Persistent computer security vulnerabilities may expose U.S. critical infrastructure and government computer systems to possible cyber attack by terrorists, possibly affecting the economy or other areas of national security. This report discusses possible cyber capabilities of terrorists and sponsoring nations, describes how computer security vulnerabilities might be exploited through a cyber terror attack, and raises some potential issues for Congress. Currently no evidence exists that terrorist organizations are actively planning to use computers as a means of attack, and there is disagreement among some observers about whether critical infrastructure computers offer an effective target for furthering terrorists' goals. However, terrorist organizations now use the Internet to communicate, and news reports have indicated that Al Qaeda and other groups may be using computer technology to help plan future terrorist attacks. At the same time, nuisance attacks against computer systems and the Internet are becoming more rapid and widespread, indicating that computer system vulnerabilities persist despite growing concerns about possible effects on national security.