A reverse shell is a fundamental technique used during the "Gaining Access" and "Maintaining Access" phases of a penetration test. In a standard (bind) shell, the attacker connects to a specific port on the victim's machine to gain command-line access. However, most modern firewalls block incoming connections to unauthorized ports. To bypass this, a reverse shell reverses the connection logic: the victim's machine is tricked into initiating anoutgoingconnection to the attacker's machine, which is "listening" for the call.

This technique is highly effective because firewalls are typically much more permissive with "egress" (outgoing) traffic than with "ingress" (incoming) traffic. For example, an attacker might host a listener on port 443 (HTTPS). Since most organizations allow internal machines to browse the web over port 443, the firewall perceives the reverse shell connection as standard web traffic and allows it to pass. Once the connection is established, the attacker has a terminal interface on the victim's machine, allowing them to execute commands remotely.

In professional pentesting, establishing a reverse shell is often the primary goal of an exploit. It provides the "foothold" needed for lateral movement and privilege escalation. Common tools used to create reverse shells include Netcat (nc), Bash, and Python scripts. To defend against this, organizations must implement "Egress Filtering," which restricts outgoing traffic to only known, necessary destinations. Security professionals also monitor for "long-lived" connections to unusual IP addresses, as these can be a tell-tale sign of an active reverse shell. Understanding how these connections manipulate network policy is crucial for any ethical hacker seeking to demonstrate how internal systems can be compromised despite robust perimeter defenses.