MiniSTRyplace was a 1-star rated ‘Web’ challenge from the HackTheBox Cyber Apocalypse CTF. The solution was pretty simple, with a vulnerable str_replace function allowing for a simple path traversal exploit.
Initially, the files for the server were supplied as part of the challenge. From a quick initial search, the index.php file stood out as being interesting due to its logic for including various PHP files within the main webpage of the site.
The vulnerable function
Looking at the code, the server will include a file from within the pages/ directory, based on the lang parameter when passed as a GET request. Typically this would be via a request such as vulnerable_site.com/index.php?lang=en.php or similar. The function will then replace any occurrences of ../ with a blank string. From a brief glance, this appears to be alright, but if an attacker uses a a string such as ....//, PHP will remove the central ../ (i.e. ....// ), leaving a final string of ../, allowing for path traversal.
We can test this exploit by using a URL such as: vulnerable_site.com/?lang=qw.php….//….//….//….//….//….//etc/passwd, which returns the contents of the passwd file as shown below.
Contents of the passwd file returned by the server, indicating we have LFI capabilities
Looking at the Dockerfile, we know that the flag is copied to the root directory, so we can simply traverse up to the root folder, and then load the flag file. As shown below, this reveals a flag of CHTB{b4d_4li3n_pr0gr4m1ng} for MiniSTRyplace!
To learn more about this, HackTheBox have a really good Academy article which covers a range of methods for identifying and exploiting LFI vulnerabilities. These commonly come up in CTFs and OSCP boxes, so it is a good skill to get comfortable with!
If you enjoyed this writeup, I have written up several other boxes at this link.
BlitzProp is a 1* rated challenge from the web category of the HTB CTF. To exploit this, you need to use a ‘prototype pollution’ vulnerability in order to gain RCE against the target. This was the first time I had exploited a target using a vulnerability such as this, so I learned a lot from this challenge!
To start off, a downloadable zip file is provided, containing the contents of the BlitzProp server and the related Docker infrastructure. The file which caught my interest initially was the routes/index.js file, which we can see below:
The routes/index.js file from the challenge
At a first glance, it looks like a pretty standard API based site, with the use of a number of node libraries. The song names seemed a little odd, though it is a cyber-space themed CTF, so I didn’t think too much into it initially. Looking at the package.json file revealed the versions of the packages in use by node.
package.json showing details on vulnerable packages
A good site I have found to lookup vulnerable packages is snyk.io. Looking up the packages above, we find a couple of vulnerabilities which look interesting:
The song names made me realise I was on the correct track – ‘Not polluting with the boys’ and ‘ASTa la vista baby’ both relate to prototype pollution! To try and exploit this, I attempted to modify the PoC included with the Flat 5.0.0 vulnerability. I modified the API call we were making from the website to attempt to pollute an object. The API call I went with was {"song.name":{'__proto__.includes': {return res.json({'test':'123'})}}}. This request then resulted in a JSON parsing error, indicating I was along the right lines.
After a lot of googling and searching for approchable tutorials on how to exploit this, I found a great blog at https://blog.p6.is/AST-Injection. One of the examples from this site includes an exploit for pug and so we now had a better PoC which should allow us to get an RCE with some modification. The target in this case is 178.x.x.x and our ‘attacker’ is on 52.x.x.x.
One of the interesting things about this attack, is that we have to submit two queries, one to pollute the prototype, and another to trigger the RCE. Executing this PoC proved that we were getting very close to a working exploit, as it returned a bash error, as shown below. This error was due to the target using /bin/ash, rather than /bin/bash.
Not wanting to get into a drawn-out battle with the various Linux shells, I decided to keep things simple and use a basic Python reverse shell which I have used a number of times during OSCP. By replacing the bash reverse shell used above with this, we end up with the updated exploit below.
We now can execute this exploit, and get a reverse shell back on our attacker VM. From this we can easily find the flag (flagsF9iN) within the current directory. Revealing a flag of CHTB{p0llute_with_styl3}.
Overall, I really enjoyed BlitzProp and learned some new techniques! If you enjoyed this writeup, I have written up several other boxes at this link.
