Exploiting Authentication Logic Flaws in Modern APIs

Exploiting Authentication Logic Flaws in Modern APIs

Introduction

Modern APIs are the lifeblood of digital services — but even well-designed interfaces can conceal dangerous authentication logic flaws.

In this case study, we demonstrate how overlooked trust assumptions and token mismanagement exposed a production SaaS platform to full privilege escalation — without exploiting a single input validation bug.

Background

The target: a SaaS provider with a microservices-based architecture utilizing REST and GraphQL APIs.

The client requested a logic-focused security assessment simulating adversaries who exploit architectural flaws, not just OWASP Top 10 issues.

Objective: Discover ways to impersonate privileged users, bypass business logic, and compromise sensitive workflows.

Recon & Trust Model Mapping

We began with API enumeration and trust mapping:

• Parsed API specs (Swagger, Postman collections) to identify high-value endpoints
• Inspected JWT structures, session headers, and authentication flows
• Analyzed trust boundaries between services and enforcement layers

Critical finding:
✅ JWT validation was limited to cryptographic checks — authorization logic was delegated to the frontend.
❌ Backend accepted any properly signed JWT, and the dev-stage private key was exposed in a public .git folder.

Weak claims enforcement + exposed signing keys = attacker-controlled trust

Exploitation Path

By manipulating JWTs, we were able to:

• Access admin-level endpoints to exfiltrate customer data
• Modify billing settings across multiple tenants
• Create persistent shadow admin accounts with MFA bypass

No brute force, no injection — just a broken trust model and insufficient validation of claims like aud and iss.

💡 Insight: JWTs are not just tokens — they’re trust assertions. If you skip validating them properly, you're handing attackers a blank check.

Business Impact

What began as a subtle logic flaw escalated into high-impact business risk:

• Exposure of PII, billing, and customer metadata
• Potential violation of GDPR, SOC 2, and PCI DSS standards
• Elevated attacker persistence through MFA-bypassed accounts
• Breakdown of tenant isolation in a multi-tenant environment

Recommendations

• Enforce strict claim validation (aud, iss, exp) on every JWT
• Avoid using long-lived tokens; implement refresh token rotation
• Never reuse or leak development keys in production environments
• Centralize access control in the backend — never rely on client-side enforcement
• Implement token anomaly detection to flag suspicious use patterns

Key Lessons

Authentication flaws aren’t always in the login form.
Real attackers look for broken logic in how systems assign and validate trust.

Effective API security testing must go beyond automated scanners and focus on:

• Business logic flow
• Trust boundaries
• Token misuse scenarios

Context-aware manual testing is essential to uncover what tools can’t.

Curious If Your APIs Are Exposing Hidden Trust Paths?

APIs are the backbone of modern applications — but they often contain silent assumptions that attackers can exploit.
Siber Ninja helps you uncover what conventional tests miss.

Through our Web Application & API Security Testing (WAST) and Secure Code Review services, we identify:

• Privilege escalation risks in multi-tenant and role-based flows

• Broken trust chains across microservices and third-party integrations

• Flawed token validation and session handling mechanisms

• Request a Web Application & API Security Testing Engagement

• Download Our Web Application & API Pentest Methodology

• Schedule a Security Assessment

Let’s uncover trust gaps before attackers do.