Special Publication 800-45 Version 2 Guidelines on Electronic Mail Security Recommendations of the National Institute of Standards and Technology Miles Tracy Wayne Jansen Karen Scarfone Jason Butterfield NIST Special Publication 800-45 Guidelines on Electronic Mail Security Version 2 Recommendations of the National Institute of Standards and Technology Miles Tracy, Wayne Jansen, Karen Scarfone, and Jason Butterfield C O M P U T E R S E C U R I T Y Computer Security Division Information Technology Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899-8930 February 2007 U .S. Department of Commerce Carlos M. Gutierrez, Secretary Technology Administration Robert C. Cresanti, Under Secretary of Commerce for Technology National Institute of Standards and Technology William Jeffrey, Director Reports on Computer Systems Technology The Information Technology Laboratory (ITL) at the National Institute of Standards and Technology (NIST) promotes the U.S. economy and public welfare by providing technical leadership for the Nation’s measurement and standards infrastructure. ITL develops tests, test methods, reference data, proof of concept implementations, and technical analysis to advance the development and productive use of information technology. ITL’s responsibilities include the development of technical, physical, administrative, and management standards and guidelines for the cost-effective security and privacy of sensitive unclassified information in Federal computer systems. This Special Publication 800-series reports on ITL’s research, guidance, and outreach efforts in computer security, and its collaborative activities with industry, government, and academic organizations. National Institute of Standards and Technology Special Publication 800-45 Version 2 Natl. Inst. Stand. Technol. Spec. Publ. 800-45 Version 2, 139 pages (Feb. 2007) Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose. iii Acknowledgements, Version 2 The authors, Wayne Jansen and Karen Scarfone of NIST, Miles Tracy of Federal Reserve Information Technology, and Jason Butterfield of Booz Allen Hamilton, wish to express their thanks to colleagues at both organizations who reviewed drafts of this document. In particular, their appreciation goes to Linda Antil, Rick Ayers, Bill Burr, Tim Grance, and Tim Polk from NIST for their research, technical support, and written contributions to this version of the document. The authors would also like to express their thanks to all those who contributed input during the public comment period and who assisted with our internal review process. Acknowledgements, Original Version The authors, Wayne Jansen of NIST and Scott Bisker and Miles Tracy of Booz Allen Hamilton (BAH), wish to express their thanks to colleagues at both organizations who reviewed drafts of this document. In particular, their appreciation goes to John Wack, Murugiah Souppaya, and Tim Grance from NIST, and Steve Allison, Alexis Feringa, Jonathan Holleran, Kevin Kuhlkin, and Mark McLarnon from BAH, for their research, technical support, and written contributions to this document. The authors would also like to express their thanks to all those who contributed input during the public comment period and who assisted with our internal review process. iv GUIDELINES ON ELECTRONIC MAIL SECURITY Table of Contents Executive Summary..............................................................................................................ES-1 1. Introduction ......................................................................................................................1-1 1.1 Authority...................................................................................................................1-1 1.2 Purpose and Scope .................................................................................................1-1 1.3 Audience and Assumptions .....................................................................................1-2 1.4 Document Organization ...........................................................................................1-2 2. Background and Standards ............................................................................................2-1 2.1 Background..............................................................................................................2-1 2.2 Multipurpose Internet Mail Extensions .....................................................................2-2 2.3 Mail Transport Standards.........................................................................................2-3 2.3.1 Simple Mail Transfer Protocol ......................................................................2-3 2.3.2 Simple Mail Transfer Protocol Extensions....................................................2-4 2.3.3 Proprietary Mail Transports ..........................................................................2-6 2.4 Client Access Standards..........................................................................................2-6 2.4.1 Post Office Protocol......................................................................................2-7 2.4.2 Internet Message Access Protocol ...............................................................2-8 2.4.3 Proprietary Mailbox Access Mechanisms.....................................................2-9 2.4.4 Web-Based Mail Access...............................................................................2-9 3. Signing and Encrypting Email Messages ......................................................................3-1 3.1 OpenPGP.................................................................................................................3-2 3.2 S/MIME ....................................................................................................................3-4 3.3 Key Management.....................................................................................................3-4 3.4 Issues with Email Encryption ...................................................................................3-5 4. Planning and Managing Mail Servers.............................................................................4-1 4.1 Installation and Deployment Planning......................................................................4-1 4.2 Security Management Staff......................................................................................4-3 4.2.1 Senior IT Management/Chief Information Officer (CIO) ...............................4-3 4.2.2 Information Systems Security Program Managers .......................................4-3 4.2.3 Information Systems Security Officers .........................................................4-4 4.2.4 Mail Server and Network Administrators ......................................................4-4 4.3 Management Practices ............................................................................................4-4 4.4 System Security Plan...............................................................................................4-5 4.5 Human Resources Requirements............................................................................4-7 4.6 General Information System Security Principles......................................................4-7 4.7 Checklist for Planning and Managing Mail Servers .................................................4-9 5. Securing the Mail Server Operating System .................................................................5-1 5.1 Updating and Configuring the Operating System ....................................................5-2 5.1.1 Patch and Upgrade Operating System.........................................................5-2 5.1.2 Remove or Disable Unnecessary Services and Applications.......................5-2 5.1.3 Configure Operating System User Authentication........................................5-4 5.1.4 Configure Resource Controls Appropriately .................................................5-6 5.1.5 Install and Configure Additional Security Controls .......................................5-6 5.2 Security Testing the Operating System ...................................................................5-7 v GUIDELINES ON ELECTRONIC MAIL SECURITY 5.3 Checklist for Securing the Mail Server Operating System .......................................5-7 6. Securing Mail Servers and Content................................................................................6-1 6.1 Hardening the Mail Server Application.....................................................................6-1 6.1.1 Securely Installing the Mail Server ...............................................................6-1 6.1.2 Configuring Operating System and Mail Server Access Controls ................6-1 6.2 Protecting Email from Malware ................................................................................6-3 6.2.1 Malware Scanning ........................................................................................6-5 6.2.2 Content Filtering ...........................................................................................6-9 6.2.3 User Awareness .........................................................................................6-12