Home > Articles > Cisco Network Technology > General Networking > Identifying and Classifying Network Security Threats

Identifying and Classifying Network Security Threats

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jun 26, 2008.

Chapter Description

This chapter presents several best practices and methodologies you can use to successfully and quickly identify and classify security threats and anomalies in the network.

This chapter covers the following topics:

  • Network Visibility
  • Telemetry and Anomaly Detection
  • Intrusion Detection and Intrusion Prevention Systems (IDS/IPS)

Worms and denial of service (DoS) attacks are used maliciously to consume the resources of your hosts and network that would otherwise be used to serve legitimate users. In some cases, misconfigured hosts and servers can send traffic that consumes network resources unnecessarily. Having the necessary tools and mechanisms to identify and classify security threats and anomalies in the network is crucial. This chapter presents several best practices and methodologies you can use to successfully and quickly identify and classify such threats.

Most people classify security attacks into two separate categories: logic attacks and resource attacks. Logic attacks exploit existing software deficiencies and vulnerabilities to cause systems to crash, to substantially degrade their performance, or to enable attackers to gain access to a system. An example of this type of attack is the exploit of the Microsoft PnP MS05-039 Overflow Vulnerability, in which the attacker exploits a stack overflow in the Windows "plug and play" (PnP) service. You can exploit this vulnerability on Windows 2000 without a valid user account. Another example is the famous and old ping-of-death, whereby an attacker sends the system Internet Control Message Protocol (ICMP) packets that exceed the maximum legal length (65535 octets). You can prevent most of these attacks by either upgrading the vulnerable software or by filtering particular packet sequences.

The second category of attacks is referred to as resource attacks. The goal with these types of attacks is to overwhelm the victim system/network resources, such as CPU and memory. In most cases, this is done by sending numerous IP packets or forged requests. An attacker can build up a more powerful attack with a more sophisticated and effective method of compromising multiple hosts and installing small attack daemon(s). This is what many call zombies or bot hosts/nets. Subsequently, an attacker can launch a coordinated attack from thousands of zombies onto a single victim. This daemon typically contains both the code for sourcing a variety of attacks and some basic communications infrastructure to allow for remote control. A zombie attack is illustrated in Figure 3-1.

Figure 3-1

Figure 3-1 Zombies and Bots

In Figure 3-1, an attacker controls compromised hosts in Company A and Company B to attack a web server farm in another organization.

You can use different mechanisms and methodologies to successfully identify and classify these threats/attacks depending on their type. In other words, depending on the threat, you can use specific techniques to identify and classify them accordingly. Following are the most common methodologies:

  • The use of anomaly detection tools
  • Network telemetry using flow-based analysis
  • The use of intrusion detection and intrusion prevention systems (IDS/IPS)
  • Analyzing network component logs (that is, SYSLOG from different network devices, accounting records, application logs, Simple Network Management Protocol (SNMP), and so on)

Complete visibility is one of the key requirements when identifying and classifying security threats. The following sections explain best practices for achieving complete network visibility and the use of the previously mentioned tools and mechanisms.

Network Visibility

The first step in the process of preparing your network and staff to successfully identify security threats is achieving complete network visibility. You cannot protect against or mitigate what you cannot view/detect. You can achieve this level of network visibility through existing features on network devices you already have and on devices whose potential you do not even realize. In addition, you should create strategic network diagrams to clearly illustrate your packet flows and where, within the network, you may enable security mechanisms to identify, classify, and mitigate the threat. Remember that network security is a constant war. When defending against the enemy, you must know your own territory and implement defense mechanisms in place. Figure 3-2 illustrates a fairly simple high-level enterprise diagram.

Figure 3-2

Figure 3-2 High-Level Enterprise Diagram

In Figure 3-2, the following sections are numbered:

  1. The Internet edge: In this example, the enterprise headquarters is connected to the Internet via redundant links. Two Cisco Adaptive Security Appliances (ASA) are configured to protect the infrastructure.
  2. Site-to-Site VPN: The headquarters office is connected to two branches via IPsec site-to-site VPN tunnels terminated on two Cisco IOS routers.
  3. End users: The headquarters building has its sales, finance, engineering, and marketing departments on four separate floors.
  4. Call center: There is a call center with more than 100 agents on the 5th floor.
  5. Data center: The data center includes e-commerce, e-mail, database, and other application servers.

You can create this type of diagram not only to understand the architecture of your organization but also to strategically identify places within the infrastructure where you can implement telemetry mechanisms like NetFlow and identify choke points where you can mitigate an incident. Notice that the access, distribution, and core layers/boundaries are clearly defined.

Look at the example illustrated in Figure 3-3. A workstation at the call center usually communicates over TCP port 80 (HTTP) to a server in the data center. This traffic is allowed within the access control lists because it is legitimate traffic to the server. However, the traffic from this specific workstation increased more than 400 percent over normal. Subsequently, performance on the server is degraded, and the infrastructure is congested with unnecessary packets.

Figure 3-3

Figure 3-3 NetFlow at the Distribution Switch

In this case, NetFlow was configured at the distribution layer switch, and the administrator was able to detect the anomaly. The administrator then configures a host-specific ACL to deny the traffic from the call center workstation, as shown in Figure 3-4. In more sophisticated environments, you can even implement remotely triggered black hole (RTBH) routing to mitigate this incident.

Figure 3-4

Figure 3-4 Abnormal Traffic Stopped

In the example illustrated in Figure 3-4, the problem was a defect within the call center workstation application. The administrator was able to perform detailed analysis and patch the machine while preventing disruption of service.

You can also develop a different type of diagram to visualize operational risks within your organization. These diagrams are based on device roles and can be developed for critical systems you want to protect. For example, identify a critical system within your organization and create a layered diagram similar to the one in Figure 3-5. In this example, a database called ABC is the most critical application/data source for this company. The diagram presents ABC Database Server in the center.

Figure 3-5

Figure 3-5 Layered Diagram for Visualizing Risk

You can use this type of diagram to audit device roles and the type of services they should be running. For example, you can decide in what devices you can run services like Cisco NetFlow or where to enforce security policies. In addition, you can see the life of a packet within your infrastructure depending on the source and destination. An example is illustrated in Figure 3-6.

Figure 3-6

Figure 3-6 Illustrating a Packet Flow

Figure 3-6 shows the packet flow that occurs when a user from the sales department accesses an Internet site. You know exactly where the packet is going based on your architecture and your security and routing policies. This is a simple example; however, you can use this concept to visualize risks and to prepare your isolation policies.

2. Telemetry and Anomaly Detection | Next Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020