IP SPOOFING
IP spoofing, also known as IP address forgery or a host file hijack, is a hijacking technique in which a cracker masquerades as a trusted host to conceal his identity,spoof a Web site, hijack browsers, or gain access to a network. Here's how it works: The hijacker obtains the IP address of a legitimate host and alters packet headers so that the legitimate host appears to be the source.When IP spoofing is used to hijack a browser, a visitor who types in the URL (Uniform Resource Locator) of a legitimate site is taken to a fraudulent Web page created by the hijacker. For example, if the hijacker spoofed the Library of Congress Web site, then any Internet user who typed in the URL www.loc.gov.com would see spoofed content created by the hijacker.If a user interacts with dynamic content on a spoofed page, the high jacker can gain access to sensitive information or computer or network resources. He could steal or alter sensitive data, such as a credit card number or password, or install malware . The hijacker would also be able to take control of a compromised computer to use it as part of a zombie army in order to send out spam.Web site administrators can minimise the danger that their IP addresses will be spoofed by implementing hierarchical or one-time passwords and data encryption/decryption techniques. Users and administrators can protect themselves and their networks by installing and implementing firewalls that block outgoing packets with source addresses that differ from the IP address of the user's computer or internal network.
Technical Discussion
To completely understand how these attacks can take place, one must examine the structure of the TCP/IP protocol suite. A basic understanding of these headers and network exchanges is crucial to the process.
Internet Protocol – IP
Internet protocol (IP) is a network protocol operating at layer 3 (network) of the OSI model. It is a connectionless model, meaning there is no information regarding transaction state, which is used to route packets on a network. Additionally, there is no method in place to ensure that a packet is properly delivered to the destination.
Examining the IP header, we can see that the first 12 bytes (or the top 3 rows of the header) contain various information about the packet. The next 8 bytes (the next 2 rows), however, contains the source and destination IP addresses. Using one of several tools, an attacker can easily modify these addresses – specifically the “source address” field. It's important to note that each datagram is sent independent of all others due to the stateless nature of IP. Keep this fact in mind as we examine TCP in the next section.
Transmission Control Protocol – TCP
IP can be thought of as a routing wrapper for layer 4 (transport), which contains the Transmission Control Protocol (TCP). Unlike IP, TCP uses a connection-oriented design. This means that the participants in a TCP session must first build a connection - via the 3-way handshake (SYN-SYN/ACK-ACK) - then update one another on progress - via sequences and acknowledgements. This “conversation”, ensures data reliability, since the sender receives an OK from the recipient after each packet exchange.
As you can see above, a TCP header is very different from an IP header. We are concerned with the first 12 bytes of the TCP packet, which contain port and sequencing information. Much like an IP datagram, TCP packets can be manipulated using software. The source and destination ports normally depend on the network application in use (for example, HTTP via port 80). What's important for our understanding of spoofing are the sequence and acknowledgement numbers. The data contained in these fields ensures packet delivery by determining whether or not a packet needs to be resent. The sequence number is the number of the first byte in the current packet, which is relevant to the data stream. The acknowledgement number, in turn, contains the value of the next expected sequence number in the stream. This relationship confirms, on both ends, that the proper packets were received. It’s quite different than IP, since transaction state is closely monitored.
Consequences of the TCP/IP Design
Now that we have an overview of the TCP/IP formats, let's examine the consequences. Obviously, it's very easy to mask a source address by manipulating an IP header. This technique is used for obvious reasons and is employed in several of the attacks discussed below. Another consequence, specific to TCP, is sequence number prediction, which can lead to session hijacking or host impersonating. This method builds on IP spoofing, since a session, albeit a false one, is built. We will examine the ramifications of this in the attacks discussed below.
Spoofing Attacks
There are a few variations on the types of attacks that successfully employ IP spoofing. Although some are relatively dated, others are very pertinent to current security concerns.
Non-Blind Spoofing
This type of attack takes place when the attacker is on the same sub net as the victim. The sequence and acknowledgement numbers can be sniffed, eliminating the potential difficulty of calculating them accurately. The biggest threat of spoofing in this instance would be session hijacking. This is accomplished by corrupting the data stream of an established connection, then re-establishing it based on correct sequence and acknowledgement numbers with the attack machine. Using this technique, an attacker could effectively bypass any authentication measures taken place to build the connection.
Blind Spoofing
This is a more sophisticated attack, because the sequence and acknowledgement numbers are unreachable. In order to circumvent this, several packets are sent to the target machine in order to sample sequence numbers. While not the case today, machines in the past used basic techniques for generating sequence numbers. It was relatively easy to discover the exact formula by studying packets and tcp sessions. Today, most of the implement random sequence number generation, making it difficult to predict them accurately. If, however, the sequence number was compromised, data could be sent to the target. Several years ago, many machines used host-based authentication services. A properly crafted attack could add the requisite data to a system (i.e. a new user account), blindly, enabling full access for the attacker who was impersonating a trusted host.
Misconceptions of IP Spoofing
While some of the attacks described above are a bit outdated, such as session hijacking for host-based authentication services, IP spoofing is still prevalent in network scanning and probes, as well as denial of service floods. However, the technique does not allow for anonymous Internet access, which is a common misconception for those unfamiliar with the practice. Any sort of spoofing beyond simple floods is relatively advanced and used in very specific instances such as evasion and connection hijacking.
Defending Against Spoofing
There are a few precautions that can be taken to limit IP spoofing risks on your network, such as:
Filtering at the Router - Implementing ingress and egress filtering on your border routers is a great place to start your spoofing defense. You will need to implement an ACL (access control list) that blocks private IP addresses on your downstream interface. Additionally, this interface should not accept addresses with your internal range as the source, as this is a common spoofing technique used to circumvent firewalls. On the upstream interface, you should restrict source addresses outside of your valid range, which will prevent someone on your network from sending spoofed traffic to the Internet.
Encryption and Authentication - Implementing encryption and authentication will also reduce spoofing threats. Both of these features are included in Ipv6, which will eliminate current spoofing threats. Additionally, you should eliminate all host-based authentication measures, which are sometimes common for machines on the same sub net. Ensure that the proper authentication measures are in place and carried out over a secure (encrypted) channel.
Conclusion
IP Spoofing is a problem without an easy solution, since it’s inherent to the design of the TCP/IP suite. Understanding how and why spoofing attacks are used, combined with a few simple prevention methods, can help protect your network from these malicious cloaking and cracking techniques.
Matt Tanase is President of Qaddisin. He and his company provide nationwide security consulting services. Additionally, he produces The Security Blog, a daily weblog dedicated to network security.
