How to evaluate OAuth 2.0 and OpenID Connect security

OAuth 2.0 underpins delegated access across web portals, SaaS platforms and internal APIs. It lets users approve access without sharing passwords with every application, but poor configuration can expose data or privileges.

OpenID Connect adds an identity layer, allowing clients to establish who signed in and receive claims. Protecting the sign-in screen is not enough if authorisation codes, access tokens or refresh tokens can be stolen, replayed or confused with credentials meant for another service.

A useful review combines architecture analysis with practical abuse-case testing. It examines redirect handling, client registration, scope design, token validation and session behaviour across sensitive integrations, rather than treating a successful login as proof of security.

For Australian enterprises, government agencies, banks, retailers and healthcare providers, identity sprawl is common. Teams in Sydney and Melbourne often combine Microsoft Entra ID, Okta, Google Workspace and specialist APIs, while local procurement discussions often use “VAPT” for a combined vulnerability assessment and penetration test with business-readable risk ratings.

Map the trust boundaries

Begin with an architecture inventory: authorisation servers, resource servers, client types, identity providers, API gateways, session stores and administrative planes. Record which clients are confidential, public, native or browser-based, because each type has a different attack surface and needs an appropriate way to protect credentials.

Then map registration, login, consent, redirect, code exchange, token use and logout. Identify every redirect URI, scope and audience, and mark boundaries where tokens move between systems. Note assumptions tied to Azure regions, AWS Sydney or Melbourne deployments, or records covered by the Privacy Act 1988 (Cth), the Australian Privacy Principles and OAIC guidance.

Exercise the authorisation flows

Exercise every relevant role and consent path with separate accounts for administrator, employee, customer and partner access. Try to substitute a weaker role, broaden a scope, skip consent, tamper with state or nonce, replay a code and redirect a response elsewhere. Confirm that codes bind to the correct client and redirect, expired artefacts fail, and mix-up or confused-deputy attacks are rejected.

Check features against deployed versions rather than assuming modern options are enabled. Public clients should use authorisation code flow with PKCE; native apps need current URI guidance; and sensitive APIs may benefit from sender-constrained tokens such as DPoP or mTLS. Assess PAR, JAR, FAPI-style profiles and resource indicators where they match the risk. For teams using Queue to organise test playlists, switching between queues quickly can maintain the sequence with less mouse use, while captures remain timestamped and linked to each test case.

Inspect tokens, clients and sessions

Treat every token as a security boundary. Validate signature, issuer, audience, authorised party, expiry, not-before and issued-at claims against trusted metadata; reject unexpected algorithms, malformed keys and tokens minted for another client. Verify JWKS rotation and cache behaviour, then test token substitution, scope escalation and algorithm-confusion cases at every API.

For refresh tokens, confirm rotation, expiry, revocation, reuse detection and device binding where promised. Browser sessions should resist fixation, and logout should invalidate the relevant server-side session. Sensitive actions may need step-up authentication and phishing-resistant factors, particularly for payments, patient records or privileged cloud administration.

Inspect client-side storage as well. Keep server-side secrets in a vault, never source control, and ensure public clients ship no reusable secret. Mobile reviews should cover keychain or keystore use, deep-link validation, universal links, certificate handling and local token storage.

Verify cloud, mobile and monitoring controls

Extend the review beyond the login screen. In cloud environments, examine workload identity, IAM trust policies, role assumptions, metadata-service exposure and CI/CD separation between development and production. API gateways, service meshes and legacy components can silently accept broader audiences or scopes than the primary application.

Monitoring should reveal unusual token behaviour without exposing credentials. SIEM rules should flag repeated authorisation failures, impossible travel, refresh-token reuse, consent changes and privilege escalation. Redact or hash tokens, codes and personal data before logs cross the authentication boundary. The ACSC and ASD Essential Eight offer useful Australian baselines for assessing this control maturity.

Regulated sectors need domain-specific checks. Healthcare organisations should review role and purpose-of-use controls, while financial institutions should consider FAPI-aligned transaction controls. Confirm privacy obligations with the relevant legal or compliance team rather than assuming one federal rule covers every record.

Turn findings into a remediation programme

Rate findings by realistic impact and exploitability, not scanner severity alone. A redirect flaw affecting every customer may outrank a theoretical cryptographic weakness in an unused endpoint. Tie each issue to affected assets, exploit steps, an owner, remediation code, retest criteria and residual risk.

• Block wildcard and open redirect URIs; allow exact HTTPS destinations with app-link controls for native clients.
• Require PKCE for public clients, rotate refresh tokens, detect reuse and cap token lifetime to business need.
• Validate issuer, audience, authorised party, algorithm and time claims centrally in every resource server.
• Bind high-risk transactions to stronger client authentication, sender-constrained tokens and step-up checks.
• Store client secrets in managed vaults, segregate cloud roles and prevent CI/CD identities from inheriting human permissions.
• Redact credentials in SIEM events, monitor consent and token anomalies, and rehearse revocation with the on-call team.

Retest the original abuse cases and adjacent paths after fixes. Evidence should show what changed, which environments were exercised and whether compensating controls remain necessary. Where the Privacy Act applies, assess whether exposed information could create a notifiable privacy incident, involving legal and privacy specialists without delaying containment.

Infoziant Security can scope this work for Australian organisations, combining OAuth 2.0 and OpenID Connect testing with vulnerability assessment, penetration testing, cloud and mobile reviews, compliance support and 24/7 SIEM monitoring. Request a free VAPT report or a trial-based engagement to benchmark the highest-risk authentication and authorisation flows.