Almost four years ago, security researcher Antirez posted an innovative new TCP port scanning technique. Idlescan, as it has become known, allows for completely blind port scanning. Attackers can actually scan a target without sending a single packet to the target from their own IP address! Instead, a clever side-channel attack allows for the scan to be bounced off a dumb "zombie" host. Intrusion detection system (IDS) reports will finger the innocent zombie as the attacker. Besides being extraordinarily stealthy, this scan type permits mapping out IP-based trust relationships between machines.

I assumed that a problem of this magnitude would generate an immediate response and patches from the OS vendors. Unfortunately, many have chosen to ignore the problem for years. Apparently, they believe this is just a "theoretical" issue which is not practical to exploit in the real world. To refute that position, and increase the pressure on vendors to fix the problem, I have released a robust Idlescan implementation in recent versions of Nmap. This paper describes the technique in detail and offers defenses that network administrators, ISPs, and OS vendors can use to mitigate the vulnerability.

Note that Idle scanning is only one of the security risks posed by predictable IPID sequence numbers. This paper describes several other information gathering attacks made possible by this characteristic.

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• Software: Nmap