How to get started with writing a post module

Post module development is a challenge to your programming skills. It's not like writing a memory corruption based exploit, where technically speaking is usually about crafting a malicious input - a string. A post module is more about proper module design, practical knowledge in Ruby and the Metasploit library. It's also a very valuable skill to have, because if you don't know what to do after popping a shell, what's the point of the penetration test, right? Also, what if a module doesn't work? Are you willing to wait days, weeks, or maybe even months for someone else to fix it for you? Probably not. If you know how to do it yourself, you can probably fix it a lot sooner, and continue with your pentest and do more things. So learn post module development! It's good for you, and your career.

Plan your module

Just like writing a software, before you start coding you should have a clear and specific goal for what your post module does. It's never a good idea to have multiple functionalities in a single module. For example: having it steal the network configuration files, steal passwd, hashes, shell history, etc. Instead, you should break it down into multiple modules.

You should also think about what session types to support: meterpreter, or shell. Ideally, support both. But if you have to choose between the two, on Windows you should favor Windows Meterpreter. On Linux, the shell session type has been a stronger candidate than the Linux Meterpreter, but hopefully this will change in the near future. On platforms don't have a Meterpreter, obviously your only choice is a shell.

Another important thing is to think about how your module will perform on different distributions/systems. For example, say you want to run a ifconfig command on Linux. On Ubuntu it's a no-brainer, simply run the ifconfig command. Well, a different Linux distro might not actually know what you're asking, so you have to be more specific and do /sbin/ifconfig instead. Same thing with Windows. Is it C:\WINDOWS\ or C:\WinNT? It's both. Is it C:\Documents and Settings\[User name], or C:\Users\User name? Both, depends on that Windows version. A better solution to that would be use an environment variable :-)

Always do your homework, and contain as many scenarios you can think of. And most importantly, get your VMs and TEST!

Categories of post modules

Post modules are classified based on their behavior. For example, if it collects data, naturally it goes to the "gather" category. If it adds/updates/or removes an user, it belongs to "manage". Here's a list as a reference:

• #gather# - Modules that involve data gathering/collecting/enumeration.
• gather/credentials - Modules that involve stealing credentials.
• gather/forensics - Modules that involve forensics data gathering.