What is Central Authentication Service?

Central Authentication Service, or CAS, is a single sign-on (SSO) protocol that allows users to access multiple applications with one set of login credentials. Developed at Yale University, CAS is an open, well-documented protocol that’s widely used in enterprise systems.

Because CAS is open-source and protocol-driven, it’s flexible. It supports several backend identity stores (like LDAP, databases, or Active Directory) and integrates easily into systems with different security and infrastructure layouts.

CAS handles authentication by redirecting users to a central login server. Here are some benefits of this approach:

• Single sign-on across multiple applications: Users log in once and gain access to all connected services without having to re-authenticate.
• Centralized user management: Admins can control access policies, password rules and account statuses from one place.
• Simplified integration: Applications don’t need to manage their own login flows; they can rely on CAS to handle authentication.
• Audit and logging: With a central system in place, it becomes much easier to track login attempts and user activity.
• Support for multiple protocols: CAS can work with different authentication protocols, like SAML, OAuth and OpenID Connect.
• Fewer login errors and less password fatigue: Users don’t have to remember separate passwords for every app, which improves usability and reduces help desk load.

OneLogin; A Centralized Authentication Service solution

OneLogin is a popular identity and access management platform that supports centralized authentication through a cloud-based approach. Here are some of its key features:

• Integrates with Active Directory and LDAP: Since OneLogin can connect with existing directories, it allows organizations to sync user data and manage authentication from their current identity store.
• Supports two-factor authentication (2FA): To improve account security, OneLogin supports 2FA using methods like SMS, mobile apps, or hardware tokens.
• Self-service password reset: Users can reset their passwords without IT involvement, using secure links sent via email or SMS.

Learn more about OneLogin’s offerings here.

Any organization that manages a large or growing application suite, to which its users require secure and seamless access, should implement a Central Authentication Service. That said, here are some scenarios where it's especially important:

• When you're about to launch a suite of internal apps that share users or data
• When onboarding a large number of new users, such as during a company merger or rapid hiring phase
• If users are regularly complaining about login issues, forgotten passwords or the hassle of multiple sign-ins
• When security requirements call for centralized control over authentication and access policies
• If your IT team is overwhelmed with password reset or access-related support tickets
• When expanding to cloud apps and needing a consistent way to manage authentication across platforms

The CAS architecture has two main components:

• CAS server: This is the core of the setup. It manages user login, handles ticket generation and communicates with backend identity stores like LDAP, databases, or Active Directory.
• CAS clients: These are the applications (web apps, services, portals) that trust the CAS server for authentication. They redirect users to the CAS server when authentication is needed.

CAS supports the following authentication protocols:

• CAS protocol versions 1.0, 2.0, and 3.0
• SAML 1.1 and SAML 2.0
• OpenID Connect
• OAuth 2.0
• WS-Federation

This wide protocol support allows CAS to work across different app stacks and identity systems.

How a typical CAS architecture is designed

Next, let’s explore how you’d set up a typical Central Authentication Service architecture:

1. You will start by installing and configuring the CAS server to handle authentication. It will have to be connected to a backend identity store like LDAP, Active Directory or a database.
2. The CAS server maintains a registry of allowed client applications (known as “services”). You’ll need to register each client app with details like its name, URL and the protocols it will use.
3. Communication between the CAS server and clients should happen over HTTPS. You’ll also secure backend communication with the identity provider.
4. Each application that needs authentication must integrate a CAS client library or plugin that knows how to redirect users to the CAS server and handle the returned ticket.
5. Once validation succeeds, the client app considers the user authenticated and establishes a local session. If SSO is enabled, other CAS-integrated apps will recognize the session and will not prompt the user to sign in.
6. Optionally, you can configure Single Logout (SLO), which allows users to end their sessions in all CAS-connected apps by logging out from one app.

Typical CAS login flow

Here’s how a standard CAS login process works:

1. The user accesses a protected application.
2. The app checks if the user is already authenticated. If not, it redirects them to the CAS server.
3. The user sees the CAS login page and enters their credentials.
4. The server checks the credentials against the backend identity provider (e.g., LDAP or AD).
5. If login is successful, CAS generates a service ticket and redirects the user back to the requesting app, with the ticket attached to the URL.
6. The application sends the ticket back to the CAS server for validation via a secure backchannel.
7. If the ticket is valid, the user is considered authenticated and granted access to the app.
8. When the user visits another CAS-enabled app, the CAS server sees the active session and skips the login prompt.

Finally, here are some best practices that will help you get the best out of your CAS setup:

• Keep the CAS server and client libraries up to date with the latest stable releases.
• Maintain a strict and updated service registry on the CAS server to limit unauthorized application access.
• Configure ticket expiration and session timeouts to balance security and user convenience.
• Implement two-factor authentication at the CAS login level, especially for high-privilege users.
• Enable detailed logging and monitoring to catch failed login attempts, ticket validation issues or unusual access patterns
• Use role-based access control (RBAC) or attribute-based access control (ABAC) when connecting CAS to external identity providers.
• Apply bot mitigation tools or rate-limiting to the CAS login endpoint to prevent automated abuse and credential stuffing attacks.
• Treat the CAS server as a tier-zero asset and apply strict access controls, network segmentation and hardening practices accordingly.

CAS is a stable and well-documented SSO protocol that makes it easier to manage authentication across multiple applications. Whether you’re running a campus-wide system or a growing set of internal tools, CAS provides the flexibility and reliability needed for scalable and strong authentication.