IdentityServer by Duende Software is a modern, flexible, and standards-compliant OpenID Connect and OAuth 2.1 framework for ASP.NET Core, designed to handle authentication and authorization for web, mobile, and native applications. It serves as a central identity provider (IdP), enabling secure access control and single sign-on (SSO) capabilities across distributed systems. IdentityServer supports a wide range of scenarios, including API security, federation, identity delegation, and token issuance, while offering extensibility for custom policies and integration with external identity providers. As a commercial solution, it combines enterprise-grade features with ongoing support and regular updates, making it a robust choice for organizations seeking secure and scalable identity management within the .NET ecosystem.