Shellcode is a sequence of machine code injected into a computer’s memory to take control of a running program. Exploiting vulnerabilities in memory allocation and input validation, it can be written in low-level languages or scripting languages. Local and remote exploits can grant access to a machine or process.
Shellcode is a sequence of machine code, or executable instructions, that is injected into a computer’s memory with the intent of taking control of a running program. In such an attack, one of the steps is to gain control of the program counter, which identifies the next instruction to execute. The program flow can then be redirected to the inserted code. The intrusive machine code is called the attack payload and is commonly referred to as shellcode. This method was often used to grant access to an attacker by opening an operating system command shell, so code injection attacks in general came to be known as shellcode.
The exploited vulnerability typically affects the way a program allocates memory, checks the validity of input data, and handles memory errors. Software developers can usually avoid this threat by strictly defining input data and rejecting improper values. If unchecked, values that require more storage space than the memory allocated for that value may be accepted. This causes a security violation called a buffer overflow, in which part of the data is written to memory locations adjacent to the space allotted for the value. If handled carefully, this anomaly can allow the introduction of intrusive code.
Shellcode is normally written in a low-level system access programming language such as assembly, C, or C++. Depending on the targeted vulnerability, however, the same result could be achieved by inserting code for an interpreted scripting language, such as PERL, or bytecode for a virtual machine, such as JAVA. The code can be implanted before, during, or after the program counter hijack. Thus, the intrusive code may be contained in a file on the target machine or transmitted over a network connection in real time.
Local shellcode exploits are designed to give an attacker control over a machine to which they have physical access. A goal here might be to create an account with administrator privileges, for example. Similarly, if a running process has a high privilege level, a successful exploit would temporarily grant the same level of access to the intruder.
Processes running on a remote machine can be exploited by using standard network protocols to communicate with the target computer. The Connect-back shell code instructs the target machine to initiate a connection to the intruder’s computer. If the intruder initiates the connection, the code is called a bindshell, as it attempts to take control of a network connection called a port on the remote machine. The connection method is more popular, as firewalls rarely prohibit outgoing connections.
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