Untrusted input reaches an interpreter that executes it as code instead of treating it as data.

* The injection model — untrusted input crosses a trust boundary into an interpreter, which executes it as code instead of treating it as data. *

An injection attack crosses a trust boundary — untrusted input reaches an interpreter and gets executed:

1. Interface allows untrusted input into the system
2. Untrusted input is forwarded to a subcomponent
3. Interpreter or parser executes the unintended malicious code

The attack crosses a trust boundary where malicious code embedded in input reaches a component with a command interface and gets executed.

Go deeper:

• OWASP Top 10 — A03: Injection — injection as untrusted data reaching an interpreter as code.
• OWASP Injection Prevention Cheat Sheet — the full injection family and the general defense pattern.