Is Microsoft's Firewall Secure?

Security experts and vendors this week welcomed the introduction of Windows Firewall, part of Windows XP Service Pack 2 (SP2), as a valuable way of protecting PCs. But while the firewall is an improvement, it falls short of the standard of protection expected of commercial firewalls, according to some industry observers.

Windows Firewall--which replaces the old Internet Connection Firewall--marks the first time all up-to-date PCs will have a firewall switched on by default, an important step in stopping the spread of viruses, according to industry analysts. However, the software suffers from two major flaws, critics say: it does not block outbound traffic, and it can be switched off by another application, possibly even by a clever worm.

Jumping the Wall

Most commercial firewalls include a feature to stop all but authorized applications from sending data to the Internet; this stops malicious code from sending unauthorized communications, and also prevents PCs from being hijacked and used to send spam or participate in distributed denial-of-service attacks. Windows Firewall, however, filters only incoming traffic, allowing any application to send outbound packets, a fact which some industry observers have said makes it less useful for serious protection.

"It still isn't as robust as many third-party host-based firewalls," writes Jeff Fellinge, information security officer at media company aQuantive, in a recent analysis of the firewall.

More seriously, rival firewall makers claim that the API used to manage the Windows Firewall could also be used by attackers to modify the software or turn it off. Major firewall makers, including Zone Labs, McAfee, and Symantec are preparing SP2-compatible versions of their applications which disable Windows Firewall when they are installed, and enable it again when they are uninstalled.

But if an installer can switch off Windows Firewall, so could an attacker, argues Zone Labs, maker of the popular ZoneAlarm firewall. The company says its own products are locked down in such a way that third-party applications can't disable firewall protection without uninstalling the software.

Defining Roles

Microsoft admits that, in some cases, malicious code could indeed switch the firewall off. However, this isn't so much a flaw as a limitation on the role firewalls should play in a company's security system, according to Microsoft.

"An attacker could misuse that (administrative) capability," says David Overton, a Microsoft technical specialist. "But you're already in a compromised state, if you're at that point." He says Windows Firewall is designed to stop malicious transmissions to the PC, rather than protecting the PC once it's been infected.

If malicious code makes it past the firewall, it is the role of anti-virus software to protect the machine, Overton adds. Likewise, it is not the firewall's place to stop malicious code from sending outbound packets--Microsoft contends that companies should use perimeter technologies to examine outbound traffic.

"The firewall is a management process, not a silver bullet," Overton says. He says Microsoft's user testing showed that asking users to approve every application trying to communicate with the Internet tends to backfire.

"If you flood the user with messages like that, they say 'yes' all the time," he says.

Rival firewall makers say they have various ways of dealing with this problem. McAfee, for example, has a "white list" of trusted applications, designed to reduce the number of messages a user receives.