Authentication Bypass: Eight Techniques Pen-Testers Use
Authentication is high-stakes, high-complexity, and high-velocity — three properties that produce predictable bugs. Many authentication bypasses fall into one of eight patterns. Here is the checklist.
Web applications · By Thimmagurudu Bathini · Published · 6 min read · Last reviewed
1. Mass assignment on signup
The signup endpoint accepts a JSON body that maps directly to a user model. The model has fields the user should not control — is_admin, tenant_id, role. The first signup with the right extra field creates a privileged account.
2. Password reset token leakage
Tokens that are short, predictable, never expire, or sent in the URL referrer to third-party trackers. Email enumeration via timing differences in the reset endpoint is a related, common finding.
3. Session fixation
The session ID does not rotate after login. An attacker who plants a session ID via a non-HTTPS subdomain or a link inherits the post-login session.
4. MFA bypass
Step-up MFA implemented client-side. The post-MFA endpoint accepts a flag like ?mfa_verified=true that the client controls. Or MFA can be skipped via a legacy mobile API endpoint that was never updated.
5. OAuth and SAML flow bugs
State parameter not validated (CSRF on the callback), redirect_uri allowlist too permissive (open redirect leaks auth code), implicit flow when authorization code would have been correct.
6. JWT misuse
Algorithm confusion (HS256 verified with the RSA public key), unverified kid header, expired tokens still accepted. Deep dive in JWT attacks.
7. Predictable session identifiers
Custom session implementations that are not cryptographically random. Session identifiers that are only base64-encoded user IDs are a known real-world pattern.
8. Account lockout missing or naive
No rate limit on login, password reset, MFA challenge — credential stuffing trivial. Or lockout per username (denial of service against any user) instead of per IP plus per username.
If any of these worry you, the web app VAPT covers authentication as a dedicated phase.
Written by Thimmagurudu Bathini, BERRY9 IT SERVICES PRIVATE LIMITED, Hyderabad. Questions about your own systems? Request a scoping call or call +91 86868 92922.
Plan your next security test
Tell us what needs testing and when you need it done, and we will propose a scope and timeline. We respond within 24 hours. If you would rather talk first, call or write to us.
- +91 86868 92922
- contact@berry9.com
- H.No: 8-3-945/A/12 & 13, Sri Venkata Sai Nest Apartments, Nagarjuna Nagar Colony, Yellareddyguda, Hyderabad 500073, Telangana