Most of us have heard about the concept of building a defense in depth in order to protect computer resources from black hat hackers. The idea revolves around the use of multiple defenses to thwart, or at least limit, the damage arising from a potential security breach.\n\nGiven the rapid pace of change in the security sector, some executives may have difficulty naming the specific safeguards that their companies deploy. This guide aims to shed some light on some of the more common aspects of computer security, and also serve as a checklist to identify potential areas upon which to improve.\n\n1. Network firewallThe first line of defense against unwelcomed visitors would surely be the firewall. At one point, the use of dual firewalls from different vendors was all the rage, though the creation of a DMZ (Demilitarized zone) appears to be more popular these days. Internet-facing servers are typically placed within the DMZ, where they are encumbered by fewer restrictions and lesser monitoring than the internal corporate network.\n\nThere are actually a few different types of firewall implementations. For example, consumer-grade routers typically make use of Network Address Translation (NAT), which was originally created to address the problem of limited IPv4 routable addresses. Because the identity of hosts is obfuscated, NAT is often said to offer firewall capabilities.\n\nAt a minimum, a proper firewall typically offers packet filter technology, which allows or denies data packets based on established rules relating to the type of data packet and its source and destination address. Stateful packet filter firewalls conduct what is known as stateful packet inspection (SPI), which tracks active connections to sieve out spoofed packets, a superior approach to the stateless packet filtering firewall. Finally, a firewall operating on the application layer understands application-level protocols to identify sophisticated intrusion attempts.\n\nA heightened security awareness and an increase in ecommerce have led more users than ever to use encryption to protect against third-party snooping. Paradoxically, this has resulted in lower visibility of network traffic at a time when more sophisticated malware varieties are resorting to encryption in order to conceal themselves from a casual inspection.\n\n2. Virtual Private NetworkEmployees who need to access company resources from unsecured locations such as public Wi-Fi hotspots are a particularly vulnerable group. Such workers will be well served by a virtual private network (VPN) connection in order to protect the confidentiality of their network access. A VPN channels all network traffic through an encrypted tunnel back to the trusted corporate network. \n\nAs a downside, a VPN can be complex for a small business to deploy, and is costly to support due to the overheads of authentication, processing and bandwidth. Moreover, it is also vulnerable to the theft of physical authentication tokens -- or authentication technology, as was the case with the compromise of RSA's SecurID technology last year. Finally, stolen and lost company laptops with preconfigured VPN settings can become potential gateways for unauthorized access.\n\n3. IDS and IPSAn intrusion detection system (IDS) is a network-centric strategy that involves monitoring traffic for suspicious activities that may indicate that the corporate network has been compromised. On its simplest level, this may entail the detection of port scans originating from within the network or excessive attempts to log into a server. The former could be indicative of a compromised host being used to perform initial reconnaissance, while the latter could well be a brute-force attempt in progress. On more advanced network switches, IDS monitoring of network traffic may be enabled by port mirroring, or via the use of passive network taps.\n\nThen an intrusion prevention system (IPS) is usually deployed in-line in order to actively prevent or block intrusions as they are detected. A specific IP address could be automatically blocked off, with an alarm sent to an administrator.\n\n4. Malware DetectionThe cat-and-mouse game of malware detection is very much a linchpin of the $22.9 billion enterprise security software market projected for 2012. Malware scanning performed on client devices relies on the processing capabilities of individual devices to check for threats. Business-centric versions typically include some form of central management used to push out new definition updates and implement simple security policies. Malware products specifically optimized for servers are also available, though they are not particularly popular, as businesses are understandably loathe to deploy anything that saps the processing cycles of expensive server hardware.\n\nGiven that most malware infestations are a direct result of a user action, the typical anti-malware package has also evolved into comprehensive suites that attempt to offer protection against multiple threat vectors. This may include a component to scrutinize a URL link prior to launching it, or email and browser plug-ins that do the same to file attachments. In addition, anti-malware suites are increasingly bundled with a software-based firewall, spyware detection and even spam filtering.\n\n5. WhitelistingWhitelisting is an anti-malware defense implemented on client devices much like traditional antivirus software. Instead of attempting to identify known malware, however, whitelisting only allows known files to be executed. This necessitates an initial baseline scan to construct a database of whitelisted applications, to which new applications can be added over time as they are installed.\n\nThough promising, whitelisting has been plagued by various practical problems that have hindered its adoption in businesses. Situations may arise, for example, in which critical file dependencies were not properly identified, resulting in application crashes or an improper installation, as they were prevented from loading. Also, whitelisting may be less useful against exploits that leverage the use of specially created documents or other non-executable files. Finally, employees who are in a hurry may simply disregard warnings and opt to add everything, including malware, into their whitelist.\n\nTo be fair, whitelisting software has seen tremendous improvements over the years. Today, most whitelisting software applications will recognize commonly used applications upon installation and are hence capable of building an initial whitelist very quickly and with minimum interaction from users. It is important to ask question whether whitelisting software can coexist with traditional antivirus software. The answer varies, though some whitelisting products do advertise their compatibility with antivirus applications.\n\n6. Spam FilteringThough spam is not traditionally considered within the domain of computer security, the lines are getting blurred given the increasing number of spear phishing attacks used by hackers to sneak Trojan or zero-day malware into corporate workstations. In addition, there is also evidence to suggest that users who deal with a high volume of emails are more susceptible to being taken in by a phishing attempt. It is clearly in the interest of the IT department to filter out as many bogus email messages as possible.\n\nThere are many ways to deal with spam, which may entail channeling all incoming email messages through a specialized cloud service provider, a server-based spam filtering software, or dedicated anti-spam appliances deployed within the DMZ.\n\n7. Keeping Software up to DateEnsuring that software updates and security patches are kept up to date is widely acknowledged to be an important defense against security breaches. The reason is simple. Though vendors do not typically release the full details of new security flaws, the proffered guidelines and the release of the security patches are often sufficient for black hats to reverse engineer a particular vulnerability. Depending on the nature of the security flaw that is identified, an exploit could potentially be written in days.\n\nThis becomes a problem in larger SMBs, which may make use of wide range of software applications or in-house tools that depend on various third-party tools or codebases. It is hence not uncommon for new software updates or security patches to be overlooked, thus opening up a window of vulnerability. The increasing variety of software that is capable of updating itself over the Internet may somewhat alleviate this problem. However, it should be noted that automatic updating may not be a desirable behavior in mission-critical production environments. To that end, businesses need to implement appropriate processes to identify and test new updates in a timely manner.\n\n8. Physical securityPhysical security is a crucial factor that cannot be overstated. After all, given physical access, practically every security or network appliance can be reset to its factory default. In addition, unsecured Ethernet ports may also offer a direct line past the firewall and other perimeter defenses, though that access can be mitigated to an extent with managed switches configured to deny access to unrecognized MAC addresses. Another concern within server rooms is the theft of hard disk drives from hot-swappable bays of storage appliances or servers. Given how passwords files can be deciphered relatively easily from stolen storage devices, server closets or server rooms should be kept locked at all times, and access granted only to authorized staffers.\n\nWe have only touched on some of the most common aspects of security deployments. There are obviously many others, such as the importance of user education, independent security audits and the value of a good IT policy. The presence of comprehensive logging and auditing will also help greatly in identifying sources of a breach. \n\nThe important point here is that security is a multi-faceted topic that is constantly evolving. Small and mid-sized businesses need to ensure that they do not rely on a single mechanism to stay secure, and that they stay up to date on the latest security offerings available.\n\n\n\nPaul Mah is a freelance writer and blogger who lives in Singapore. You can reach Paul at email@example.com and follow him on Twitter at @paulmah.