Exploit Exercises – Nebula – Level 04

The information about this level says:

This level requires you to read the token file, but the code restricts the files that can be read. Find a way to bypass it 🙂
To do this level, log in as the level04 account with the password level04 . Files for this level can be found in /home/flag04.

It also contains some source code:

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#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>
 
int main(int argc, char **argv, char **envp)
{
    char buf[1024];
    int fd, rc;
 
    if(argc == 1) {
        printf("%s [file to read]\n", argv[0]);
        exit(EXIT_FAILURE);
    }
 
    if(strstr(argv[1], "token") != NULL) {
        printf("You may not access '%s'\n", argv[1]);
        exit(EXIT_FAILURE);
    }
 
    fd = open(argv[1], O_RDONLY);
    if(fd == -1) {
        err(EXIT_FAILURE, "Unable to open %s", argv[1]);
    }
 
    rc = read(fd, buf, sizeof(buf));
 
    if(rc == -1) {
        err(EXIT_FAILURE, "Unable to read fd %d", fd);
    }
 
    write(1, buf, rc);
}

Its fairly clear from looking at the files and the source code (I will admit I had to use a lot of man to help me understand the source code) that I want to read the contents of “token”, but the program won’t allow it. I tried things like ./token and ../flag04/token, but that didn’t work because the program is just searching for the string “token” anywhere in the first argument. Well… how do I get the contents of that file “into” another file without having permission to read the file? Symbolic link! Here’s what I did:

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ln -s ~flag04/token /tmp/link
~flag04/flag04 /tmp/link

It turns out that the contents of the token file is the password for the flag04 account so I just did su flag04 and used that password. I ran getflag and violà!

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