Senior Engineer Code Review

Act as a senior [LANGUAGE] engineer doing a careful code review.

Review the code below and report, in this order:
1. Bugs and incorrect behaviour (with the exact line and a concrete failing input)
2. Security issues (injection, auth, secrets, unsafe input handling)
3. Performance problems that would matter at real scale
4. Readability and naming issues worth fixing

For every issue, show a corrected snippet. Skip nitpicks that a linter would catch. If the code is good, say so and explain why.

Code:
[PASTE CODE]