Shells
Types of Shells
At a high level, we are interested in two kinds of shell when it comes to exploiting a target: Reverse shells, and bind shells.
List Installed Shells in Linux
cat /etc/shells
Reverse shells
are when the target is forced to execute code that connects back to your computer. On your own computer you would use one of the tools mentioned to set up a listener which would be used to receive the connection. Reverse shells are a good way to bypass firewall rules that may prevent you from connecting to arbitrary ports on the target; however, the drawback is that, when receiving a shell from a machine across the internet, you would need to configure your own network to accept the shell. This, however, will not be a problem on the TryHackMe network due to the method by which we connect into the network.
Bind shells
are when the code executed on the target is used to start a listener attached to a shell directly on the target. This would then be opened up to the internet, meaning you can connect to the port that the code has opened and obtain remote code execution that way. This has the advantage of not requiring any configuration on your own network, but may be prevented by firewalls protecting the target.
Interactive Shells
If you've used Powershell, Bash, Zsh, sh, or any other standard CLI environment then you will be used to interactive shells. These allow you to interact with programs after executing them. For example, take the SSH login prompt
it's asking interactively that the user type either yes or no in order to continue the connection. This is an interactive program, which requires an interactive shell in order to run.
Non-Interactive shells
In a non-interactive shell you are limited to using programs which do not require user interaction in order to run properly. Unfortunately, the majority of simple reverse and bind shells are non-interactive, which can make further exploitation trickier. Let's see what happens when we try to run SSH in a non-interactive shell:
Notice that the whoami command (which is non-interactive) executes perfectly, but the ssh command (which is interactive) gives us no output at all. As an interesting side note, the output of an interactive command does go somewhere (redirect the input, output, error descriptors to a file). Suffice to say that interactive programs do not work in non-interactive shells.
Takeaway from Shells
Reverse and Bind shells are an essential technique for gaining remote code execution on a machine, however, they will never be as fully featured as a native shell. Ideally we always want to escalate into using a "normal" method for accessing the machine, as this will invariably be easier to use for further exploitation of the target.
Linux
On Linux ideally we would be looking for opportunities to gain access to a user account. SSH keys stored at /home/<user>/.ssh are often an ideal way to do this. In CTFs it's also not infrequent to find credentials lying around somewhere on the box. Some exploits will also allow you to add your own account. In particular something like Dirty C0w or a writeable /etc/shadow or /etc/passwd would quickly give you SSH access to the machine, assuming SSH is open.
Windows
On Windows the options are often more limited. It's sometimes possible to find passwords for running services in the registry. VNC servers, for example, frequently leave passwords in the registry stored in plaintext. Some versions of the FileZilla FTP server also leave credentials in an XML file at C:\Program Files\FileZilla Server\FileZilla Server.xml or C:\xampp\FileZilla Server\FileZilla Server.xml. These can be MD5 hashes or in plaintext, depending on the version.
Ideally on Windows you would obtain a shell running as the SYSTEM user, or an administrator account running with high privileges. In such a situation it's possible to simply add your own account (in the administrators group) to the machine, then log in over RDP, telnet, winexe, psexec, WinRM or any number of other methods, dependent on the services running on the box. The syntax for this is as follows:
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