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Special Publication 800- Revision 1

Guidelines on Firewalls and

Firewall Policy

Recommendations of the National Institute

of Standards and Technology

Karen Scarfone

Paul Hoffman

Guidelines on Firewalls and Firewall

Policy

Recommendations of the National

Institute of Standards and Technology

Karen Scarfone

Paul Hoffman

NIST Special Publication 800- Revision 1

C O M P U T E R S E C U R I T Y

Computer Security Division Information Technology Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899-

September 2009

U.S. Department of Commerce

Gary Locke, Secretary National Institute of Standards and Technology

Patrick D. Gallagher, Deputy Director

Acknowledgments

The authors, Karen Scarfone of the National Institute of Standards and Technology (NIST) and Paul Hoffman of the Virtual Private Network Consortium, wish to thank their colleagues who reviewed drafts of this document and contributed to its technical content. The authors would like to acknowledge Tim Grance, Murugiah Souppaya, Sheila Frankel, and Gale Richter of NIST, and Matthew Goche, David Klug, Logan Lodge, John Pearce, Noel Richards, Anne Roudabush, and Steven Sharma of Booz Allen Hamilton, for their keen and insightful assistance throughout the development of the document. Special thanks go to Brahim Asfahani of Booz Allen Hamilton for his contributions to early drafts of the document. The authors also thank all the reviewers who provided feedback during the public comment period, particularly Joel Snyder (Opus One), Ron Colvin (National Aeronautics and Space Administration [NASA]), Dean Farrington (Wells Fargo), Raffael Marty (Splunk), and David Newman (Network Test).

The authors also wish to express their thanks to the individuals and organizations that contributed to the original version of the publication, including John Wack of NIST and Ken Cutler and Jamie Pole of the MIS Training Institute, who authored the original version, and other contributors and reviewers— particularly Peter Batista and Wayne Bavry (U.S. Treasury); Harriet Feldman (Integrated Computer Engineering, Inc.); Rex Sanders (U.S. Geological Survey); and Timothy Grance, D. Richard Kuhn, Peter Mell, Gale Richter, and Murugiah Souppaya (NIST).

iv

Table of Contents

Executive Summary

Firewalls are devices or programs that control the flow of network traffic between networks or hosts that employ differing security postures. At one time, most firewalls were deployed at network perimeters. This provided some measure of protection for internal hosts, but it could not recognize all instances and forms of attack, and attacks sent from one internal host to another often do not pass through network firewalls. Because of these and other factors, network designers now often include firewall functionality at places other than the network perimeter to provide an additional layer of security, as well as to protect mobile devices that are placed directly onto external networks.

Threats have gradually moved from being most prevalent in lower layers of network traffic to the application layer, which has reduced the general effectiveness of firewalls in stopping threats carried through network communications. However, firewalls are still needed to stop the significant threats that continue to work at lower layers of network traffic. Firewalls can also provide some protection at the application layer, supplementing the capabilities of other network security technologies.

There are several types of firewalls, each with varying capabilities to analyze network traffic and allow or block specific instances by comparing traffic characteristics to existing policies. Understanding the capabilities of each type of firewall, and designing firewall policies and acquiring firewall technologies that effectively address an organization’s needs, are critical to achieving protection for network traffic flows. This document provides an overview of firewall technologies and discusses their security capabilities and relative advantages and disadvantages in detail. It also provides examples of where firewalls can be placed within networks, and the implications of deploying firewalls in particular locations. The document also makes recommendations for establishing firewall policies and for selecting, configuring, testing, deploying, and managing firewall solutions.

This document does not cover technologies that are called “firewalls” but primarily examine only application layer activity, not lower layers of network traffic. Technologies that focus on activity for a particular type of application, such as email firewalls that block email messages with suspicious content, are not covered in detail in this document.

To improve the effectiveness and security of their firewalls, organizations should implement the following recommendations:

Create a firewall policy that specifies how firewalls should handle inbound and outbound network traffic.

A firewall policy defines how an organization’s firewalls should handle inbound and outbound network traffic for specific IP addresses and address ranges, protocols, applications, and content types based on the organization’s information security policies. Organizations should conduct risk analysis to develop a list of the types of traffic needed by the organization and how they must be secured—including which types of traffic can traverse a firewall under what circumstances. Examples of policy requirements include permitting only necessary Internet Protocol (IP) protocols to pass, appropriate source and destination IP addresses to be used, particular Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) ports to be accessed, and certain Internet Control Message Protocol (ICMP) types and codes to be used. Generally, all inbound and outbound traffic not expressly permitted by the firewall policy should be blocked because such traffic is not needed by the organization. This practice reduces the risk of attack and can also decrease the volume of traffic carried on the organization’s networks.

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Identify all requirements that should be considered when determining which firewall to implement.

There are many considerations that organizations should include in their firewall selection and planning processes. Organizations need to determine which network areas need to be protected, and which types of firewall technologies will be most effective for the types of traffic that require protection. Several important performance considerations also exist, as well as concerns regarding the integration of the firewall into existing network and security infrastructures. Additionally, firewall solution design involves requirements relating to physical environment and personnel as well as consideration of possible future needs, such as plans to adopt new IPv6 technologies or virtual private networks (VPN).

Create rulesets that implement the organization’s firewall policy while supporting firewall performance.

Firewall rulesets should be as specific as possible with regards to the network traffic they control. To create a ruleset involves determining what types of traffic are required, including protocols the firewall may need to use for management purposes. The details of creating rulesets vary widely by type of firewall and specific products, but many firewalls can have their performance improved by optimizing firewall rulesets. For example, some firewalls check traffic against rules in a sequential manner until a match is found; for these firewalls, rules that have the highest chance of matching traffic patterns should be placed at the top of the list wherever possible.

Manage firewall architectures, policies, software, and other components throughout the life of the firewall solutions.

There are many aspects to firewall management. For example, choosing the type or types of firewalls to deploy and their positions within the network can significantly affect the security policies that the firewalls can enforce. Policy rules may need to be updated as the organization’s requirements change, such as when new applications or hosts are implemented within the network. Firewall component performance also needs to be monitored to enable potential resource issues to be identified and addressed before components become overwhelmed. Logs and alerts should also be continuously monitored to identify threats—both successful and unsuccessful. Firewall rulesets and policies should be managed by a formal change management control process because of their potential to impact security and business operations, with ruleset reviews or tests performed periodically to ensure continued compliance with the organization’s policies. Firewall software should be patched as vendors provide updates to address vulnerabilities.

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 Section 5 provides an overview of firewall planning and implementation. It lists factors to consider when selecting firewall solutions, and provides recommendations for firewall configuration, testing, deployment, and management.

The document also contains appendices with supporting material:

 Appendices A and B contain a glossary and an acronym and abbreviation list, respectively.

 Appendix C lists print and online resources that may be of use in gaining a better understanding of firewalls.

2. Overview of Firewall Technologies

Firewalls are devices or programs that control the flow of network traffic between networks or hosts that employ differing security postures. While firewalls are often discussed in the context of Internet connectivity, they may also have applicability in other network environments. For example, many enterprise networks employ firewalls to restrict connectivity to and from the internal networks used to service more sensitive functions, such as accounting or personnel. By employing firewalls to control connectivity to these areas, an organization can prevent unauthorized access to its systems and resources. Inclusion of a proper firewall provides an additional layer of security. Organizations often need to use firewalls to meet security requirements from mandates (e.g., FISMA); some mandates, such as the Payment Card Industry (PCI) Data Security Standard,^1 specifically require firewalling.

Several types of firewall technologies are available. One way of comparing their capabilities is to look at the Transmission Control Protocol/Internet Protocol (TCP/IP) layers that each is able to examine. TCP/IP communications are composed of four layers that work together to transfer data between hosts. When a user wants to transfer data across networks, the data is passed from the highest layer through intermediate layers to the lowest layer, with each layer adding more information. The lowest layer sends the accumulated data through the physical network, with the data then passed upwards through the layers to its destination. Simply put, the data produced by a layer is encapsulated in a larger container by the layer below it. The four TCP/IP layers, from highest to lowest, are shown in Figure 2-1.

Application Layer. This layer sends and receives data for particular applications, such as Domain Name System (DNS), Hypertext Transfer Protocol (HTTP), and Simple Mail Transfer Protocol (SMTP). The application layer itself has layers of protocols within it. For example, SMTP encapsulates the Request for Comments (RFC) 2822 message syntax, which encapsulates Multipurpose Internet Mail Extensions (MIME), which can encapsulate other formats such as Hypertext Markup Language (HTML). Transport Layer. This layer provides connection-oriented or connectionless services for transporting application layer services between networks, and can optionally ensure communications reliability. Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are commonly used transport layer protocols.^2 IP Layer (also known as the Network Layer). This layer routes packets across networks. Internet Protocol version 4 (IPv4) is the fundamental network layer protocol for TCP/IP. Other commonly used protocols at the network layer are Internet Protocol version 6 (IPv6), ICMP, and Internet Group Management Protocol (IGMP). Hardware Layer (also known as the Data Link Layer). This layer handles communications on the physical network components. The best known data link layer protocol is Ethernet.

Figure 2-1. TCP/IP Layers

Addresses at the data link layer, which are assigned to network interfaces, are referred to as media access control (MAC) addresses —an example of this is an Ethernet address that belongs to an Ethernet card. Firewall policies rarely concern themselves with the data link layer. Addresses at the network layer are referred to as IP addresses. The transport layer identifies specific network applications and communication sessions as opposed to network addresses; a host may have any number of transport layer sessions with other hosts on the same network. The transport layer may also include the notion of ports — a destination port number generally identifies a service listening on the destination host, and a source port usually identifies the port number on the source host that the destination host should reply to. Transport protocols such as TCP and UDP have ports, while other transport protocols do not. The combination of

(^1) The PCI Data Security Standard may apply to some Federal agencies. It is defined at https://www.pcisecuritystandards.org/. (^2) The differences between TCP and UDP are explained by several of the print resources listed in Appendix C.

 The network or transport protocol being used to communicate between source and destination hosts, such as TCP, UDP, or ICMP

 Possibly some characteristics of the transport layer communications sessions, such as session source and destination ports (e.g., TCP 80 for the destination port belonging to a web server, TCP 1320 for the source port belonging to a personal computer accessing the server)

 The interface being traversed by the packet, and its direction (inbound or outbound).

Filtering inbound traffic is known as ingress filtering. Outgoing traffic can also be filtered, a process referred to as egress filtering. Here, organizations can implement restrictions on their internal traffic, such as blocking the use of external file transfer protocol (FTP) servers or preventing denial of service (DoS) attacks from being launched from within the organization against outside entities. Organizations should only permit outbound traffic that uses the source IP addresses in use by the organization—a process that helps block traffic with spoofed addresses from leaking onto other networks. Spoofed addresses can be caused by malicious events such as malware infections or compromised hosts being used to launch attacks, or by inadvertent misconfigurations.

Stateless packet filters are generally vulnerable to attacks and exploits that take advantage of problems within the TCP/IP specification and protocol stack. For example, many packet filters are unable to detect when a packet’s network layer addressing information has been spoofed or otherwise altered, or uses options that are permitted by standards but generally used for malicious purposes, such as IP source routing. Spoofing attacks, such as using incorrect addresses in the packet headers, are generally employed by intruders to bypass the security controls implemented in a firewall platform. Firewalls that operate at higher layers can thwart some spoofing attacks by verifying that a session is established, or by authenticating users before allowing traffic to pass. Because of this, most firewalls that use packet filters also maintain some state information for the packets that traverse the firewall.

Some packet filters can specifically filter packets that are fragmented. Packet fragmentation is allowed by the TCP/IP specifications and is encouraged in situations where it is needed. However, packet fragmentation has been used to make some attacks harder to detect (by placing them within fragmented packets), and unusual fragmentation has also been used as a form of attack. For example, some network- based attacks have used packets that should not exist in normal communications, such as sending some fragments of a packet but not the first fragment, or sending packet fragments that overlap each other. To prevent the use of fragmented packets in attacks, some firewalls have been configured to block fragmented packets.

Today, fragmented packets on the Internet often occur not because of attacks, but because of virtual private networking (VPN) technologies that encapsulate packets within other packets. If encapsulating a packet would cause the new packet to exceed the maximum permitted size for the medium it will be transmitted on, the packet must be fragmented. Fragmented packets being blocked by firewalls is a common cause of VPN interoperability issues.

Some firewalls can reassemble fragments before passing them to the inside network, although this requires additional firewall resources, particularly memory. Firewalls that have this reassembly feature must implement it carefully, otherwise someone can readily mount a denial-of-service attack. Choosing whether to block, reassemble, or pass fragmented packets is a tradeoff between overall network interoperability and full system security. Given this, automatic blocking of all fragmented packets is not recommended because of the legitimate and necessary uses of fragmentation on the Internet.

2.1.2 Stateful Inspection

Stateful inspection improves on the functions of packet filters by tracking the state of connections and blocking packets that deviate from the expected state. This is accomplished by incorporating greater awareness of the transport layer. As with packet filtering, stateful inspection intercepts packets at the network layer and inspects them to see if they are permitted by an existing firewall rule, but unlike packet filtering, stateful inspection keeps track of each connection in a state table. While the details of state table entries vary by firewall product, they typically include source IP address, destination IP address, port numbers, and connection state information.

Three major states exist for TCP traffic—connection establishment, usage, and termination (which refers to both an endpoint requesting that a connection be closed and a connection with a long period of inactivity.) Stateful inspection in a firewall examines certain values in the TCP headers to monitor the state of each connection. Each new packet is compared by the firewall to the firewall’s state table to determine if the packet’s state contradicts its expected state. For example, an attacker could generate a packet with a header indicating it is part of an established connection, in hopes it will pass through a firewall. If the firewall uses stateful inspection, it will first verify that the packet is part of an established connection listed in the state table.

In the simplest case, a firewall will allow through any packet that seems to be part of an open connection (or even a connection that is not yet fully established). However, many firewalls are more cognizant of the state machines for protocols such as TCP and UDP, and they will block packets that do not adhere strictly to the appropriate state machine. For example, it is common for firewalls to check attributes such as TCP sequence numbers and reject packets that are out of sequence. When a firewall provides NAT services, it often includes NAT information in its state table.

Table 2-1 provides an example of a state table. If a device on the internal network (shown here as 192.168.1.100) attempts to connect to a device outside the firewall (192.0.2.71), the connection attempt is first checked to see if it is permitted by the firewall ruleset. If it is permitted, an entry is added to the state table that indicates a new session is being initiated, as shown in the first entry under “Connection State” in Table 2-1. If 192.0.2.71 and 192.168.1.100 complete the three-way TCP handshake, the connection state will change to “established” and all subsequent traffic matching the entry will be allowed to pass through the firewall.

Table 2-1. State Table Example

Source Address Source Port DestinationAddress^ DestinationPort Connection State

192.168.1.100 1030 192.0.2.71 80 Initiated 192.168.1.102 1031 10.12.18.74 80 Established 192.168.1.101 1033 10.66.32.122 25 Established 192.168.1.106 1035 10.231.32.12 79 Established

Because some protocols, most notably UDP, are connectionless and do not have a formal process for initializing, establishing, and terminating a connection, their state cannot be established at the transport layer as it is for TCP. For these protocols, most firewalls with stateful inspection are only able to track the source and destination IP addresses and ports. UDP packets must still match an entry in the state table based on source and destination IP address and port information to be permitted to pass—a DNS response from an external source would be permitted to pass only if the firewall had previously seen a corresponding DNS query from an internal source. Since the firewall is unable to determine when a

Application-Proxy Gateways

An application-proxy gateway is a feature of advanced firewalls that combines lower-layer access control with upper-layer functionality. These firewalls contain a proxy agent that acts as an intermediary between two hosts that wish to communicate with each other, and never allows a direct connection between them. Each successful connection attempt actually results in the creation of two separate connections—one between the client and the proxy server, and another between the proxy server and the true destination. The proxy is meant to be transparent to the two hosts—from their perspectives there is a direct connection. Because external hosts only communicate with the proxy agent, internal IP addresses are not visible to the outside world. The proxy agent interfaces directly with the firewall ruleset to determine whether a given instance of network traffic should be allowed to transit the firewall.

In addition to the ruleset, some proxy agents have the ability to require authentication of each individual network user. This authentication can take many forms, including user ID and password, hardware or software token, source address, and biometrics.

Like application firewalls, the proxy gateway operates at the application layer and can inspect the actual content of the traffic. These gateways also perform the TCP handshake with the source system and are able to protect against exploitations at each step of a communication. In addition, gateways can make decisions to permit or deny traffic based on information in the application protocol headers or payloads. Once the gateway determines that data should be permitted, it is forwarded to the destination host.

Application-proxy gateways are quite different than application firewalls. First, an application-proxy gateway can offer a higher level of security for some applications because it prevents direct connections between two hosts and it inspects traffic content to identify policy violations. Another potential advantage is that some application-proxy gateways have the ability to decrypt packets (e.g., SSL-protected payloads), examine them, and re-encrypt them before sending them on to the destination host. Data that the gateway cannot decrypt is passed directly through to the application. When choosing the type of firewall to deploy, it is important to decide whether the firewall actually needs to act as an application proxy so that it can match the specific policies needed by the organization.

Firewalls with application-proxy gateways can also have several disadvantages when compared to packet filtering and stateful inspection. First, because of the “full packet awareness” of application-proxy gateways, the firewall spends much more time reading and interpreting each packet. Because of this, some of these gateways are poorly suited to high-bandwidth or real-time applications—but application- proxy gateways rated for high bandwidth are available. To reduce the load on the firewall, a dedicated proxy server (discussed in Section 2.1.5) can be used to secure less time-sensitive services such as email and most web traffic. Another disadvantage is that application-proxy gateways tend to be limited in terms of support for new network applications and protocols—an individual, application-specific proxy agent is required for each type of network traffic that needs to transit a firewall. Many application-proxy gateway firewall vendors provide generic proxy agents to support undefined network protocols or applications. Those generic agents tend to negate many of the strengths of the application-proxy gateway architecture because they simply allow traffic to “tunnel” through the firewall.

Dedicated Proxy Servers

Dedicated proxy servers differ from application-proxy gateways in that while dedicated proxy servers retain proxy control of traffic, they usually have much more limited firewalling capabilities. They are described in this section because of their close relationship to application-proxy gateway firewalls. Many dedicated proxy servers are application-specific, and some actually perform analysis and validation of common application protocols such as HTTP. Because these servers have limited firewalling capabilities,

such as simply blocking traffic based on its source or destination, they are typically deployed behind traditional firewall platforms. Typically, a main firewall could accept inbound traffic, determine which application is being targeted, and hand off traffic to the appropriate proxy server (e.g., email proxy). This server would perform filtering or logging operations on the traffic, then forward it to internal systems. A proxy server could also accept outbound traffic directly from internal systems, filter or log the traffic, and pass it to the firewall for outbound delivery. An example of this is an HTTP proxy deployed behind the firewall—users would need to connect to this proxy en route to connecting to external web servers. Dedicated proxy servers are generally used to decrease firewall workload and conduct specialized filtering and logging that might be difficult to perform on the firewall itself.

In recent years, the use of inbound proxy servers has decreased dramatically. This is because an inbound proxy server must mimic the capabilities of the real server it is protecting, which becomes nearly impossible when protecting a server with many features. Using a proxy server with fewer capabilities than the server it is protecting renders the non-matched capabilities unusable. Additionally, the essential features that inbound proxy servers should have (logging, access control, etc.) are usually built into the real servers. Most proxy servers now in use are outbound proxy servers, with the most common being HTTP proxies.

Figure 2-2 shows a sample diagram of a network employing a dedicated HTTP proxy server that has been placed behind another firewall system. The HTTP proxy would handle outbound connections to external web servers and possibly filter for active content. Requests from users first go to the proxy, and the proxy then sends the request (possibly changed) to the outside web server. The response from that web server then comes back to the proxy, which relays it to the user. Many organizations enable caching of frequently used web pages on the proxy to reduce network traffic and improve response times.

Figure 2-2. Application Proxy Configuration

2.1.6 Virtual Private Networking

Firewall devices at the edge of a network are sometimes required to do more than block unwanted traffic. A common requirement for these firewalls is to encrypt and decrypt specific network traffic flows between the protected network and external networks. This nearly always involves virtual private networks (VPN), which use additional protocols to encrypt traffic and provide user authentication and integrity checking. VPNs are most often used to provide secure network communications across untrusted networks. For example, VPN technology is widely used to extend the protected network of a multi-site

 Latest updates to antimalware and personal firewall software

 Configuration settings for antimalware and personal firewall software

 Elapsed time since the previous malware scan

 Patch level of the operating system and selected applications

 Security configuration of the operating system and selected applications

These health checks require software on the user’s system that is controlled by the firewall. If the user has acceptable credentials but the device does not pass the health check, the user and device may get only limited access to the internal network for remediation purposes.

2.1.

Unified Threat Management (UTM)

Many firewalls combine multiple features into a single system, the idea being that it is easier to set and maintain policy on a single system than on many systems that are deployed at the same location on a network. A typical unified threat management (UTM) system has a firewall, malware detection and eradication, sensing and blocking of suspicious network probes, and so on. There are pros and cons to merging multiple, not-completely-related functions into a single system. For example, deploying a UTM reduces complexity by making a single system responsible for multiple security objectives, but it also requires that the UTM have all the desired features to meet every one of the objectives. Another tradeoff is in performance: a single system handling multiple tasks has to have enough resources such as CPU speed and memory to handle every task assigned to it. Some organizations will find the balance favors a UTM, while other organizations will use multiple firewalls at the same location in their network.

Web Application Firewalls

The HTTP protocol used in web servers has been exploited by attackers in many ways, such as to place malicious software on the computer of someone browsing the web, or to fool a person into revealing private information that they might not have otherwise. Many of these exploits can be detected by specialized application firewalls called web application firewalls that reside in front of the web server.

Web application firewalls are a relatively new technology, as compared to other firewall technologies, and the type of threats that they mitigate are still changing frequently. Because they are put in front of web servers to prevent attacks on the server, they are often considered to be very different than traditional firewalls.

Firewalls for Virtual Infrastructures

Many virtualization solutions allow more than one operating system to run on a single computer simultaneously, each appearing as if it were a real computer. This has become popular recently because it allows organizations to make more efficient use of computer hardware. Most of these types of virtualization systems include virtualized networking , which allows the multiple operating systems to communicate as if they were on a standard Ethernet, even though there is no actual networking hardware.

Network activity that passes directly between virtualized operating systems within a host cannot be monitored by an external firewall. However, some virtualization systems offer built-in firewalls or allow third-party software firewalls to be added as plug-ins. Using firewalls to monitor virtualized networking is a relatively new area of firewall technology, and it is likely to change significantly as virtualization usage continues to increase.

2.2 Firewalls for Individual Hosts and Home Networks

Although firewalls at a network’s perimeter provide some measure of protection for internal hosts, in many cases additional network protection is required. Network firewalls are not able to recognize all instances and forms of attack, allowing some attacks to penetrate and reach internal hosts—and attacks sent from one internal host to another may not even pass through a network firewall. Because of these and other factors, network designers often include firewall functionality at places other than the network perimeter to provide an additional layer of security. This section describes firewalls specifically designed for deployment onto individual hosts and home networks.

2.2.1 Host-Based Firewalls and Personal Firewalls

Host-based firewalls for servers and personal firewalls for desktop and laptop personal computers (PC) provide an additional layer of security against network-based attacks. These firewalls are software-based, residing on the hosts they are protecting—each monitors and controls the incoming and outgoing network traffic for a single host. They can provide more granular protection than network firewalls to meet the needs of specific hosts.

Host-based firewalls are available as part of server operating systems such as Linux, Windows, Solaris, BSD, and Mac OS X Server, and they can also be installed as third-party add-ons. Configuring a host- based firewall to allow only necessary traffic to the server provides protection against malicious activity from all hosts, including those on the same subnet or on other internal subnets not separated by a network firewall. Limiting outgoing traffic from a server may also be helpful in preventing certain malware that infects a host from spreading to other hosts.^11 Host-based firewalls usually perform logging, and can often be configured to perform address-based and application-based access controls. Many host-based firewalls can also act as intrusion prevention systems (IPS) that, after detecting an attack in progress, take actions to thwart the attacker and prevent damage to the targeted host.

A personal firewall is software that runs on a desktop or laptop PC with a user-focused operating system such as Microsoft Windows Vista or Macintosh OS X. A personal firewall is similar to a host-based firewall, but because the computer being protected is meant for end users, the interface is usually different (and presumably easier for the typical user to understand). A personal firewall provides an additional layer of security for PCs located both inside and outside perimeter firewalls (e.g., mobile laptop users), because it can restrict inbound communications and can often limit outbound communications as well. This not only allows personal firewalls to protect PCs from incoming attacks, but also limits the spread of malware from infected PCs and the use of unauthorized software such as peer-to-peer file sharing utilities. Personal firewalls are often packaged with antimalware programs, intrusion detection software, and other security utilities.^12

Some personal firewalls allow creation of different profiles based on location, such as a profile for use inside the organization’s network and a different profile for use when at a remote location. This is particularly important when a computer is used on an untrusted external network, because having a separate firewall profile for use on such networks can restrict network activity more tightly and provide stronger protection than having a single profile for all networks.

(^11) If an attacker compromises a host and gains administrator-level privileges, the attacker can disable or circumvent the host-

12 based firewall. For additional information about personal firewalls, see NIST SP 800-114, User's Guide to Securing External Devices for Telework and Remote Access (http://csrc.nist.gov/publications/PubsSPs.html).