Chapter 1: Understanding Network Security Principles

Cisco Press

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For an attacker to “hijack” a session being set up between a legitimate originator and a destination, the attacker needs to know the TCP sequence numbers used in the TCP segments. If the attacker successfully guesses or predicts the correct TCP sequence numbers, he can send a properly constructed ACK segment to the destination. If the attacker’s ACK segment reaches the destination before the originator’s ACK segment does, the attacker becomes trusted by the destination, as illustrated in Figure 1-4.

Figure 1-4

Figure 1-4

IP Spoofing

How an attacker guesses or predicts correct TCP sequence numbers depends on the type of IP spoofing attack being launched. Table 1-7 describes two categories of IP spoofing attacks.

Table 1-7  Types of IP Spoofing Attacks

Type of Attack


Nonblind spoofing

Nonblind spoofing occurs when the attacker and the destination are on the same subnet. By being on the same subnet, the attacker might be able to use a packet-capture utility to glean sequence numbers.

Blind spoofing

Blind spoofing occurs when the attacker is not on the same subnet as the destination. Therefore, obtaining correct TCP sequence numbers is more difficult. However, using techniques such as IP source routing (described next), an attacker can accurately determine those sequence numbers.

Launching a Remote IP Spoofing Attack with IP Source Routing

If an attacker uses a feature known as IP source routing, he can specify a complete routing path to be taken by two endpoints. Consider Figure 1-5. The attacker is on a different subnet than the destination host. However, the attacker sends an IP packet with a source route specified in the IP header, which causes the destination host to send traffic back to the spoofed IP address via the route specified. This approach can overcome the previously described challenge that an attacker might have when launching a remote IP spoofing (blind spoofing) attack.

Figure 1-5

Figure 1-5

IP Source Routing

Source routing has two variations:

  • Loose: The attacker specifies a list of IP addresses through which a packet must travel. However, the packet could also travel through additional routers that interconnect IP addresses specified in the list.

  • Strict: The IP addresses in the list specified by the attacker are the only IP addresses through which a packet is allowed to travel.

Launching a Local IP Spoofing Attack Using a Man-in-the-Middle Attack

If an attacker is on the same subnet as the target system, he might launch a man-in-the-middle attack. In one variant of a man-in-the-middle attack, the attacker convinces systems to send frames via the attacker’s PC. For example, the attacker could send a series of gratuitous ARP (GARP) frames to systems. These GARP frames might claim that the attacker’s Layer 2 MAC address was the MAC address of the next-hop router. The attacker could then capture traffic and forward it to the legitimate next-hop router. As a result, the end user might not notice anything suspicious.

Another variant of a man-in-the-middle attack is when the attacker connects a hub to a network segment that carries the traffic the attacker wants to capture, as shown in Figure 1-6. Alternatively, an attacker could connect to a Switch Port Analyzer (SPAN) port on a Catalyst switch, which makes copies of specified traffic and forwards them to the configured SPAN port. The attack could then use a packet-capture utility to capture traffic traveling between end systems. If the captured traffic is in plain text, the attacker might be able to obtain confidential information, such as usernames and passwords.

Figure 1-6

Figure 1-6

Man-in-the-Middle Attack

Protecting Against an IP Spoofing Attack

The following approaches can be used to mitigate IP spoofing attacks:

  • Use access control lists (ACL) on router interfaces. As traffic comes into a router from an outside network, an ACL could be used to deny any outside traffic claiming to be addressed with IP addressing used internally on the local network. Conversely, ACLs should be used to prevent traffic leaving the local network from participating in a DDoS attack. Therefore, an ACL could deny any traffic leaving the local network that claimed to have a source address that was different from the internal network’s IP address space.

  • Encrypt traffic between devices (for example, between two routers, or between an end system and a router) via an IPsec tunnel. In Figure 1-7, notice that the topology is now protected with an IPsec tunnel. Even though the attacker can still capture packets via his rogue hub, the captured packets are unreadable, because the traffic is encrypted inside the IPsec tunnel.

Figure 1-7

Figure 1-7

Protecting Traffic in a Tunnel

  • Use cryptographic authentication. If the parties involved in a conversation are authenticated, potential man-in-the-middle attackers can be thwarted. Potential attackers will not be successfully authenticated by the other party in the conversation.

Understanding Confidentiality Attacks

A confidentiality attack (see Figure 1-8) attempts to make “confidential” data (such as personnel records, usernames, passwords, credit card numbers, and e-mails) viewable by an attacker. Because an attacker often makes a copy of the data, rather than trying to manipulate the data or crash a system, confidentiality attacks often go undetected. Even if auditing software to track file access were in place, if no one suspected an issue, the audit trail might never be examined.

Figure 1-8

Figure 1-8

Confidentiality Attack

In Figure 1-8, a web server and a database server have a mutual trust relationship. The database server houses confidential customer information, such as credit card information. As a result, Company A decides to protect the database server (for example, patching known software vulnerabilities) better than the web server. However, the attacker leverages the trust relationship between the two servers to obtain customer credit card information and then make a purchase from Company B using the stolen information. The procedure is as follows:

Step 1

The attacker exploits a vulnerability in Company A’s web server and gains control of that server.

Step 2

The attacker uses the trust relationship between the web server and the database server to obtain customer credit card information from the database server.

Step 3

The attacker uses the stolen credit card information to make a purchase from Company B.

Table 1-8 identifies several methods that attackers might use in a confidentiality attack.

Table 1-8 Confidentiality Attack Strategies



Packet capture

A packet-capture utility (such as Wireshark, available at can capture packets visible by a PC’s network interface card (NIC) by placing the NIC in promiscuous mode. Some protocols (for example, Telnet and HTTP) are sent in plain text. Therefore, an attacker can read these types of captured packets, perhaps allowing him to see confidential information.

Ping sweep and port scan

A confidentiality attack might begin with a scan of network resources, to identify attack targets on a network. A ping sweep could be used to ping a series of IP addresses. Ping replies might indicate to an attacker that network resources can be reached at those IP addresses. As soon as a collection of IP addresses is identified, the attacker might scan a range of UDP and/or TCP ports to see what services are available on the host at the specified IP addresses. Also, port scans often help attackers identify the operating system running on the target system.

Dumpster diving

Because many companies throw away confidential information, without proper shredding, some attackers rummage through company dumpsters in hopes of discovering information that could be used to compromise network resources.

Electromagnetic interference (EMI) interception

Because data is often transmitted over wire (for example, unshielded twisted-pair), attackers can sometimes copy information traveling over the wire by intercepting the EMI being emitted by the transmission medium. These EMI emissions are sometimes called “emanations.”


If an attacker gains physical access to a wiring closet, he might physically tap into telephone cabling to eavesdrop on telephone conversations. Or he might insert a shared media hub inline with a network cable. This would let him connect to the hub and receive copies of packets flowing through the network cable.

Social engineering

Attackers sometimes use social techniques (which often leverage people’s desire to be helpful) to obtain confidential information. For example, an attacker might pose as a member of the IT department and ask a company employee for her login credentials “for the IT staff to test the connection.”

Sending information over overt channels

An attacker might send or receive confidential information over a network using an overt channel. An example of using an overt channel is tunneling one protocol inside another (for example, sending instant messaging traffic via HTTP). Steganography is another example of sending information over an overt channel. An example of steganography is sending a digital image made up of millions of pixels, with “secret” information encoded in specific pixels. Only the sender and receiver know which pixels represent the encoded information.

Sending information over covert channels

An attacker might send or receive confidential information over a network using a covert channel, which can communicate information as a series of codes and/or events. For example, binary data could be represented by sending a series of pings to a destination. A single ping within a certain period of time could represent a binary 0, and two pings within that same time period could represent a binary 1.

Understanding Integrity Attacks

Integrity attacks attempt to alter data (that is, compromise its integrity). Figure 1-9 shows an example of an integrity attack.

Figure 1-9

Figure 1-9

Integrity Attack

In the figure, an attacker has launched a man-in-the-middle attack (as previously described). This attack causes data flowing between the banking customer and the banking server to be sent via the attacker’s computer. The attacker then can not only intercept but also manipulate the data. In the figure, notice that the banking customer attempts to deposit $500 into her account. However, the attacker intercepts and changes the details of the transaction, such that the instruction to the banking server is to deposit $5,000 in the attacker’s account.

The following list describes methods that attackers might leverage to conduct an integrity attack:

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