OAuth 2.0
An authorisation framework that lets an application obtain limited access to a resource on a user's behalf without handling their password, by exchanging a user consent step for an access token with a defined scope and lifetime. It is an authorisation protocol, not an authentication one -- OpenID Connect is the identity layer built on top of it. OAuth 2.0 and OIDC underpin most modern SaaS and API access and are examined in CISSP, CCSP, SC-200, AZ-500 and appsec curricula.
Why It Matters
In practice the common failure is treating an access token as proof of who the user is: OAuth answers what an application may do, and only an OIDC ID token, properly validated, answers who is present. Real-world abuse concentrates on consent rather than cryptography -- illicit consent grant attacks phish a user into approving a malicious application, which then holds a refresh token that survives a password reset and often MFA, so incident response must revoke tokens and application consent rather than merely rotate the credential. Other recurring findings are the implicit flow returning tokens in the URL fragment (deprecated in favour of authorisation code with PKCE), missing or unvalidated redirect URI allow-lists enabling token theft, over-broad scopes requested once and held indefinitely, and refresh tokens stored where client-side code can read them. On exams such as CISSP, CCSP and SC-200, expect questions separating authentication from authorisation, identifying the right flow for a public client, and responding to a malicious OAuth application.
Practice this topic
Test your knowledge of OAuth 2.0 concepts with exam-style practice questions.
Related Access Control terms
Access Control List (ACL)
A list of permissions attached to an object that specifies which users or system processes are granted access to resources and what operations they can perform. ACLs are implemented in routers, firewalls, and operating systems to filter traffic and restrict file access. They can be standard (filtering by source IP only) or extended (filtering by source, destination, port, and protocol). In cybersecurity certifications like CISSP and Security+, understanding ACLs is essential for network security and access management domains.
Authentication
The process of verifying the identity of a user, device, or system before granting access to resources. Authentication typically relies on one or more factors: something you know (password), something you have (token or smart card), or something you are (biometric). Modern systems often combine multiple factors (MFA) to strengthen security. Common authentication protocols include Kerberos, LDAP, OAuth 2.0, and SAML. Authentication is a foundational concept tested across nearly every cybersecurity certification.
Authorization
The process of determining what resources or actions an authenticated user is permitted to access. While authentication verifies identity, authorization enforces permissions based on policies, roles, or attributes. Common models include discretionary access control (DAC), mandatory access control (MAC), role-based access control (RBAC), and attribute-based access control (ABAC). Authorization failures are a leading cause of data breaches, making this a critical topic in CISSP, Security+, and OWASP Top 10 studies.
Multi-Factor Authentication (MFA)
A security mechanism that requires two or more independent credentials to verify a user's identity, combining factors from different categories: knowledge (passwords, PINs), possession (hardware tokens, smartphones), and inherence (fingerprints, facial recognition). MFA significantly reduces the risk of account compromise, Microsoft reports it blocks over 99.9% of automated attacks. It is required by many compliance frameworks including PCI DSS, HIPAA, and NIST 800-63. MFA implementation is a key topic in Security+, CISSP, and cloud security certifications.
Single Sign-On (SSO)
An authentication scheme that allows a user to log in with a single set of credentials to access multiple applications and services without re-authenticating. SSO improves user experience and reduces password fatigue, but creates a single point of failure if the identity provider is compromised. Common SSO protocols include SAML 2.0, OAuth 2.0, and OpenID Connect. Enterprise SSO solutions often integrate with Active Directory or cloud identity providers like Okta and Azure AD. SSO is covered in CISSP Domain 5 (Identity and Access Management) and Security+.
Zero Trust
A security model that requires strict identity verification for every person and device attempting to access resources, regardless of whether they are inside or outside the network perimeter. Zero Trust operates on the principle of 'never trust, always verify' and assumes that breaches are inevitable. Key pillars include micro-segmentation, least privilege access, continuous verification, and real-time monitoring. The NIST SP 800-207 framework provides guidance for implementing Zero Trust architectures. This model has become a dominant security strategy and is increasingly tested in CISSP, Security+, and cloud security certifications.