Application Security Best Practices — The Complete Guide

Almost every application breach traces back to a small, well-understood set of mistakes: an unescaped input, a query built by string concatenation, a session that never expires, an API endpoint that trusts the caller's user ID. The attackers know this list by heart. The good news is that so can you, and each item has a concrete, boring fix. This guide is the map: the vulnerabilities that actually get exploited, what each one looks like in code, and where the full fix lives.

The single most useful mental shift is to stop trusting input. Every value that crosses a trust boundary (a form field, a header, a URL parameter, a JSON body, a webhook) is attacker-controlled until you validate and encode it. Most of what follows is that one idea applied in different places.

The injection family #

SQL injection: the classic, still in the OWASP Top 10 because ORMs get bypassed for "just one raw query." The fix is parameterized queries every time, covered in preventing SQL injection.

Cross-site scripting (XSS): untrusted data rendered into a page as markup instead of text. Contextual output encoding and a strong Content-Security-Policy shut it down, walked through in preventing XSS.

Trusting the browser too much #

CSRF: the browser helpfully attaches cookies to requests your site did not intend to make. SameSite cookies plus anti-CSRF tokens fix it, explained in CSRF protection done right.

Missing security headers: a handful of response headers (CSP, HSTS, X-Content-Type-Options, and friends) turn off whole classes of attack for free. The full set is in the HTTP security headers guide.

Identity and access #

Broken authentication: weak password storage, no MFA, sessions that live forever, JWTs verified badly. The secure defaults are in authentication and session security.

API security: broken object-level authorization (IDOR/BOLA) is the number-one API bug, where the endpoint checks that you are logged in but not that the record is yours. That plus rate limiting and input validation is in API security best practices. For eliminating long-lived API keys and cloud credentials entirely, see keyless cloud authentication.

Knowing what you ship and what's in it #

Vulnerable dependencies: most of your code is other people's code. Continuous SCA scanning, plus where SAST and DAST fit, is in dependency and SCA scanning, which pairs with signed provenance in supply-chain security with SBOMs and attestation.

Finding problems before shipping: threat modeling for engineers is how you catch design-level flaws that no scanner will ever find. When something does slip through, handling vulnerabilities in production is the response playbook.

Deeper into specific vulnerabilities #

Each item above is a category. These go one level deeper into the specific bugs that keep showing up in real breaches:

Broken access control: the OWASP #1 category, and IDOR is its most common shape. What it looks like and how to close it: broken access control and IDOR explained.

SSRF: tricking your server into making requests on an attacker's behalf, often straight at the cloud metadata endpoint. Covered in SSRF explained.

Insecure deserialization: turning a serialized object back into attacker-controlled code execution. The mechanics and the fix are in insecure deserialization explained.

Security misconfiguration: default credentials, verbose errors, and unnecessary features left on. The checklist is in security misconfiguration.

JWT pitfalls: alg: none, weak signing secrets, and tokens that never expire. The specific failure modes are in JWT security vulnerabilities.

SAST vs DAST: which security testing catches which bugs, and why you need both. Compared in SAST vs DAST.

Secrets in git history: the leak that keeps happening even with good secrets management elsewhere. Prevention in secret scanning.

GraphQL-specific risks: introspection, query depth, and batching attacks that REST-focused API security advice misses. Covered in GraphQL security best practices.

Business logic flaws: the vulnerabilities no scanner finds because the code runs exactly as designed. Explained in business logic vulnerabilities.

The mental model #

Application security is not a product you buy, it's a set of defaults you make automatic: parameterize every query, encode every output, validate every input against an allowlist, authorize every object access, expire every session, and scan every dependency. Build these into frameworks, linters, and CI so the secure path is the easy path. A note on AI features: LLM-backed apps add prompt injection to the list, which is the same "don't trust input" lesson in a new place.

The call we'd make #

Start from the OWASP Top 10 as your checklist, not because it's complete but because it's what actually gets exploited: read the OWASP Top 10 explained first, then work down the specific fixes. Automate the boring parts (scanning, headers, encoding) so humans only spend attention on the design-level risks a tool can't see. Security done this way is cheap and durable; done reactively after a breach, it's neither.