Microsoft® SharePoint® Portal Server 2003 Service Pack 1 (SP1) contains significant security enhancements, in addition to stability and performance improvements. SP1 includes stability improvements developed as a result of user input from the Microsoft Online Crash Analysis in Office SharePoint Portal Server 2003 and from Microsoft Product Support feedback. You can get specific information about this update in the Microsoft Knowledge Base article Description of the SharePoint Portal Server 2003 Service Pack 1.
Download: SharePoint Portal Server 2003 Service Pack 1
News source: Microsoft Download Center