The attacker uploads a Python reverse shell script to disguising it as an image by calling the file test.jpg.The attacker discovers a remote code execution (RCE) vulnerability in and also establishes that lets users upload their own images without testing whether the uploaded file is a valid image.Since getting a reverse shell is just one stage of an attack, here is an example chain of events that involves the use of a reverse shell: All the attacker needs is a machine that has a public (routable) IP address and a tool such as netcat to create the listener and bind shell access to it. On the other hand, firewalls usually do not limit outgoing connections at all, and neither do NATs, so there is nothing to stop an attacker from setting up a server on their own machine and establishing a reverse connection. Another reason might be that the server is located behind a NAT (network address translation) layer. This makes it impossible to establish a shell listener on the attacked server. The targeted servers usually allow connections only on specific ports, so, for example, a dedicated web server will only accept connections on ports 80 and 443. Reverse shells are often used by attackers because they are not stopped by the most common firewall configurations. In the case of a reverse shell, these roles are switched: the target machine initiates the connection to the user while the user’s computer listens for incoming connections on a specified port. The user initiates a remote shell connection while the target system listens for such connections. In a typical remote system access scenario, the user has the role of a client, and the target machine acts as a server. Note that many shells are network-enabled (for example, telnet or SSH) and let users send commands to the operating system remotely. The name shell comes from the fact that this is the outer layer of the operating system. This term is typically used to describe command-line interfaces created for that purpose, such as cmd.exe in Windows, but it can also be used to describe graphical user interfaces (GUIs). What is a shell?Ī shell is a program that lets users (or other programs) use operating system services. Malicious hackers often use reverse shells as a means to send commands to a compromised system. A reverse shell is a script or executable program that makes it possible to gain interactive shell access to a system through an outgoing connection from that system.
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