Code Execution

In computer security, Code Execution is used to describe an attacker's ability to execute any commands of the attacker's choice on a target machine or in a target process. It is commonly used in arbitrary code execution vulnerability to describe a software bug that gives an attacker a way to execute arbitrary code.
A program that is designed to exploit such a vulnerability is called an arbitrary code execution exploit. Most of these vulnerabilities allow the execution of machine code and most exploits therefore inject and execute shellcode to give an attacker an easy way to manually run arbitrary commands.
The ability to trigger arbitrary code execution from one machine on another (especially via a wide-area network such as the Internet) is often referred to as remote code execution.
It is the worst effect a bug can have because it allows an attacker to completely take over the vulnerable process. From there the attacker can potentially take complete control over the machine the process is running on.
Arbitrary code execution vulnerabilities are commonly exploited by malware to run on a computer without the owner's consent or by an owner to run homebrew software on a device without the manufacturer's consent.
Arbitrary code execution is commonly achieved through control over the program counter (also known as the instruction pointer) of a running process. The instruction pointer points to the next instruction in the process that will be executed. Control over the value of the instruction pointer therefore gives control over which instruction is executed next.
In order to execute arbitrary code, many exploits inject code into the process (for example by sending input to it which gets stored in an input buffer) and use a vulnerability to change the instruction pointer to have it point to the injected code. The injected code will then automatically get executed.
This type of attack exploits the fact that Von Neumann architecture computers do not make a general distinction between code and data, so that malicious code can be camouflaged as harmless input data. Many newer CPUs have mechanisms to make this harder, such as a no-execute bit.
Once the invader can execute arbitrary code directly on the OS, there is often an attempt at a privilege escalation exploit in order to gain additional control. This may involve the kernel itself or an account such as Administrator, SYSTEM, or root. With or without this enhanced control, exploits have the potential to do severe damage or turn the computer into a zombie - but privilege escalation helps with hiding the attack from the legitimate administrator of the system.
An arbitrary remote code execution with privilege escalation vulnerability in widely-deployed software is thus the worst vulnerability sub-type of them all. If bugs of this kind become known, fixes are usually made available within a few hours.
A lesser kind of vulnerability is the denial of service vulnerability, where the target machine can be prevented from doing what it's supposed to do, but can't be made to positively do something the attacker wants it to do.

Preventing Code Execution Vulnerabilities
It is never advisable to utilize user supplied input for system calls. In cases where it is absolutely necessary it is vital to sanitize user input whenever passing values to system calls.
Any application that utilizes system calls should be suspect, as it is a good indicator that the web application is attempting to do something for which another solution is probably more appropriate. If user input is allowed to be passed to system calls it should be strictly controlled and limited to a predefined set of values.

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