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System Design Masterclass · Security Testing Operations
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In February 2014, Apple shipped a catastrophic SSL/TLS vulnerability known as "goto fail." A duplicated line of code caused the SSL verification function to always succeed, meaning any attacker on the same Wi-Fi network could intercept encrypted traffic from any iOS or macOS device. The fix was literally deleting one line. A code review would have caught it in seconds.
That single duplicated goto fail; statement bypassed the entire certificate chain validation. Millions of devices were exposed for months. Apple's security team was world-class, but the code shipped without a second pair of eyes on that particular diff.
IBM Systems Sciences Institute research found that defects caught during design and code review cost 6.5x less to fix than defects found during testing, and 100x less than defects found after release. Microsoft's internal data from the Security Development Lifecycle showed that code reviews catch 60-65% of all defects before testing even begins.
Code reviews are not just about finding bugs. They spread security knowledge across teams. When a senior engineer flags a injection pattern in a PR, the author learns to avoid it forever. Over six months, the entire team starts writing more secure code. Google's engineering practices documentation calls this the "teaching effect," and they measure it by tracking how often the same reviewer needs to flag the same pattern for the same author.
Not every reviewer knows what to look for. A checklist turns security review from "use your judgment" into a repeatable process.
| Review Area | What to Look For | Common Vulnerabilities |
|---|---|---|
| Input handling | User input used without validation or sanitization | injection, XSS, command injection |
| Authentication | Hardcoded credentials, weak token generation, missing MFA checks | Credential exposure, broken auth |
| Authorization | Missing access checks, IDOR vulnerabilities, role bypass | Privilege escalation |
| Cryptography | Weak algorithms (MD5, SHA-1 for passwords), hardcoded keys, Math.random() for tokens | Data exposure |
| Error handling | Stack traces in responses, verbose error messages, caught exceptions silently swallowed | Information disclosure |
| Dependencies | Outdated libraries, known vulnerable versions, unnecessary packages | Supply chain attacks |
| Data exposure | Logging PII, returning sensitive fields in API responses, storing passwords in plaintext | Privacy violations, GDPR fines |
| File handling | Path traversal in uploads, unrestricted file types, missing size limits | Remote code execution |
Google requires every changelist (their term for PR) to be reviewed by at least one person with readability certification in that language. For security-sensitive code (auth, crypto, payment), they require a security-focused reviewer on top of the normal reviewer.
Shopify's security review process requires a dedicated security review for any PR that touches authentication, authorization, payment processing, or PII handling. Their internal data shows that security-focused reviews catch 3x more vulnerabilities than general reviews on the same code.