Top 5 open source SSO solutions: Pros, cons, + key tips

These days, single sign-on (SSO) is a must-have — nobody wants to remember yet another password for your app. The good news is that thanks to open-source SSO solutions, SaaS developers like yourself can add SSO to their apps without breaking the bank.

But with so many options available, how do you choose the right one for your app?

In this article, we'll walk you through:

• What to watch out for when choosing an open-source SSO solution
• A rundown of five providers worth considering
• Why a hosted SSO provider can be a better option

Let’s start by explaining what an open-source SSO provider is.

What is an open-source SSO provider?

An open-source single sign-on (SSO) provider lets users log in once and access multiple apps without re-entering credentials. 

The key difference between open-source and commercial providers is that they share their source code publicly and allow anyone to inspect, modify, or improve it.

Open-source SSO providers are typically free, though some may offer advanced features or support through paid versions. It's worth noting that they may have fewer features compared to their commercial counterparts — the specific features will depend on the individual provider. Always check the docs, licensing terms, and the distinctions between the free and paid versions (if any) of the provider you want to support in your SaaS.

What to look for in an open-source SSO solution

Here are the key factors to keep in mind when choosing an open-source SSO provider:

• Protocol support: Your customer's identity providers may use different SSO protocols. For broad compatibility, go for a provider that supports the most widely used standards, like SAML and OpenID Connect.
• Security features: Look beyond basic login capabilities. The best solutions offer extras like multi-factor authentication (MFA) for added security.
• Specific IdP support: You’ll want a solution that supports the IdPs your customers use. Pre-built integrations and straightforward setup options are a big plus. Also, ensure the documentation is clear so you’re not digging around for answers.
• User-friendly login flows: A seamless login flow across the web, mobile, and desktop is a must. Bonus points if you customize the login UI to match your brand so the experience feels like a natural part of your app.
• Regular updates: Look for a solution backed by a vibrant community or, even better, dedicated developer support that regularly updates the software to address security vulnerabilities. Check forums, mailing lists, and GitHub repositories for activity levels.
• High availability: Choose an SSO solution designed for high availability and capable of scaling graciously with your user base.
• Easy integration: The solution should integrate smoothly with your existing infrastructure. Accessible APIs, SDKs, and solid documentation make a huge difference in smooth setup, customization, and troubleshooting.
• Cost-effectiveness: Even if it’s a free SSO provider, don’t overlook hidden costs like deployment, customization, and maintenance. Make sure it fits into your budget long-term.

The top 5 open-source SSO providers

1. ‍Keycloak for end-to-end identity management beyond SSO.
2. Central Authentication Service for enterprise-grade SSO for web apps.
3. FreeIPA for SSO in Linux systems.
4. Authelia for a fast and lightweight SSO provider.
5. IdentityServer for headless SSO in .NET apps.

Below are 5 open-source SSO providers you should consider:

1. Keycloak

Keycloak is an open-source identity management solution developed by Red Hat. Beyond SSO, it enables user federation, identity brokering (connecting with external identity providers), and fine-grained authorization. You can integrate Keycloak into your app using the APIs and client libraries (for languages such as Java and JavaScript) it provides.

Pros of Keycloak

• Comprehensive protocol support: Keycloak supports two of the most popular SSO protocols — OpenID Connect (OIDC) and SAML 2.0, making it highly versatile and compatible with a wide range of apps, including web and mobile.
• Third-party integrations: Keycloak has many out-of-the-box third-party integrations (like Google, FB, Twitter, Stack Overflow, etc.).
• External identity source sync: If your customer uses a user directory, Keycloak allows you to synchronize with it—whether it's standard ones like Active Directory (which it supports by default) or custom databases (which you can support using the Keycloak User storage API).
• Customization: Keycloak lets you customize all user-facing login pages. With support for the FreeMarker Template Language (or .ftl files), you can tweak the UI using HTML, CSS styles, or JS scripts until it matches your brand identity.

Cons of Keycloak

• Steep learning curve: The extensive functionality and configuration options can be overwhelming if you don’t know much about configuring IAM systems. You’ll need to invest considerable hours to comb through the documentation and test your integration.
• Scalability issues: Scalability may require additional setup and optimization, such as setting up multi-site deployments and having processes to sync data between sites if they go out of sync. Luckily, a whole section in the docs is dedicated to ensuring high availability.
• Documentation and examples: Although Keycloak's documentation is thorough, it’s difficult to navigate, especially if you’re searching for particular information. It also lacks practical and end-to-end examples from other SSO providers.

There are several Keycloak distributions to choose from:

• Server: A standalone application downloadable as a .tar or .zip file.
• Container image: Distribution appropriate for Docker, Podman, Kubernetes, and OpenShift.
• Operator: Distribution for Kubernetes and OpenShift based on Operator SDK.

All three can be downloaded from the Keycloak downloads page.

2. Central Authentication Service (CAS)

The Central Authentication Service (CAS) is an open-source, enterprise-grade single sign-on solution for web apps. Yale University created it, and it has since evolved into a widely adopted service managed by the Apereo Foundation. CAS also refers to the CAS SSO protocol, which this provider supports.

Pros of CAS

• Integration capabilities: CAS offers various libraries for various languages and frameworks, including Java, .Net, PHP, Perl, Apache, uPortal, and others.
• Support for multiple authentication protocols: CAS supports various authentication protocols, including SAML, OpenID Connect, CAS, and WS-Fed. You can even use it to delegate authentication to social IdPs like Facebook or Twitter.
• Admin UIs: It provides an admin console for administering and managing the CAS server deployment.
• Customizable login pages: CAS allows you to edit the CSS and HTML files or add JavaScript effects to match your branding and UI guidelines. You can also localize messages to suit your customer’s language needs.

Cons of CAS

• Complexity in setup and configuration: Deploying and configuring CAS can be complex, especially if you don’t have a dedicated engineering team with expertise in identity management.
• Learning curve: It has a very steep learning curve. You’ll need to study its architecture and the multiple configuration options.
• Operational overhead: Maintaining a CAS server, especially in a high-availability environment, can introduce operational overhead in monitoring, scaling, and updating it. Fortunately, like Keycloak, it has a high-availability guide you can refer to.

Where to get CAS

CAS recommends building and deploying locally using the WAR Overlay method, which you can find in the docs.

3. FreeIPA

FreeIPA (Identity, Policy, Audit) is an integrated security information management solution that combines identity management (directory services), policy (configuration management), and audit into a single, cohesive framework. It’s a bit different from the providers on this list since it focuses on managing identities specifically for Linux users.

Pros of FreeIPA

• It’s an all-in-one authentication service: Besides SSO, it allows admins to define and enforce policies for access control. It also has a built-in Certificate Authority (CA) that can be used to implement certificate-based authentication.
• Multiple management interfaces: It provides a user-friendly GUI and command-line interface, a JSON-RPC API, and a Python SDK for managing identities and policies — you can choose which works best for you.
• Support for standard protocols: It supports SSO protocols like OpenID and Kerberos.

Cons of FreeIPA

• Complexity in setup and maintenance: Even though it’s not as complicated as CAS, the initial setup and ongoing maintenance of FreeIPA can be complex, particularly in large or distributed environments.
• Focused on Linux/UNIX: While it offers significant advantages for Linux/UNIX systems, you'll find it limiting if your environment doesn’t use Linux or has a mix of other OSs.

Where to get FreeIPA

Download and start using the latest FreeIPA release from the downloads page.

4. Authelia

Authelia describes itself as the single sign-on, multi-factor portal for web apps. It enables SSO via session cookies, OpenID Connect, or trusted headers.

Compared to the providers we’ve discussed so far, Authelia is very lightweight, mostly because it offers fewer features. Beyond SSO, it also enables granular policy definitions, MFA, and identity validation via email.

Pros of Authelia

• Flexible authentication sources: Authelia can integrate with your customer’s existing LDAP directories or Active Directory.
• Intuitive UI: It has a simple login portal whose workflow is completely transparent to your users.
• Fast authentication process: It was created in React and Go, and according to Authelia, the login portal loads in 100 milliseconds.

Cons of Authelia

• Limited protocol support: The only identity provider implementation it currently supports is OpenID Connect 1.0 (even then, it doesn’t support the OpenID Connect 1.0 relying party role), unlike other SSO solutions that directly support protocols like SAML (a protocol widely used by enterprises).
• Focus on web apps: Authelia is specifically designed for web apps, potentially limiting its applicability to desktop or mobile apps without additional configuration.

Where to get Authelia

See the configuration and deployment documentation to get started with Authelia.

5. IdentityServer

IdentityServer is an open-source identity and access management framework for .NET and offers headless SSO through a developer-friendly API.

It primarily focuses on token-based SSO through the OpenID Connect protocol.

Pros of IdentityServer

• Standards compliant: IdentityServer fully complies with OpenID Connect and OAuth 2.0 standards.
• More than just an SSO provider: IdentityServer also supports access control for APIs and identity federation.
• Flexible and extensible: Its modular architecture makes it easy to extend and integrate with your existing system.

Cons of IdentityServer

• .NET Ecosystem: Because it is tightly coupled with .NET, IdentityServer might be less appealing if you use other technology stacks.
• Complex to set up and configure: Unlike an alternative like Keycloak, IdentityServer is not a ready-to-run product. You have to set it up and configure it, which can get complicated if you don’t usually configure IAM services.
• Pricing: While IdentityServer is open source, it has two versions: the free one and the commercial one. The paid version offers security fixes and regular updates, but you’ll have to make annual payments.

Where to get IdentityServer

Visit the IdentityServer homepage to get started.

Challenges of open-source providers 

While open-source solutions offer plenty of benefits, such as cost savings and flexibility, they also have their own set of limitations.

• Limited functionality: Depending on the project, some open-source SSO providers might lack certain features found in commercial products and may have slower release cycles for new features and updates.
• Security concerns: Security audits and updates depend heavily on the community’s activity level and resources, which means fixes and improvements are often less predictable.
• Integration efforts: Setting up and managing open-source SSO can require deep expertise, which can be a time-sink if your team isn’t already skilled in this area.
• Documentation: While some open-source projects have excellent documentation, others may suffer from outdated, sparse, or overly technical documentation.
• Professional support: They typically don’t come with dedicated support. You’ll likely rely on forums, Stack Overflow, or GitHub discussions for help, which isn’t ideal when you need fast, reliable assistance.

Benefits of commercial SSO providers

Commercial SSO providers solve these issues by offering enterprise-grade uptime, reliability, and ongoing support — saving your team time and effort.

Here’s what you can expect from a paid SSO provider:

• Simple to set up: No need for complex installations. Most paid SSO providers offer a web-based UI, SDKs, and APIs that make integration simple and accessible, even if SSO is new to your team.
• Professional support and high reliability: Paid SSO solutions typically include dedicated support via email, Slack, or even direct calls, so help is just a message away when you need it.
• Continuous updates and feature releases: With a paid solution, you’ll benefit from ongoing updates, new feature rollouts, and critical security patches, all managed for you.
• Integration ease: Paid SSO solutions often come with pre-built integrations for popular identity providers and HRIS platforms, reducing the time and effort needed to build an SSO integration.

Next steps with a reliable, cost-effective SSO solution: WorkOS

If you’re building an enterprise app and are looking for a reliable SSO solution, you’ll love WorkOS:

• Get started fast: With SDKs in every popular language, easy-to-follow documentation, and Slack-based support, you can implement SSO in minutes rather than weeks.
• Support every protocol: With OAuth 2.0 integrations to popular providers like Google and Microsoft, compatibility with every major IdP, and full support for custom SAML/OIDC connections, WorkOS can support any enterprise customer out of the box.
• Avoid the back-and-forth: WorkOS’ Admin Portal takes the pain out of onboarding your customers’ IT teams and configuring your app to work with their identity provider.
• Pricing that makes sense: Unlike competitors who price by monthly active users, WorkOS charges a flat rate for each company you onboard — whether they bring 10 or 10,000 SSO users to your app.

Explore Unified SSO by WorkOS.

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