Understanding reverse shells
Shell Game
Firewalls block shell access from outside the network. But what if the shell is launched from the inside?
Recently, I've thoroughly enjoyed brushing up my offensive security skills. I've worked in the defensive security field for longer than I care to remember, and gaining more insight into how attackers perceive the world has really opened my eyes. My background is two-and-half decades of Linux and securing containers over the last seven years or so. An area that always piques my interest is Linux-based local privilege escalation. Once you have found a way of gaining access to a machine, the Holy Grail is elevating your privileges to the root user so you have full control.
Sometimes achieving root can take a little time. As an attacker, it is important to be able to return at a later date if you haven't achieved root user privileges yet or you want to monitor changeable data on a machine. Penetration testers and attackers would call this ongoing access persistence, which is the ability to gain a foothold and then maintain access; you might also call it creating a backdoor.
Attackers have a multitude of ways for ensuring that, if a machine reboots or some other event occurs, a backdoor is re-established automatically. This article looks at reverse shells and provides some examples of how to achieve persistence once you have gained access to a Linux machine. It should go without saying that you should use the following information for testing, practicing, and improving your knowledge and not for some nefarious purpose.
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