Securing Unit Tests for Legacy Code: Principles and Practices Quiz

Explore the fundamentals and challenges of applying unit-testing techniques to legacy code with a focus on security testing. This quiz assesses knowledge of secure unit test strategies, code refactoring risks, and best practices for improving legacy system testability.

1. Identifying Challenges in Legacy Code

   Which challenge most commonly hinders effective unit testing for legacy codebases containing potential security flaws?

   1. Lack of modular design and hard-coded dependencies
   2. Having descriptive variable names
   3. Abundant code documentation
   4. High test coverage already present

   Explanation: Legacy code often lacks modular design and contains hard-coded dependencies, making it difficult to isolate units for testing, especially when diagnosing security vulnerabilities. While descriptive variable names and abundant documentation help maintain code, they do not typically impede unit testability. High test coverage would actually make testing easier, not harder. Therefore, the core issue is the lack of modularity and use of hard-coded dependencies.

2. Refactoring for Secure Unit Testing

   When introducing unit tests to legacy code, what is the safest initial approach to minimize introducing new security risks?

   1. Refactor code in small, incremental steps with thorough testing at each step
   2. Rewrite the entire codebase at once
   3. Immediately add external libraries for testing
   4. Disable security checks to simplify testing

   Explanation: Refactoring in small, incremental steps enables easier identification and elimination of issues, reducing the risk of introducing new security vulnerabilities. Rewriting the codebase at once greatly increases risk and can introduce more security flaws. Adding external libraries without caution may lead to compatibility or security issues, and disabling security checks, even temporarily, is a poor practice that can expose the system. Incremental and tested changes are safest for legacy code.

3. Mocking and Isolation in Security Testing

   Why is using mocking frameworks important when unit testing legacy code that interacts with sensitive external resources such as databases?

   1. They simulate external resources and prevent exposure of real data
   2. They generate production-ready code automatically
   3. They increase the performance of production deployment
   4. They update database schemas directly in tests

   Explanation: Mocking frameworks simulate external resources, enabling safer and more focused unit tests by preventing unintentional exposure or modification of sensitive data. They do not generate production code or increase runtime performance, and they should not be used to change production database schemas during tests. Properly used, mocking enhances test reliability and protects data.

4. Tests for Input Validation in Legacy Systems

   Which type of unit test is most effective for verifying that a legacy function correctly handles untrusted input to prevent security vulnerabilities such as injection attacks?

   1. Boundary value tests with special characters
   2. Happy path tests with valid input
   3. Testing code formatting functions
   4. Smoke tests after deployment

   Explanation: Testing with boundary values and special characters, especially those used in injection attacks, directly examines a function's resilience to untrusted inputs. Happy path tests do not cover malicious inputs, code formatting tests do not focus on input validation, and smoke tests are too high-level to catch function-specific vulnerabilities. Therefore, boundary value tests with relevant input are most effective.

5. Measuring Unit Test Coverage for Security

   What does code coverage analysis primarily help identify when security-testing legacy code with new unit tests?

   1. Portions of the codebase not exercised by tests
   2. The speed at which the tests run
   3. The amount of documentation per function
   4. How recent the code changes are

   Explanation: Code coverage analysis reveals which parts of the legacy code remain untested, highlighting possible zones where vulnerabilities may go undetected. While coverage does not indicate test performance speed, documentation volume, or code change recency, it is critical for assessing the thoroughness of security testing. This information helps target future security-related test development.