The following is an excerpt from the book Designing Secure Software: A Guide for Developers by Loren Kohnfelder, Copyright 2022, No Starch Press
“Testing leads to failure, and failure leads to understanding.”—Burt Rutan
To begin, it’s important to define what I mean by security testing. Most testing consists of exercising code to check that functionality works as intended. Security testing simply flips this around, ensuring that operations that should not be allowed aren’t (an example with code will shortly make this distinction clear).
Security testing is indispensable, because it ensures that mitigations are working. Given that coders reasonably focus on getting the intended functionality to work with normal use, attacks that do the unexpected can be difficult to fully anticipate. The material covered in the preceding chapters should immediately suggest numerous security testing possibilities. Here are some basic kinds of security test cases corresponding to the major classes of vulnerabilities (covered in detail in the book):
Integer overflows: Establish permitted ranges of values and ensure that detection and rejection of out-of-range values works.
Memory management problems: Test that the code handles extremely large data values correctly, and rejects them when they’re too big.
Untrusted inputs: Test various invalid inputs to ensure they are either rejected or converted to a valid form that is safely processed.
Web security: Ensure that HTTP downgrade attacks, invalid authentication and CSRF tokens, and XSS attacks fail (see the previous chapter for details on these).
Exception handling flaws: Force the code through its various exception handling paths (using dependency injection for rare ones) to check that it recovers reasonably.
What all of these tests have in common is that they are off the beaten path of normal usage, which is why they are easily forgotten. And since all these areas are ripe for attack, thorough testing makes a big difference. Security testing makes code more secure by anticipating such cases and confirming that the necessary protection mechanisms always work. In addition, for security-critical code, I recommend thorough code coverage to ensure the highest possible quality, since bugs in those areas tend to be devastating.
Security testing is likely the best way you can start making real improvements to application security, and it isn’t difficult to do. There are no public statistics for how much or how little security testing is done in the software industry, but the preponderance of recurrent vulnerabilities strongly suggests that it’s an enormous missed opportunity.
The Limits of Security Tests
Security testing aims to detect the potential major points of failure in code, but it will never cover all of the countless ways for code to go wrong. It’s possible to introduce a vulnerability that the tests we just wrote won’t detect, but it’s unlikely to happen inadvertently. Unless test coverage is extremely thorough the possibility of crafting a bug that slips through the tests remains; however, the major threat here is inadvertent bugs, so a modest set of security test cases can be quite effective.
Determining how thorough the security test cases need to be requires judgment, but the rules of thumb are clear:
Security testing is more important for code that is crucial to security.
The most important security tests often check for actions such as denying access, rejecting input, or failing (rather than success).
Security test cases should ensure that each of the key steps (in our example, the three hashes and the comparison of hashes) works correctly.
Having closely examined a real security vulnerability with a simple (if unexpected) cause, and how to security test for such eventualities, let’s consider the general case and see how we could have anticipated this sort of problem and proactively averted it.
Writing Security Test Cases
“A good test case is one that has a high probability of detecting an as yet undiscovered error.”—Glenford Myers
A security test case confirms that a specific security failure does not occur. These tests are motivated by the second of the Four Questions: what can go wrong? This differs from penetration testing, where honest people ethically pound on software to find vulnerabilities so they can be fixed before bad actors find them, in that it does not attempt to scope out all possible exploits. Security testing also differs from penetration testing in providing protection against future vulnerabilities being introduced.
A security test case checks that protective mechanisms work correctly, which often involves the rejection or neutralization of invalid inputs and disallowed operations. While nobody would have anticipated the GotoFail bug specifically, it’s easy to see that all of the if statements in the VerifyServerKeyExchange function are critical to security. In the general case, code like this calls for test coverage on each condition that enforces a security check. With that level of testing in place, when the extraneous goto creates a vulnerability, one of those test cases will fail and call the problem to your attention.
You should create security test cases when you write other unit tests, not as a reaction to finding vulnerabilities. Secure systems protect valuable resources by blocking improper actions, rejecting malicious inputs, denying access, and so forth. Create security test cases wherever such security mechanisms exist to ensure that unauthorized operations indeed fail.
General examples of commonplace security test cases include testing that login attempts with the wrong password fail, that unauthorized attempts to access kernel resources from user space fail, and that digital certificates that are invalid or malformed in various ways are always rejected. Reading the code is a great way to get ideas for good security test cases.
Security Regression Tests
“What regresses, never progresses.”—Umar ibn al-Khattâb
Once identified and fixed, security vulnerabilities are the last bugs we want to come back and bite us again. Yet this does happen, more often than it should, and when it does it’s a clear indication of insufficient security testing. When responding to a newly discovered security vulnerability, an important best practice is to create a security regression test that detects the underlying bug or bugs. This serves as a handy repro (a test case that reproduces the bug or bugs), as well as to confirm that the fix actually eliminates the vulnerability.
That’s the idea, anyway, but this practice seems to be less than diligently followed, even by the largest and most sophisticated software makers. For example, when Apple released iOS 12.4 in 2019, it reintroduced a bug identical to one already found and fixed in iOS 12.3, immediately re-enabling a vulnerability after that door should have been firmly closed. Had the original fix included a security regression test case, this should never have happened.
It’s notable that in some cases security regressions can be far worse than new vulnerabilities. That iOS regression was particularly painful because the bug was already familiar to the security research community, so they quickly adapted the existing jailbreak tool built for iOS 12.3 to work on iOS 12.4 (a jailbreak is an escalation of privilege circumventing restrictions imposed by the maker limiting what the user can do on their device).
I recommend writing the test case first, before tackling the actual fix. In an emergency, you might prioritize the fix if it’s clear-cut, but unless you’re working solo, having someone develop the regression test in parallel is a good practice. In the process of developing an effective regression test, you may learn more about the issue, and even get clues about related potential vulnerabilities.
A good security regression test should try more than a single specific test case that’s identical to a known attack; it should be more general. In addition to addressing the newly discovered vulnerability, it’s common that the investigation will suggest similar vulnerabilities elsewhere in the system that might also be exploitable. Use your superior knowledge of system internals and familiarity with the source code to stay ahead of potential adversaries. If possible, probe for the presence of similar bugs immediately, so you can fix them as part of the update that closes the original vulnerability. This can be important, since you can bet that attackers will also be thinking along these lines, and releasing a fix will be a big clue about new ways they might target your system. If there is no time to explore all the leads, file away the details for investigation later, when time permits.