As intruders become more sophisticated, they identify new and increasingly complex methods of attack. For example, intruders are developing sophisticated techniques to monitor the Internet for new connections. Newly connected systems are often not fully configured from a security perspective and are, therefore, vulnerable to attacks. The most widely publicized of the newer types of intrusion is the use of the packet sniffers described in the section above on packet sniffers. Other tools are used to construct packets with forged addresses; one use of these tools is to mount a denial-of-service attack in a way that obscures the source of the attack. Intruders also "spoof" computer addresses, masking their real identity and successfully making connections that would not otherwise be permitted. In this way, they exploit trust relationships between computers. With their sophisticated technical knowledge and understanding of the network, intruders are increasingly exploiting network interconnections. They move through the Internet infrastructure, attacking areas on which many people and systems depend. Infrastructure attacks are even more threatening because legitimate network managers and administrators typically think about protecting systems and parts of the infrastructure rather than the infrastructure as a whole. In the first quarter of 1996, 7.5% of 346 incidents handled by the CERT Coordination Center involved these new and sophisticated methods, including packet sniffers, spoofing, and infrastructure attacks. A full 20% involved the total compromise of systems, in which intruders gain system-level, or root, privileges. This represents a significant increase in such attacks over previous years' attacks, and the numbers are still rising. Of 341 incidents in the third quarter of 1996, nearly 9% involved sophisticated attacks, and root compromises accounted for 33%.