Firewall Security Requirements Guide

Information flow control regulates where information is allowed to travel within a network and between interconnected networks. Blocking or restricting detected harmful or suspicious communications between interconnected networks enforces approved authorizations for controlling the flow of traffic. The firewall that filters traffic outbound to interconnected networks with different security policies must be configured with filters (i.e., rules, access control lists [ACLs], screens, and policies) that permit, restrict, or block traffic based on organization-defined traffic authorizations. Filtering must include packet header and packet attribute information, such as IP addresses and port numbers. Configure filters to perform certain actions when packets match specified attributes, including the following actions: - Apply a policy - Accept, reject, or discard the packets - Classify the packets based on their source address - Evaluate the next term in the filter - Increment a packet counter - Set the packets’ loss priority - Specify an IPsec SA (if IPsec is used in the implementation) - Specify the forwarding path - Write an alert or message to the system log

To prevent malicious or accidental leakage of traffic, organizations must implement a deny-by-default security posture at the network perimeter. Such rulesets prevent many malicious exploits or accidental leakage by restricting the traffic to only known sources and only those ports, protocols, or services that are permitted and operationally necessary. As a managed boundary interface, the firewall must block all inbound and outbound network traffic unless a filter is installed to explicitly allow it. The allow filters must comply with the Ports, Protocols, and Services Management (PPSM) Category Assurance List (CAL) and Vulnerability Assessment (VA).

Not configuring a key boundary security protection device such as the firewall against commonly known attacks is an immediate threat to the protected enclave because they are easily implemented by those with little skill. Directions for the attack are obtainable on the Internet and in hacker groups. Without filtering enabled for these attacks, the firewall will allow these attacks beyond the protected boundary. Configure the perimeter and internal boundary firewall to guard against the three general methods of well-known DoS attacks: flooding attacks, protocol sweeping attacks, and unauthorized port scanning. Flood attacks occur when the host receives too much traffic to buffer and slows down or crashes. Popular flood attacks include ICMP flood and SYN flood. A TCP flood attack of SYN packets initiating connection requests can overwhelm the device until it can no longer process legitimate connection requests, resulting in denial of service. An ICMP flood can overload the device with so many echo requests (ping requests) that it expends all its resources responding and can no longer process valid network traffic, also resulting in denial of service. An attacker might use session table floods and SYN-ACK-ACK proxy floods to fill up the session table of a host. In an IP address sweep attack, an attacker sends ICMP echo requests (pings) to multiple destination addresses. If a target host replies, the reply reveals the target’s IP address to the attacker. In a TCP sweep attack, an attacker sends TCP SYN packets to the target device as part of the TCP handshake. If the device responds to those packets, the attacker gets an indication that a port in the target device is open, which makes the port vulnerable to attack. In a UDP sweep attack, an attacker sends UDP packets to the target device. If the device responds to those packets, the attacker gets an indication that a port in the target device is open, which makes the port vulnerable to attack. In a port scanning attack, an unauthorized application is used to scan the host devices for available services and open ports for subsequent use in an attack. This type of scanning can be used as a DoS attack when the probing packets are sent excessively.