Code injection is the exploitation of a computer bug that is caused by processing invalid data. The injection is used by an attacker to introduce (or "inject") code into a vulnerable computer program and change the course of execution. The result of successful code injection can be disastrous, for example, by allowing computer viruses or computer worms to propagate.

Code injection vulnerabilities occur when an application sends untrusted data to an interpreter. Injection flaws are most often found in SQL, LDAP, XPath, NoSQL queries, OS commands, XML parsers, SMTP headers, program arguments, etc. Injection flaws tend to be easier to discover when examining source code than via testing. Scanners and fuzzers can help find injection flaws.

Injection can result in data loss or corruption, lack of accountability, or denial of access. Injection can sometimes lead to complete host takeover.

Certain types of code injection are errors in interpretation, giving special meaning to user input. Similar interpretation errors exist outside the world of computer science such as the comedy routine Who's on First?. In the routine, there is a failure to distinguish proper names from regular words. Likewise, in some types of code injection, there is a failure to distinguish user input from system commands.

Code injection techniques are popular in system hacking or cracking to gain information, privilege escalation or unauthorized access to a system. Code injection can be used malevolently for many purposes, including:

• Arbitrarily modifying values in a database through SQL injection. The impact of this can range from website defacement to serious compromise of sensitive data.
• Installing malware or executing malevolent code on a server by injecting server scripting code (such as PHP or ASP).
• Privilege escalation to root permissions by exploiting Shell Injection vulnerabilities in a setuid root binary on UNIX, or Local System by exploiting a service on Microsoft Windows.
• Attacking web users with HTML/script injection (Cross-site scripting).

In 2008, 5.66% of all vulnerabilities reported that year were classified as Code Injection, the highest year on record. In 2015, this had decreased to 0.77%.