In SharePoint 2010 a variety of security methods have been introduced.
Claims-Based Authentication - Claims based authentication is based on identity. and trust.
Code Access Security - in which you can specify your own code access
security (CAS) policy for your web parts.
Sandbox Solutions - Sandbox Solutions which when deployed to the server, SharePoint runs in a special process that has limited permissions.
Cross-Site Scripting - Introduced to prevent Cross - Site Scripting (XSS) attacks.