TryHackMe: Complete Beginner

author: Nathan Acks
date: 2021-10-08

Upload Vulnerabilities

Overwriting Existing Files

One reason to randomize upload names is to prevent file overwrites. If a webapp isn’t doing this (or using some other form of isolating upload files), then it’s possible to inject arbitrary files into it via its upload mechanism (of course, whether or not those do anything is a separate question…).

Remote Code Execution

The gobuster tool is fuzzing/enumeration tool that can be used to find upload directories. Example invocation:

gobuseter dir -u $TARGET_URL -w $DIR_WORDLIST

The /usr/share/wordlists/dirbuster directory holds lists of various common directory names.

The simplest PHP web shell:

<?php  
    echo system($_GET["cmd"]);  
?>

This takes a GET variable cmd and executes it (semicolons, output redirection, and the like are allowed).

[PHP system() command documentation.(https://www.php.net/manual/en/function.system.php)]

Where web shells get the web server to execute commands for us, reverse shells actually fire up a true shell and connect back to the client machine; this is the trick we’ve used a couple of times now with nc -lvnp $PORT.

The Kali Linux webshells package provides a variety of webshells, some of which are reverse shells. A common option is /usr/share/webshells/php/php-reverse-shell.php.

Filtering

Client-side filtering – obviously easy to bypass!

Server-side filtering – take your best guess at a conforming upload.

Common types of filtering:

Apparently PHPv5 and higher has a poison null byte resistant extension filter.