Best Practices
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Make sure you have a security policy in place -- The security policy is the formal statement of rules on how security will be implemented in your organization. A security policy should define the level of security and the roles and responsibilities of users, administrators and managers.
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Make sure all of your operating systems and applications are patched with the latest service packs and hotfixes -- Keeping your systems patched will close vulnerabilities that can be exploited by hackers.
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Keep an inventory of your network devices -- Develop and maintain a list of all hardware/software components, and understand which default software installations provide weak security configurations.
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Scan TCP/UDP services -- Turn off or remove unnecessary services. Unneeded services can be the entry point attackers use to gain control of your system.
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Establish a strong password policy -- Weak passwords could mean a compromised user account.
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Don't trust code from non-trusted sources.
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Block certain e-mail attachment types -- This list includes .bas, .bat, .exe and .vbs.
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Don't provide more rights to system resources than necessary -- Implement the concept of "least privilege".
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Perform your own network security testing -- Find the holes before the attackers do!
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Implement "defense-in-depth" -- Don't rely on just one control or system to provide all the security you need.
You can find the full guide Here.
SANS Top-20

Top Ten Network Security Tips
Wondering what the "best practices" are for securing your network?
There is a very informative document called "The 60 Minute Network
Security Guide" on the National Security Agency Web site (www.nsa.gov).
Here's a brief summary ...