Most successful attacks on web applications don't involve a sophisticated zero-day exploit — they exploit well-known, well-documented mistakes that simply weren't fixed. The good news is that a fairly short list of practices closes off the overwhelming majority of real-world risk.
Validate and sanitize every input
Never trust data coming from a user, a form, or an API request. SQL injection, cross-site scripting, and a long list of other classic attacks all boil down to the same root cause: user-supplied input being treated as trusted code or trusted markup instead of as raw data. Validate types and formats server-side (never rely on client-side validation alone), and sanitize any HTML you render back to users.
Use parameterized queries, always
Building SQL queries by concatenating strings with user input is how SQL injection happens, full stop. Parameterized queries (or an ORM that handles this for you) treat user input strictly as data, never as executable query structure — this single habit eliminates an entire category of attack.
Enforce HTTPS everywhere
Every page, every API endpoint, every asset. Without it, data in transit — including session cookies and login credentials — can be intercepted on any network between the user and your server, no advanced skills required.
Hash passwords properly
Never store passwords in plain text, and never use fast general-purpose hashes like MD5 or SHA-1 for them. Use a purpose-built algorithm (bcrypt, scrypt, or Argon2) designed to be slow enough to resist brute-force attempts even if your database is ever exposed.
Keep dependencies up to date
A huge share of real-world breaches exploit a known vulnerability in an outdated library — one that already had a patch available. Run dependency audits regularly (most package managers have a built-in command for this) and actually act on what they surface.
Security isn't a feature you add at the end — it's a set of habits you apply continuously. The projects that get breached are almost always the ones that treated it as a checklist item instead.
Limit what each account can access
Apply the principle of least privilege: a user account, an API key, or a service should only be able to access exactly what it needs, nothing more. When (not if) a credential is eventually compromised, this principle determines whether the damage is contained or catastrophic.
Log and monitor
You can't respond to an incident you don't know is happening. Log authentication attempts, unusual access patterns, and errors, and actually review them — automated alerting on suspicious patterns is worth setting up early, not after the first incident.