Key security concepts (threats, vulnerabilities, exploits, and mitigation techniques)

Key security concepts (threats, vulnerabilities, exploits, and mitigation techniques)

Because modern enterprise networks are usually made up of many parts that all work together, securing them can become a very complex task. As with the simple box analogy, you cannot effectively try to secure it until you have identified many of the vulnerabilities, assessed the many exploits that exist, and realized where the threats might come from. Only then can the appropriate countermeasures and mitigations be put in place.

You should also consider some important attributes of enterprise resources that should be protected and preserved. As you work through the many threats that are discussed in this chapter, think about the vulnerability and exploit that makes the threat possible. Notice how many different parts of the enterprise network exhibit vulnerabilities and how the threats are crafted to take advantage of the weaknesses.

Attacks That Spoof Addresses

When systems behave normally, parameters and services can be trusted and used effectively. For example, when a machine sends an IP packet, everyone expects the source IP address to be the machine’s own IP address. The source MAC address in the Ethernet frame is expected to be the sender’s own MAC address. Even services like DHCP and DNS should follow suit; if a machine sends a DHCP or DNS request, it expects any DHCP or DNS reply to come from a legitimate, trusted server. Spoofing attacks focus on one vulnerability; addresses and services tend to be implicitly trusted. Attacks usually take place by replacing expected values with spoofed or fake values. Address spoofing attacks can be simple and straightforward, where one address value is substituted for another.

For example, an attacker can send packets with a spoofed source IP address instead of its own. When the target receives the packets, it will send return traffic to the spoofed address, rather than the attacker’s actual address. If the spoofed address exists, then an unsuspecting host with that address will receive the packet. If the address does not exist, the packet will be forwarded and then dropped further out in the network.

An attacker can send spoofed MAC addresses too, to add false information to the forwarding tables used by Layer 2 switches or ARP tables used by other hosts and routers. DHCP requests with spoofed MAC addresses can also be sent to a legitimate DHCP server, filling its address lease table and leaving no free IP addresses for normal use.

Denial-of-Service Attacks

In the normal operation of a business application, clients open connections to corporate servers to exchange information. This might occur in the form of web-based sessions that are open to internal users as well as external users on the public Internet. The process is simple: users open a web browser to the corporate site, which then opens a TCP connection with the corporate web server; then some transaction can take place. If all the users are well behaved and conduct legitimate transactions, the corporate servers are (hopefully) not stressed and many clients can do business normally.

Now suppose a malicious user finds a way to open an abnormal connection to the same corporate server. The TCP connection begins with the malicious user sending a SYN flag to the server, but the source IP address is replaced with a fake address. The server adds the TCP connection to its table of client connections and replies to the fake address with a SYN-ACK. Because the fake address is not involved in the TCP connection, there is no ACK reply to complete the TCP three-way handshake. The incomplete connection stays in the server’s table until it eventually times out and is removed. During this time, the attacker can try to open many, many more abnormal connections at such a rate that the server’s connection table fills. At that point, the server is no longer able to maintain TCP connections with legitimate users, so their business transactions all halt.

When an attacker is able to deplete a system resource, services and systems become unavailable or crash. This is called a denial-of-service (DoS) attack because it denies service to legitimate users or operations. DoS attacks can involve something as simple as ICMP echo (ping) packets, a flood of UDP packets, and TCP connections, such as the TCP SYN flood attack previously described. Such attacks can be successful provided a system has a vulnerability with the protocol or type of traffic that is exploited.

Attackers can carry the DoS idea even further by enlisting many other systems to participate. To do this, the attacker sets up a master control computer somewhere on the Internet. Next, many computers must first be infected with malicious code or malware by leveraging vulnerabilities present in those machines. Each machine then silently becomes a “bot,” appearing to operate normally, while awaiting commands from the master control. When the time comes for an attack to begin, the master control sends a command to every bot and tells it to initiate a denial-of-service attack against a single target host. This is called a distributed denial-of-service (DDoS) attack because the attack is distributed across a large number of bots, all flooding or attacking the same target.

Reflection and Amplification Attacks

Recall that in a spoofing attack, the attacker sends packets with a spoofed source address to a target. The goal is to force the target to deal with the spoofed traffic and send return traffic toward a nonexistent source. The attacker does not care where the return traffic goes or that it cannot be delivered successfully. In a somewhat related attack, the attacker again sends packets with a spoofed source address toward a live host. However, the host is not the intended target; the goal is to get the host to reflect the exchange toward the spoofed address that is the target. This is known as a reflection attack, and the host reflecting the traffic toward the target is called the reflector. The attacker might also send the spoofed packets to multiple reflectors, causing the target to receive multiple copies of the unexpected traffic.

The impact of a reflection attack might seem limited because a single target host is the victim, and the amount of traffic being reflected to the target is in proportion to the packets sent by the attacker. If an attacker is able to send a small amount of traffic to a reflector and leverage a protocol or service to generate a large volume of traffic toward a target, then an amplification attack has occurred. In effect, such an attack amplifies the attacker’s efforts to disrupt the target. Another result is that large amounts of network bandwidth can be consumed forwarding the amplified traffic toward the target, especially if many reflectors are involved. Some mechanisms of DNS and NTP have been exploited in the past to set new records for enormous bandwidth consumption during an amplification attack.

Man-in-the-Middle Attacks

Many types of attacks are meant to disrupt or directly compromise targeted systems, often with noticeable results. Sometimes an attacker might want to eavesdrop on data that passes from one machine to another, avoiding detection. A man-in-the-middle attack does just that, by allowing the attacker to quietly wedge itself into the communication path as an intermediary between two target systems. One type of man-in-the-middle attack exploits the ARP table that each host maintains to communicate with other hosts on its local network segment. Normally, if one host needs to send data to another, it looks for the destination host in its ARP table. If an entry is found, the Ethernet frame can be sent directly to the destination MAC address; otherwise, the sender must broadcast an ARP request containing the destination’s IP address and wait for the destination to answer with an ARP reply and its own MAC address.

In step 1, a client broadcasts an ARP request to find out what MAC address is used by the host with IP address In step 2, the ARP request is flooded to all hosts in the broadcast domain. This allows the attacker to overhear the ARP request and prepare to exploit the information learned. The legitimate owner of may indeed respond with its own ARP reply and real MAC address, as expected. However, in step 3, the attacker simply waits a brief time and then sends a spoofed ARP reply containing its own MAC address, rather than that of the actual destination. The goal is for the attacker to send the last ARP reply so that any listening host will update its ARP table with the most recent information.

This process effectively poisons the ARP table entry in any system receiving the spoofed ARP reply. From that point on, a poisoned system will blindly forward traffic to the attacker’s MAC address, which now masquerades as the destination. The attacker is able to know the real destination’s MAC address because he received an earlier ARP reply from the destination host. Figure 4-8 depicts the end result. The attacker can repeat this process by poisoning the ARP entries on multiple hosts and then relaying traffic between them without easy detection.

Once an attacker has inserted herself between two hosts, she can passively eavesdrop on and inspect all traffic passing between them. The attacker might also take an active role and modify the data passing through.

Reconnaissance Attacks

When an attacker intends to launch an attack on a target, that attacker might want to identify some vulnerabilities so the attack can be focused and more effective. A reconnaissance attack can be used to discover more details about the target and its systems prior to an actual attack.

During a reconnaissance attack, the attacker can use some common tools to uncover public details like who owns a domain and what IP address ranges are used there. For example, the nslookup command exists in many operating systems and can perform a DNS lookup to resolve an IP address from a fully qualified domain name. If an attacker knows the domain name of a business, nslookup can reveal the owner of the domain and the IP address space registered to it. The whois and dig commands are complementary tools that can query DNS information to reveal detailed information about domain owners, contact information, mail servers, authoritative name servers, and so on.

Then the attacker can progress to using ping sweeps to send pings to each IP address in the target range. Hosts that answer the ping sweep then become live targets. Port scanning tools can then sweep through a range of UDP and TCP ports to see if a target host answers on any port numbers. Any replies indicate that a corresponding service is running on the target host.

Keep in mind that a reconnaissance attack is not a true attack because nothing is exploited as a result. It is used for gathering information about target systems and services so that vulnerabilities can be discovered and exploited using other types of attacks.

Buffer Overflow Attacks

Operating systems and applications normally read and write data using buffers and temporary memory space. Buffers are also important when one system communicates with another, as IP packets and Ethernet frames come and go. As long as the memory space is maintained properly and data is placed within the correct buffer boundaries, everything should work as expected.

However, some systems and applications have vulnerabilities that can allow buffers to overflow. This means some incoming data might be stored in unexpected memory locations if a buffer is allowed to fill beyond its limit. An attacker can exploit this condition by sending data that is larger than expected. If a vulnerability exists, the target system might store that data, overflowing its buffer into another area of memory, eventually crashing a service or the entire system. The attacker might also be able to specially craft the large message by inserting malicious code in it. If the target system stores that data as a result of a buffer overflow, then it can potentially run the malicious code without realizing.


Some types of security threats can come in the form of malicious software or malware. For example, a trojan horse is malicious software that is hidden and packaged inside other software that looks normal and legitimate. If a well-meaning user decides to install it, the trojan horse software is silently installed too. Then the malware can run attacks of its own on the local system or against other systems. Trojan horse malware can spread from one computer to another only through user interaction such as opening email attachments, downloading software from the Internet, and inserting a USB drive into a computer.

In contrast, viruses are malware that can propagate between systems more readily. To spread, virus software must inject itself into another application, then rely on users to transport the infected application software to other victims. One other type of malware is able to propagate to and infect other systems on its own. An attacker develops worm software and deposits it on a system. From that point on, the worm replicates itself and spreads to other systems through their vulnerabilities, then replicates and spreads again and again.

Human Vulnerabilities

Many types of attack must take advantage of a vulnerability in an operating system, service, or other types of application software. In other words, an attacker or the malware involved must find a weakness in the target computer system. There are still many other attacks that can succeed by exploiting weaknesses in the humans that use computer systems. One rather straightforward attack is called social engineering, where human trust and social behaviors can become security vulnerabilities. For example, an attacker might pose as an IT staff member and attempt to contact actual end users through phone calls, emails, and social media. The end goal might be to convince the users to reveal their credentials or set their passwords to a “temporary” value due to some fictitious IT maintenance that will take place, allowing the attacker to gain easy access to secure systems. Attackers might also be physically present and secretly observe users as they enter their credentials.

Phishing is a technique that attackers use to lure victims into visiting malicious websites. The idea is to either disguise the invitation as something legitimate, frighten victims into following a link, or otherwise deceive users into browsing content that convinces them to enter their confidential information. Phishing comes in many forms. Spear phishing targets a group of similar users who might work for the same company, shop at the same stores, and so on, who all receive the same convincing email with a link to a malicious site. Whaling is similar but targets high-profile individuals in corporations, governments, and organizations. Phishing can also occur over traditional communications, such as voice calls (vishing) and SMS text messages (smishing).

Password Vulnerabilities

Most systems in an enterprise network use some form of authentication to grant or deny user access. When users access a system, a username and password are usually involved. It might be fairly easy to guess someone’s username based on that person’s real name. If the user’s password is set to some default value or to a word or text string that is easy to guess, an attacker might easily gain access to the system too. An attacker can launch an online attack by actually entering each password guess as the system prompts for user credentials. In contrast, an offline attack occurs when the attacker is able to retrieve the encrypted or hashed passwords ahead of time, then goes offline to an external computer and uses software there to repeatedly attempt to recover the actual password.

Attackers can also use software to perform dictionary attacks to discover a user’s password. The software will automatically attempt to log in with passwords taken from a dictionary or word list. It might have to go through thousands or millions of attempts before discovering the real password. In addition, the software can perform a brute-force attack by trying every possible combination of letter, number, and symbol strings. Brute-force attacks require very powerful computing resources and a large amount of time. To mitigate password attacks, an enterprise should implement password policies for all users. Such a policy might include guidelines that require a long password string made up of a combination of upper- and lowercase characters along with numbers and some special characters. The goal is to require all passwords to be complex strings that are difficult to guess or reveal by a password attack. As well, password management should require all passwords to be changed periodically so that even lengthy brute-force attacks would not be able to recover a password before it is changed again .

Controlling and Monitoring User Access

You can manage user activity to and through systems with authentication, authorization, and accounting (AAA, also pronounced “triple-A”) mechanisms. AAA uses standardized methods to challenge users for their credentials before access is allowed or authorized. Accounting protocols also can record user activity on enterprise systems. AAA is commonly used to control and monitor access to network devices like routers, switches, firewalls, and so on .

In a nutshell, you can think of AAA in the following manner:

  • Authentication: Who is the user?
  • Authorization: What is the user allowed to do?
  • Accounting: What did the user do?

A more scalable solution is to leverage AAA functions that are centralized, standardized, resilient, and flexible. For example, a centralized authentication server can contain a database of all possible users and their passwords, as well as policies to authorize user activities. As users come and go, their accounts can be easily updated in one place. All switches and routers would query the AAA server to get up-to-date information about a user. For greater security, AAA servers can also support multifactor user credentials and more. Cisco implements AAA services in its Identity Services Engine (ISE) platform.

AAA servers usually support the following two protocols to communicate with enterprise resources:

  • TACACS+: A Cisco proprietary protocol that separates each of the AAA functions. Communication is secure and encrypted over TCP port 49.
  • RADIUS: A standards-based protocol that combines authentication and authorization into a single resource. Communication uses UDP ports 1812 and 1813 (accounting) but is not completely encrypted.

Both TACACS+ and RADIUS are arranged as a client/server model, where an authenticating device acts as a client talking to a AAA server. Below shows a simplified view of the process, where a user is attempting to connect to a switch for management purposes. In the AAA client role, the switch is often called Network Access Device (NAD) or Network Access Server (NAS). When a user tries to connect to the switch, the switch challenges the user for credentials, then passes the credentials along to the AAA server. In simple terms, if the user passes authentication, the AAA server returns an “accept” message to the switch. If the AAA server requires additional credentials, as in multifactor authentication, it returns a “challenge” message to the switch. Otherwise, a “reject” message is returned, denying access to the user.

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