Many forms of code injection (for instance cross-site scripting and SQL injection) rely upon the instantaneous execution of the embedded code to carry out the attack (e.g. stealing a user's current session information or executing a modified SQL query). In some cases it may be possible for an attacker to inject their malicious code into a data storage area that may be executed at a later date or time. Depending upon the nature of the application and the way the malicious data is stored or rendered, the attacker may be able to conduct a second-order code injection attack.

By Gunter Ollmann at WindowSecurity.com

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