A host-based firewall is a piece of software running on a single host that can restrict incoming and outgoing network activity for that host only. They can prevent a host from becoming infected and stop infected hosts from spreading malware to other hosts.
Host-based firewalls for servers typically use rulesets similar to those of network firewalls. Some host-based firewalls for desktops and laptops also use similar rulesets, but most allow or deny activity based on lists of applications. Activity involving any application not on the lists is either denied automatically, or permitted or denied on the basis of the user's response to a prompt asking for a decision about the activity. To prevent malware incidents, organizations should configure host-based firewalls with deny-by-default rulesets for incoming traffic.
In addition to restricting network activity based on rules, many host-based firewalls for workstations incorporate antivirus software and intrusion prevention software capabilities, as well as suppressing Web browser pop-up windows, restricting mobile code, blocking cookies, and identifying potential privacy issues within Web pages and e-mails. Host-based firewalls that integrate these functions can be very effective not only at preventing most types of malware incidents, but also at stopping the spread of malware infections.
For example, a host-based firewall with antivirus capabilities can monitor inbound and outbound e-mails for signs of mass mailing viruses or worms and temporarily shut off e-mail services if such activity is detected. Accordingly, host-based firewalls for workstations that offer several types of malware prevention capabilities typically offer the best single, host-based technical control for malware threat mitigation, as long as they are configured properly and kept up-to-date at all times with the latest signatures and software updates.
Host-based firewalls are particularly important for systems that are network-connected but are not protected by network firewalls and other network-based security controls. Systems that are directly accessible from the Internet should be protected whenever possible through host-based firewalls to prevent network service worms and other threats from connecting to and attacking them.