Building Secure Authentication Systems
Password handling, session management, and multi-factor authentication done correctly — the fundamentals that most authentication vulnerabilities trace back to.
Authentication is one of the few systems where a subtle mistake can compromise every user at once. Getting the fundamentals right matters more than adding exotic security features on top of a weak foundation.
Never store passwords in a reversible form
Passwords should be hashed with a modern, slow hashing algorithm designed for this purpose (bcrypt, Argon2), never encrypted (reversible) or hashed with a fast general-purpose algorithm. This is table stakes, not a nice-to-have.
Session management fundamentals
- Session tokens are long, random, and unpredictable — never sequential or guessable.
- Sessions expire, and expiry is enforced server-side, not just client-side.
- Sessions are invalidated on password change and logout, not left valid.
Multi-factor authentication as a real second factor
A second factor needs to be something genuinely independent of the first (a separate device or channel), not a second password-like field. TOTP apps, WebAuthn/passkeys, and SMS (with its known weaknesses) are common options — WebAuthn is generally the strongest choice available today.
Rate limit authentication endpoints specifically
Login, password reset, and MFA verification endpoints are common targets for automated attacks. Rate limiting by IP and by account specifically on these endpoints prevents credential-stuffing and brute-force attempts.
Key Takeaways
- Hash passwords with a modern, slow algorithm — never store or encrypt them reversibly.
- Enforce session expiry and invalidation server-side, not just in the client.
- Rate limit authentication endpoints specifically, not just the API broadly.
Fastly Engineering Team
This article represents the collective engineering knowledge and standards of the Fastly team, not a single author.
Fastly Engineering
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Frequently Asked Questions
Is SMS-based multi-factor authentication secure enough?
It is better than no second factor, but has known weaknesses (SIM-swapping). WebAuthn/passkeys or an authenticator app are stronger choices where user adoption allows it.
Should we build authentication ourselves or use a provider?
For most teams, a well-audited authentication provider reduces risk compared to building password/session handling from scratch — the fundamentals above still matter for evaluating any provider or custom build.
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