Traditional database security mechanisms are very limited in defending successful data attacks. Authorized but malicious transactions can make a database useless by impairing its integrity and availability. This paper presents the design of a real-time data attack isolation system, denoted DAIS. DAIS isolates likely suspicious actions before a definite determination of intrusion is reported. In this way, the database can be immunized from many malicious transactions. DAIS is a COTS-DBMS-specific implementation of a general isolation algorithm that we developed in [19]. In this paper, the design of the first DAIS prototype, which is