Stolen credit card numbers, hacked federal computer systems and other high-profile online assaults have put many users on their guards and focused the attention of security managers on high-level intrusion-detection systems, chains of firewalls, and other high-level defenses. But many forget that, no matter how hard they try to secure a site, vulnerabilities built into the fabric of the Internet still leave them at risk -- even though measures to shut down the most glaringly common vulnerabilities are easily available.

Simple functions like requesting a connection between two machines can create openings that are to blame in about 15 percent of the attacks that are reported each year, said Fred Baker, chairman of the Internet Engineering Task Force. That's because TCP/IP hasn't changed much since the days of its acceptance as the Arpanet transport protocol.

"[Internet Protocol] was originally written among a cohesive community that had significant internal trust. By default, IP applications assume they should trust people," Baker says.

Denial-of-service and data hijacking attacks using functions of TCP/IP can be prevented using security functions that can be turned on in most server operating systems, filters built into routers or a new version of IP (Version 6), which is a standard for the public-key infrastructure IPSec protocol.

But those security measures are often ignored.

Take the TCP attack that was "rediscovered" in March by security services firm Guardent Inc. in Waltham, Mass. Guardent researchers figured out a new way to exploit an old problem with TCP: the ability of a hacker to hijack a session if he can guess the random initial sequence number (ISN) that two machines use to start a sequence of packets.

Once an attacker guesses the ISN, he can redirect the packets or inject anything into the data stream. Software vendors were thought to have averted this problem with random packet sequence generators. Turns out those random sequences aren't so random after all and actually contain patterns that make the ISN easy to guess, said Jerry Brady, vice president of research and development at Guardent.

Another ancient exploit, spoofing IP addresses, is also common today, said Paul Raines, head of global information risk management at Barclays Capital, an investment bank in New York.

"Classic TCP/IP attacks such as IP spoofing and denial-of-service attacks using buffer overflows are still out there," he said. "Take the [distributed denial-of-service] attack executed by Mafia Boy last year. He planted Trojan horses in unsuspecting servers. Those servers then flooded e-commerce sites with a load of service requests that contained bogus-source IP addresses. The e-commerce sites couldn't keep up. That caused many of their servers to crash."

Non-IP attacks typically go after vulnerable ports, services in server software or functions like address books and automailers.

Traditional attacks against TCP/IP fall into two categories: denial of service and data hijacking, said Frank Heidt, managing security architect at security consulting firm @stake, Inc. in Cambridge, Mass.

In the 1980s, dozens of types of attacks against TCP/IP ravaged the Arpanet. Of those that still exist today,

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