ISA Server 2004 achieves Common Criteria Level EAL 4+ for security.

Microsoft Internet Security and Acceleration (ISA) Server has achieved CC Evaluation Assurance Level 4+ (EAL 4+). Level 4 is the highest level possible that is mutually recognized by all countries participating in CC certification. This level provides the deepest evaluation and testing possible from an independent testing laboratory. In addition, ISA Server passed an even more thorough review, earning Level 4+. You can find the supporting resources and documentation on this page. This CC certificate assures you that the evaluated security features of ISA Server Standard Edition are effective and implemented correctly.

http://www.microsoft.com/isaserver/techinfo/deployment/commoncrit.mspx