3 min readJul 15, 2021


Hello everyone , in this post I will be sharing my writeup for HTB Shocker which was an easy linux machine, there were only 2 port http and ssh . On http we were presented with a static web page and running gobuster or dirsearch didn’t helped us . Searching the apache version for any exploits , it came back with shellshock vulnerability so it means that there must be cgi-bin directory on webserver , of fuzzing that directory it found a bash script file, on that we can do remote code execution to give us a reverse shell. On having the shell we can just check which files we can run as sudo so with sudo -l we were able to see that we have permissions to run perl as sudo which can give us the root shell.


80/tcp open http syn-ack ttl 63 Apache httpd 2.4.18 ((Ubuntu))
| http-methods:
|_ Supported Methods: GET HEAD POST OPTIONS
|_http-server-header: Apache/2.4.18 (Ubuntu)
|_http-title: Site doesn't have a title (text/html).
2222/tcp open ssh syn-ack ttl 63 OpenSSH 7.2p2 Ubuntu 4ubuntu2.2 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey:
| 2048 c4:f8:ad:e8:f8:04:77:de:cf:15:0d:63:0a:18:7e:49 (RSA)
| ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQD8ArTOHWzqhwcyAZWc2CmxfLmVVTwfLZf0zhCBREGCpS2WC3NhAKQ2zefCHCU8XTC8hY9ta5ocU+p7S52OGHlaG7HuA5Xlnihl1INNsMX7gp
| 256 22:8f:b1:97:bf:0f:17:08:fc:7e:2c:8f:e9:77:3a:48 (ECDSA)
| 256 e6:ac:27:a3:b5:a9:f1:12:3c:34:a5:5d:5b:eb:3d:e9 (ED25519)
|_ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIC/RjKhT/2YPlCgFQLx+gOXhC6W3A3raTzjlXQMT8Msk
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel


On the web server we only get a static html page

But there wasn’t anything on this page , so after searching exploits for apache 2.4.18 , I saw a vulnerability called shellshock

So maybe there’s a folder cgi-bin on web server but on running gobuster and dirsearch it wasn't showing so I assumed it's on the webserver and I fuzz for files with the extensions pl,rb,sh,py

We can see user.sh is in cgi-bin directory ,cgi-bin is a folder used to house scripts that will interact with a Web browser to provide functionality for a Web page or website. Common Gateway Interface (CGI) is a resource for accommodating the use of scripts in Web design.

Now let’s check if it’s vulnerable to shellshock

curl -H "user-agent: () { :; }; echo; echo; /bin/bash -c 'cat /etc/passwd'" \

We can get a bash reverse shell

Now to escalate to a user , in this case we have only a root user to escalate to , we can check what we can run as sudo so in order to do that we’ll run the command sudo -l

We can run perl as root so let's visit GTFOBINS to see how we can abuse this




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