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10 tips for improving security inside the firewall

By Thomas Ptacek, Arbor Networks
November 13, 2003 12:00 PM ET

Computerworld - Large companies have significantly improved the security of the network perimeter, but despite their investments in that area, most large networks remain vulnerable at their core. Techniques that have proved successful at defending the perimeter have not been effective internally, as a result of both scalability and perspective issues. However, security practitioners can make major strides in fortifying their internal networks by aligning their tactics with the realities of internal network security.
The following 10 tips illustrate ways to address the security challenges of large, active internal networks. Additionally, since they involve defensive tactics, they provide a game plan for improving the security of a large enterprise network.

1. Remember that internal security is different from perimeter security.
The threat model for internal security differs from that of perimeter security. Perimeter security defends your networks from Internet attackers, armed with zero-day exploits of common Internet services like HTTP and SMTP. However, the access a janitor has to your network, simply by plugging in to an Ethernet jack, dwarfs the access a sophisticated hacker gains with scripts. Deploy "hacker defenses" at the perimeter; configure and enforce policy to address internal threats.

2. Lock down VPN access.
Virtual private network clients are an enormous internal security threat because they position unhardened desktop operating systems outside the protection of the corporate firewall. Be explicit about what VPN users are allowed to access. Avoid giving every VPN user carte blanche for the entire internal network. Apply access-control lists to limit classes of VPN users' access to only what they need, such as mail servers or select intranet resources.

3. Build Internet-style perimeters for partner extranets.
Partner networks contribute to the internal security problem. Although savvy security administrators know how to configure their firewalls to block MS-SQL, the Slammer worm brought down networks because companies had given their partners access to internal resources. Since you can't control the security policies and practices of your partners, create a DMZ for each partner, place resources they need to access in that DMZ, and disallow any other access to your network.

4. Automatically track security policy.
Intelligent security policy is the key to effective security practice. The challenge is that changes in business operations greatly outpace the ability to adapt security policy manually. This reality demands that you devise automated methods of detecting business practice changes that require reconciliation with security policy. This can be as in-depth as tracking when employees are hired and fired, and as simple as tracking network usage and noting which computers talk to which file servers. Above all, make sure that whatever



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