Understanding Cookie Authentication Challenges in ASP.NET Core and Best Practices for Handling Redirects

If you’re building a secure web application with ASP.NET Core and integrating it with SAML-based SSO, managing authentication cookies and handling authorization failures can be complex. Many developers encounter issues where custom cookie settings interfere with proper redirection flows, especially when attempting to implement absolute expiration alongside sliding expiration.

The Context: Cookie Authentication in a .NET 8 Web API

Imagine you’re developing a Web API using ASP.NET Core 8, leveraging SAML for Single Sign-On (SSO) login and logout processes. Upon successful authentication with the Identity Provider (IDP), your backend issues an authentication cookie to the client. Your frontend, built with Vue.js, detects an unauthorized access attempt (typically a 401 response) and redirects the user to the IDP login page.

To enhance security, you decide to set both sliding expiration (which keeps users logged in as long as they’re active) and absolute expiration (which forces logout after a fixed period, say 12 hours). However, implementing these together introduces challenges, especially when customizing cookie events, which sometimes prevent the API from returning proper responses on authentication failure.

Common Challenges and Misconceptions

• Cookie Events and Response Handling: Custom cookie event handlers can modify or suppress default behaviors, such as issuing 401 responses on failed authentication attempts. This can break the frontend’s ability to detect the need for redirection.

• Expiration Settings Conflicts: In ASP.NET Core, setting both ExpiresUTC and SlidingExpiration can conflict. Generally, configuring one overrides the other, making it tricky to implement a hybrid expiration strategy.

• Redirects on Authorization Failures: By default, ASP.NET Core handles 302 redirects for applications with UI but may be less straightforward in API scenarios, especially when using cookie authentication with custom events.

Recommended Approaches for Effective Authentication and Redirection

1. Use Standard Authentication Events for Proper Response Handling

Instead of fully customizing cookie events to prevent responses, leverage the OnChallenge and OnRedirectToLogin events within your cookie authentication middleware. These handlers allow you to define the exact behavior when authentication fails, ensuring that your API returns a 401 Unauthorized status rather than redirecting, which is crucial for API communication.

“`csharp
services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
.AddCookie(options =>
{
options.Events = new CookieAuthenticationEvents
{