Securing OAuth 2.0 / OpenID Connect (OIDC) SSO Implementations

Read full article here: https://www.slashid.com/blog/oauth-security/?source=nhimg

Social logins and enterprise Single Sign-On (SSO) powered by OpenID Connect (OIDC) have become a standard for reducing signup friction, improving user experience, and enabling secure connections to enterprise directories. However, OAuth 2.0 and OIDC are complex protocols, and poor implementation can open the door to account takeover, privilege escalation, and impersonation attacks. This article examines the most common security pitfalls in OIDC flows and outlines best practices to build secure, compliant SSO integrations.

The OIDC / OAuth 2.0 Risk Landscape

While OIDC extends OAuth 2.0 to provide authentication alongside authorization, vulnerabilities often emerge from:

• Trusting non-authoritative claims (e.g., email addresses that are not verified).

• Overly permissive redirect URI configurations with wildcards.

• Use of the implicit grant flow, which exposes tokens in browser redirects.

• Failure to validate ID tokens against the correct client ID.

• Unverified or untrusted identity providers (IdPs) that can enable impersonation.

• Automatic account merging based on unverified identifiers.

Real-World Exploitation Examples

• Azure AD / Microsoft Entra – Attackers using falsified email claims to impersonate legitimate accounts.

• Google OAuth – Insider threats via shadow accounts and privilege escalation through magic link abuse.

• AWS Cognito – Email claim manipulation leading to full account takeover (Flickr case).

Security Best Practices for Robust SSO

1. Enforce Authoritative Claims Only – Validate critical attributes (like email) against verified claims such as email_verified.

2. Lock Down Redirect URIs – Use exact path matching, no wildcards, to prevent token redirection to attacker-controlled domains.

3. Avoid the Implicit Grant Flow – Prefer Authorization Code Flow with PKCE to prevent CSRF and code interception attacks.

4. Always Verify Tokens – Confirm they were issued for the correct client_id and from the expected IdP.

5. Vet Your Providers – Support only trusted IdPs and perform security reviews before integration.

6. Manage Account Linking Carefully – Prevent auto-merging accounts based on unverified identifiers.

SlashID’s Secure-by-Design Approach

• No support for implicit grant flow.

• PKCE enforced by default.

• Email claims only returned if verified.

• Only vetted, trusted IdPs supported.

• SDK-based email verification after SSO login.

Conclusion

Implementing OIDC securely is not just a development task—it’s a security-critical process. Misconfigurations can silently expose users to identity theft, data breaches, and unauthorized access. By enforcing strict claim verification, securing token flows, and controlling IdP trust, organizations can reap the usability benefits of SSO without sacrificing security.