OAuth 2.0, OpenID Connect and SAML 2.0: Choosing the right Auth protocol

Choosing an authentication protocol may seem like a technical formality, just another checkbox in your architecture. But the reality is different: picking the wrong protocol (or misapplying the right one) can lead to serious issues from broken user experiences and integration failures to data breaches and compliance risks.

Many organisations rely on APIs, mobile apps, third-party integrations and federated identity. In these complex environments, how you manage authentication and authorisation becomes critical to both security and usability. Yet too often, decisions around protocols like OAuth 2.0, OpenID Connect, or SAML are made without fully understanding what each one does or doesn't do.

Are you using OAuth for login? Should your mobile app rely on SAML? Is OpenID Connect necessary if you already have an identity provider? These questions often arise during implementation, not at the design stage, when it’s already harder and more costly to adjust.

In this article, we break down the key differences between OAuth 2.0, OpenID Connect and SAML 2.0, explain the real purpose of each and show you how to choose the one that actually fits your use case, not just the one that’s most familiar.

If you're planning a new digital service, modernising legacy systems, or trying to connect applications securely, this guide is for you.

What are you trying to secure? Defining What you’re securing and How

Before choosing a protocol, it’s important to take a step back and define what exactly needs to be protected and how it will be accessed. Authentication and authorisation are not abstract concepts, they must respond to specific, real-world requirements. Misalignment here is one of the main reasons why implementations become overcomplicated or insecure.

Start by asking these key questions:

• Are users logging into a web or mobile application?
  If so, you need to manage user identity, session handling and possibly single sign-on (SSO) across applications.
• Do services or applications need to call APIs on behalf of a user or themselves?
  This is a common scenario in backend services, third-party integrations and mobile apps, and often misunderstood when choosing between token types.
• Will external partners or identity providers be involved?
  In B2B environments or enterprises with multiple identity sources, federation becomes a requirement and protocol support must be carefully planned.
• Is this access interactive or machine-to-machine?
  A human logging in to a web app is very different from an automated process calling an internal API.
• Are there regulatory, compliance, or audit requirements?
  Depending on your sector, you may need to choose a protocol that offers stronger traceability, token revocation, or alignment with frameworks like ISO 27001 or GDPR.

Clarifying the answers to these questions will help narrow down the appropriate authentication and authorisation strategy and avoid trying to force a protocol into a use case it wasn’t designed for.

Comparing OAuth 2.0, OpenID Connect and SAML: A closer look at the most common Auth protocols

When choosing an authentication protocol, it’s important to understand not just what each one does, but when it makes the most sense to use it. Below we explore the three most widely adopted protocols (OAuth 2.0, OpenID Connect and SAML 2.0) explaining their purpose, how they work and where they fit best.

OAuth 2.0

OAuth 2.0 is a framework designed for authorisation, not authentication. It allows applications to request access to resources (like APIs) on behalf of a user or service, without needing to handle usernames and passwords directly.

This is done through access tokens, which represent permission rather than identity.

Best suited for:

• Granting limited access to APIs
• Enabling secure communication between services
• Allowing mobile or third-party apps to act on a user’s behalf
• Machine-to-machine scenarios using client credentials

Important to note: OAuth 2.0 does not verify who the user is, only what access is allowed. If you need user identity, you’ll need to add a layer on top.

OpenID Connect

OpenID Connect (OIDC) builds on OAuth 2.0 by adding authentication. It introduces an ID token, which contains information about the user (such as email, name or roles) and defines a standard way to log users in securely.

Because it extends OAuth 2.0, it supports the same token-based model, but with identity added to the equation.

Best suited for:

• Single sign-on (SSO) across web and mobile apps
• User login and profile retrieval
• Federated identity between applications or domains
• Scenarios where you need both access control and user information

OIDC has become the default protocol for most modern web and mobile applications. It is lightweight, JSON-based and widely supported.

SAML 2.0

SAML 2.0 is an XML-based protocol used for federated authentication, especially in enterprise environments. It allows users to log in to one system and gain access to another without re-authenticating.

SAML uses assertions, not tokens, and is often found in older systems or organisations with a strong reliance on traditional identity providers.

Best suited for:

• Enterprise single sign-on (SSO) across legacy applications
• Integrating with corporate identity providers like ADFS
• Scenarios where browser-based authentication is required and APIs are not the focus

While SAML is still widely used in government, finance and education sectors, it's less practical for mobile or API-based systems and generally not recommended for greenfield projects.

OAuth 2.0 vs. OpenID Connect vs. SAML: Key Technical Differences

Understanding the technical distinctions between OAuth 2.0, OpenID Connect (OIDC) and SAML 2.0 helps ensure you choose the right protocol for your architecture. Below is a side-by-side comparison covering the most relevant aspects for architects, developers and security teams:

Summary: 

• Use OAuth 2.0 when the goal is to delegate access to APIs, especially in service-to-service communication.
• Use OpenID Connect when you also need to identify the user and enable secure login flows.
• Use SAML when integrating older applications or working in enterprise environments that already rely on it.

Common challenges in Auth Protocol implementation

Selecting the right protocol is only the first step. Even the most widely adopted standards can lead to security issues or integration headaches when implemented poorly.

What really makes the difference is how you configure, monitor and govern authentication and authorisation in your environment.

Here are some of the most common challenges we see, regardless of protocol:

• Incorrect grant types
  Using the wrong OAuth 2.0 grant type (e.g. password grant in production) can expose credentials or break compliance standards.
• Overly long token lifetimes
  Leaving access tokens valid for hours (or days) increases the risk window in case of compromise.
• Inadequate scope or role enforcement
  Without well-defined scopes or role-based access control, you might grant more access than intended, a common oversight in API security.
• Token storage in insecure locations
  Storing tokens in local storage, URL parameters, or unencrypted session stores can make them easy to intercept or reuse.
• Lack of revocation or logout handling
  Especially in SSO scenarios, failing to manage session logout correctly can lead to ghost sessions or unauthorised reuse.
• No visibility or alerting
  Without audit logs or monitoring, detecting abuse or token misuse becomes extremely difficult.

Implementing OAuth 2.0, OpenID Connect, or SAML securely requires more than just following a quick-start guide. It involves understanding the context, mapping access flows and applying best practices for session management, consent, trust relationships and auditing.

Conclusion

Choosing between OAuth 2.0, OpenID Connect and SAML isn’t about picking a “better” protocol, it’s about selecting the right one for your specific context. Each protocol serves a different purpose. However, the real challenge begins after choosing a protocol. 

At Claria , we help organisations design and implement secure authentication and access strategies tailored to their systems, users and business goals. Our consultants work with a wide range of technologies and environments, including hybrid cloud, containerised platforms and legacy systems.

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