인증 흐름 결함

Authentication failure modes don't fit neatly into one bug name. They're a cluster of small implementation mistakes that, individually, look minor and, collectively, are how account takeovers actually happen. Real attackers don't grind a single password — they walk every flow the auth system exposes (login, signup, password reset, OAuth, magic link, account merge) looking for the path with the weakest checks. The bug usually isn't 'no auth at all' — it's a token that lives 24 hours when it should live 30 minutes, an email check that races, an OAuth callback that doesn't validate state, a magic link that works the second time too. Each one is a single PR away from fixed; together they're how cover-the-NYT-cybersecurity-section breaches start.

Auth-flow weaknesses appear when login, reset, signup, or session flows reveal too much state or miss key abuse controls. They often compound with rate-limit and account-enumeration issues.

Account takeover without password theft. The attacker doesn't need credentials — they exploit the auth flow itself. Once in, they change email-on-file (locking out the victim), grant themselves admin permissions if any role-elevation flow exists, exfiltrate or modify data. In B2B SaaS, attacker takes over an admin account and pivots to org-level damage.

Use single-use, short-lived (≤30 min), high-entropy reset tokens generated from a CSPRNG. Bind reset tokens to the email and invalidate on first use. For signup atomicity, rely on a database unique constraint on email — let the DB enforce single-use, not application code. Always validate the OAuth state parameter on callback (the parameter should be a CSPRNG token bound to the originating session). Use PKCE for OAuth public clients (mobile apps, SPAs) — the verifier defeats authorization-code interception. Magic links: single-use, short expiry, bound to the requesting device fingerprint where possible. For account-merge scenarios, require email verification before merging (don't merge automatically when an OAuth flow returns an unverified email match). For new-user signups, always send confirmation email and don't allow the account to do anything until confirmation. Audit your auth flows by walking them yourself with the eye of someone trying to break in — the bugs usually surface in five minutes.

Auth flow security is the sum of small choices, not one big control. Get the small choices right and credential stuffing is your only real worry — which is a much smaller worry than the alternative.