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 practice you develop to maintain your security policy is lightweight enough to be kept in day-to-day operational use.
5. Shut off unused network services.
A large corporate network might have four or five servers actively enlisted in delivering e-mail, but a typical corporate network might also have 95 other servers listening on the SMTP port. Guess which 95 hosts are most likely to harbor latent mail server vulnerabilities. Audit the network for services that shouldn't be running. If a box is acting as a Windows file server but has never been used as a file server, turn off file-sharing protocols.
6. Defend critical resources first.
On a network with 30,000 machines, it is not realistic to expect that every host can be kept locked down and patched. A typical large network has a triage security challenge. Perform a cost-benefit analysis. It might take one month to find, catalog, patch and harden every Web server on the network. That fact shouldn't keep you from finding critical Web servers (for instance, the one tracking all your sales leads) and locking them down first. You can identify your organization's most critical assets fairly quickly. Locate them on the network and lock them down.
7. Build secure wireless access.
Audit your network for wireless. Eliminate rogue wireless access points. Recognize that wireless network access is a genuinely compelling and useful facility, and offer secure wireless access. Position an access point outside your perimeter firewalls and allow users to VPN through it. It is much less likely that users will go out of their way to build rogue wireless access points if your network already provides wireless access.
8. Build secure visitor access.
Visitors should not be given open access to the internal network. Many security engineers attempt to enforce a "no Internet access from the conference room" policy. This can force employees to give illicit access to visitors from other desks that are harder to track. Build visitor network segments for conference rooms, outside the perimeter firewalls.
9. Create virtual perimeters.
Hosts will remain vulnerable to attack as long as human beings operate them. Instead of creating unrealistic goals like "no host should ever be compromised," make it the goal that no one host give an attacker complete access to the network if it is compromised. Figure out how your network is used and build virtual perimeters around business units. If a marketing user's machine is compromised, the attacker should not get access to corporate R&D. So implement access control in between R&D and marketing. We know how to build perimeters between the Internet and the internal network. It's time to figure out how to build perimeters between the different business user groups on the network.
10. Justify security decisions.
Network users are a critical partner in efforts to improve network security. Typical users may not know the difference between RADIUS and TACACS, or proxy and packet filtering firewalls, but they are likely to cooperate if you are honest and straightforward with them. Make the network easy to use for typical users. If users never have painful run-ins with cumbersome security practices, they will be more responsive to security requirements.
Thomas Ptacek is product manager at Arbor Networks Inc. He can be reached at ptacek@arbor.net.