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When Good Scanners Go Bad

Crackers are turning some scanning tools against the networks they
should protect

By Ann Harrison

03/22/99 Network scanning tools help information technology managers find security holes such as open ports or lists of running services on a host. But crackers are using a new generation of "stealth" scanners to plot attacks on the networks they were designed to protect.

Stealth scanners are dangerous because they can be modified to fall under the threshold of audit trails and intrusion-detection systems, making the attempted probes harder to detect.

"with the traditional thresholds you set up and patterns you look for, you are not going to be able to pick it up," said Karen Evans, a senior network security official at the U.S. Department of Justice.

One stealth scanner, a shareware tool known as Nmap (available at www.insecure.org/nmap), was recently identified by the Shadow intrusion-detection team at the Naval Surface Warfare Center as the likely source of recent highly publicized attacks on Pentagon computer systems.

Although Nmap isn't new, the recently released Version 2.08 gained TCP/IP fingerprinting capability, which allows crackers to identify almost 200 separate operating systems remotely and thus target their attacks. Nmap also sends out decoy packets of data over the network to mask the true source of a scan. When Department of Defense officials suggested that the Pentagon attacks were the work of overseas crackers, the Shadow team pointed to Nmap.

Nmap probes networks by sending data packets to ports, or entry points to network servers, using TCP and Unreliable Datagram Protocol. Crackers can also use Nmap to automatically probe many sites on a network for vulnerabilities.

Analysts from the Shadow team recognized Nmaps from its distinctive use of syn packets, which are the first packets sent from client to server when a TCP connection is opened. Nmap typically sends syn packets to a random range of destination ports, then sends packets to ports with high numbers, and finally more syn packets to a single port.
                              
Instead of scanning network ports at random, Nmap lets crackers launch precisely planned attacks. Nmap is relatively easy to use and can scan a network in seconds. According to John Green, a member of the Shadow team who participated in an online Nmap briefing this month, the tool brings greater sophistication to a wider number of crackers.

"The intelligence that can be garnered using Nmap is extensive," Green said. "Everything that a wiley hacker needs to know about your system is there."

Security consultants recommend running your own Nmap scans to find unsecured servers that can be accessed around the
corporate firewall.
                              
The Shadow team has integrated Nmap into its scanning as has NASA's Ames Research Center at Moffett Field, Calif.

"Right now, it's basically a suffer-along scenario," Green said. But Nmap lets security managers "know what the hackers know about you," he added.

David Remnitz, managing partner at IFsec, a New York computer security consulting firm, said network managers should also adjust intrusion-detection thresholds and place their intrusion-detection tools on the same network segment as the machines they protect. Though that approach screens out random network traffic and highlights suspicious packets, Remnitz acknowledged that it may also generate false alarms.

"If you make it too sensitive, you start tripping a lot of false positives, and then you become immune to false positives and you miss something," Evans observed.

Jacob Carlson, IFsec's senior security consultant, said Nmap is a particular threat to Linux users because many developers have access to the source code and because security holes are rapidly disseminated. Carlson said proxy firewalls keep scanners from evaluating network architecture, and he urges the use of host-based and network-based intrusion-detection systems.
                              
"It takes constant monitoring," Evans said. "It's not one tool over another; it's the mind-set of the staff who review our systems, read information, put in proper patches and do proper testing."

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