How to reduce risks with URL filtering

• Al Berg, CISSP, CISM
• Published: 14 Jun 2005

Here are three ways filtering URLs on their way out of your network can make you safer:

1. Require users to access the Internet via a proxy server. A Web proxy gives you a single point for monitoring and controlling your outbound traffic. If you configure the proxy to cache frequently used pages and graphics, it can also help make the most of your bandwidth. Proxy servers are available from the open source community (http://www.squid-cache.org/ is one very popular option), as well as a variety of commercial vendors.

2. Consider filtering outbound URLs to enforce compliance with corporate Internet acceptable usage policies. By checking URLs against lists of known "inappropriate" sites, you reduce the risk of HR problems due to non-work-related Web content.

3. URL filters can also be valuable tools in the fight against spyware, worms and Trojan horse software. In addition to allowing you to block access to sites harboring harmful code, they can help you eliminate the use of Web-based e-mail services, file sharing sites and other Web resources that allow files into your network without the proper virus scanning.

1. Many attacks on Web-based applications rely on the attacker's ability to feed programs unexpected input from parameters passed in URLs. The first line of defense is having well-written Web applications that validate inputs and protect themselves against attack. If you are responsible for Web applications, make it a point to get to know the Open Web Application Security Project (www.owasp.org) and their tools and documents. OWASP has excellent information on URL attacks and the best practices you can apply to protect against them.

2. Add an application level firewall to create defense in-depth. When packets try to enter your network, subject them to rules that insure they should be admitted. Attackers have moved up the stack, targeting applications, and so should you. If you are running the Apache http server, consider adding the open source 'mod_security' application firewall (www.modsecurity.org), or one of the commercial alternatives, to your defense plan.

While URL filtering is not a complete solution to the problems posed by malware and inappropriate content, when properly used, it is a key component of a "defense in-depth" strategy for corporate networks.

• Are your Web servers secure? Take this quiz to find out.
• Learn more about combating spyware and Trojan threats in our resource center
• Protect your organization from insider threats with five simple measures.