Learn from published fixes responsibly

1. Choose a published, already-fixed security issue in a training program or codebase you own.

2. Read the maintainer's advisory before inspecting the patch diff.

3. Identify which input check, boundary or permission rule changed.

4. Write a harmless regression test for the corrected behavior in your own local copy.

5. Exercise: describe the difference between root cause, symptom and security impact.

Scope: Education and testing on owned or explicitly authorized systems only.