc, char* argv[], char* envp[]){
if(argc<2){
printf("pass argv[1] a number\n");
return 0;
}
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);
}
printf("learn about Linux file IO\n");
return 0;
}
It seems that string compare is being used here. I will run the code locally so that way I am able to manipulate it as I want:
I edited the source code on my local machine:
The lines highlighted in red are my additions.
I looked over the source code, and noticed that if I entered all 0s, then this is the output I got:
I was curious to see what 4660 was. It turns out that is what 0x1234 is in hex:
I then ran the code (on the pwnable.kr site) with this number:
It seemed that the code was waiting for a response. I entered "LETMEWIN", since I saw that in the source code, and I then got the flag: