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Oct 6 2017

SharePoint Authentication and Session Management #saml #federated #authentication


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SharePoint Authentication and Session Management

What is authentication?

1. A security measure designed to protect a communications system against acceptance of a fraudulent transmission or simulation by establishing the validity of a transmission, message, or originator.
2. A means of identifying individuals and verifying their eligibility to receive specific categories of information.

Authentication is essentially the process of validating a user is who they say they are, such that they can gain access to a system – in this context, the system is SharePoint. Authentication is not authorization, which is the process in determine if a known user is permitted access to certain data in the system, after successful authentication.

SharePoint, much like any content management system, relies on user authentication to provide user access to secured content. Pre-SharePoint 2010, SharePoint relied on NTLM, Kerberos, or basic (forms-based) authentication protocols (their discussion out of scope of this text). SharePoint 2010 introduced Claims-based-Authentication (CBA), also present in SharePoint 2013. CBA consists of authentication abstraction, using a Secure Token Service (STS), and identification of users with multiple attributes –claims – not just the traditional username and password pair.

A Secure Token Service implements open standards. A typical STS implementation communicates over HTTPS, and packages user identity information (claim data) via signed and encrypted XML – Secure Assertion Markup Language (SAML). Examples of STS implementations are the STS engine in SharePoint 2010/2013, ADFS, and third party applications build using the Windows Identity Framework.

SharePoint Session Management

A user session in SharePoint 2010/2013 is the time in which a user is logged into SharePoint without needing to re-authenticate. SharePoint, like most secure systems, implements limited lifespan sessions – i.e. users may authentication with a SharePoint system, but they re not authenticated with the system indefinitely. The length of user sessions falls under the control of session management, configured for each SharePoint Web Application.

SharePoint handles session management differently, depending on the authentication method in play (Kerberos, NTLM, CBA, Forms, etc.). This article discusses how SharePoint works with Active Directory Federated Services (ADFS) – an STS – to maintain abstracted user authentication and user session lifetime. The following is a sequence diagram of the default authentication and session creation process in SharePoint 2010/2013 when using CBA with ADFS.

The following is a summary of the authentication process, shown in the sequence diagram.

  1. A user requests a page in SharePoint from their browser this might be the home page of the site.
  2. SharePoint captures the request and determines that no valid session exists, by the absence of the FEDAUTH cookie.
  3. SharePoint redirects the user to the internal STS – this is important because the internal STS handles all authentication requests for SharePoint and is the core of the CBA implementation in SharePoint 2010/2013.
  4. Since we have configured SharePoint to use ADFS as a trusted login provider, the internal STS redirects the user to the ADFS login page.
  5. ADFS acquires credentials and authenticates the user.
  6. ADFS creates a SAML token, containing the user s claims, as encrypted and signed.
  7. ADFS posts the SAML token to the internal SharePoint STS.
  8. The Internal STS saves the SAML token in the SAML Token Cache.
  9. SharePoint creates the FEDAUTH cookie, which contains a reference to the SAML token in the cache.
  10. The Internal STS redirects the user back to SharePoint, and then back to the original requested page.

Session Lifetime

The lifetime of a SharePoint session, when using ADFS, is the topic of much confusion. Ultimately, SharePoint determines whether a user has a current session by the presence of the FEDAUTH cookie. The default behavior of SharePoint is to store this persistent cookie on the user s disk, with fixed expiration date. Before sending a new FEDAUTH cookie back to the user s browser, SharePoint calculates the expiration of the cookie with the following formula:

SAML Token Lifetime – Logon Token Cache Expiration Window

The above values are important since they govern the overall lifetime of the FEDAUTH cookie, and hence the session lifetime. The following table describes each value and its source:



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