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. The 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 was developed previously (P. Liu et al., 2000). The design of the first DAIS prototype, which is for Oracle Server 8.1.6, is discussed. DAIS uses triggers and transaction profiles to keep track of the items read and written by transactions, isolates attacks by rewriting user SQL statements, and is transparent to end users. The DAIS design is very general. In addition to Oracle, it can be easily adapted to support many other database application platforms such as Microsoft SQL Server, Sybase, and Informix.