Come June, high-volume merchants and payment processors that do business on the Web and want to work with MasterCard International had better be conducting quarterly vulnerability assessments of their Web sites. MasterCard warned last fall that it won't do business with them otherwise."We believe the majority of events we read about with worms could be averted through vulnerability assessment," says John Verdeschi, vice president of e-business and emerging technologies at MasterCard, which has drawn up a list of a dozen approved network-based scanning services, including\u00a0Ubizen, its preferred provider.With an estimated 7% of all of MasterCard's $921.6 billion in annual card purchases now taking place on the Web, it's no wonder that the global payments company is making vulnerability assessment mandatory.And with companies like MasterCard requiring its partners to use such offerings, it's also no wonder there are dozens of vendors competing in the market, which makes choosing vulnerability-assessment options a challenge.One way to narrow down the choice is to determine whether a host- or network-based offering best meets your needs.Host-based tools reside on servers and desktops as software agents that can log on to a host to report back to a management console. Competitors in this market include Bindview, Computer Associates, Harris, IBM, Internet Security Systems, Sanctum and Symantec.On the other hand, network-based tools scan remotely in ways similar to a hacker probe. Some of the players are the same as in host-based vulnerability assessment, such as IBM, ISS and Symantec. Others, such as nCircle and Qualys, specialize in network-based offerings, and freeware such as\u00a0Nessus\u00a0also has supporters.There are pros and cons to each approach.Host-based tools are generally more expensive, says John Pescatore, a security analyst at Gartner. The products can cost $750 to $1,000 per server, which adds up across a big network.But host-based offerings provide rich information, which might explain why a third more host-based products than network-based ones are sold annually."They can do several things you can't do from the network, such as check user access logs or who touched the financial data on a server," Pescatore says.The downside of host-based offerings is that computers have to be idle to do security checks, says Bill Kline, senior security engineer at Paymentech, one of the high-volume credit card processors asked by MasterCard (and Visa) to follow new guidelines this year. Staff must carefully schedule inspections, and if host-based tools aren't used correctly they can damage data on a server, he adds. Paymentech uses Symantec's host-based\u00a0Enterprise Security Manager\u00a0and network-based offerings from\u00a0KaVaDo\u00a0and\u00a0TruSecure\u00a0to keep its 600 servers secure.Network-based vulnerability assessment comes in the form of hardware, software or services. As with many offerings, vulnerability-assessment services are a good option for organizations short on IT staff or security specialists."We had constraints with resources," says Dan Klinger, manager of information security at Hershey Foods. Using the\u00a0Qualys\u00a0service lets Hershey centrally scan 325 Windows NT servers and 125 Unix servers without adding staff. Hershey also is developing a way to transfer the Qualys vulnerability-assessment information directly into its in-house trouble-ticketing system from Remedy to build an integrated workflow process, which will address patching or other problems.Network-based offerings can cost from a few hundred to thousands of dollars per month, depending on the number of IP addresses and other variables. And custom services can usher in customer-based pricing. MCI's Digex Web-hosting unit utilizes\u00a0Sanctum's\u00a0AppScan\u00a0application scanner and Symantec's\u00a0NetRecon\u00a0as well as the host-based Enterprise Security Manager as part of a custom service. There's no set monthly fee, but setup starts at $10,000.Deploying network-based vulnerability-assessment tools can present some pitfalls.Problems can arise when scanning internal servers, desktops and public-access Web servers. That's because firewalls and other means of blocking off sub-nets are often already in place to deter this kind of inspection.In addition, if there's no coordinated staff approach, network administrators might think the network is somehow under attack because of the scans."It takes special coordination with the intrusion-detection systems and the firewall people," says Don Jankowski, manager of technology risk management at Xcel Energy, about his recent experience in deploying the\u00a0proVizor Security Risk Measurement\u00a0network-based scanner from Black Dragon Software.It took Xcel, the fourth-largest electricity and gas energy company in the U.S., a few months to install and train IT staff to begin using the four proVizor appliances it purchased to scan for vulnerabilities across its network of 800 servers and 14,000 desktops across 12 states.Jankowski says the company has plans to do a complete scan of its environment every quarter, but far more frequently for critical workstations. Operating systems are being scanned first, applications next.Chuck Bianco, an IT examination manager in Dallas for the U.S. Treasury Department's Office of Thrift Supervision agency, warns that network scans can cause disruptions, and says it's best to get the support of upper management before any scan. Bianco says he's seen people get fired "because it blew up."Robert Geiger, director of Unix administration and security at Reader's Digest Association, says it's a good idea to have the testing provider clearly define in advance what it plans to do, including any denial-of-service attacks. And if the question comes up, say 'no' to hiring hackers past or present as a matter of trust, he adds.Assessing the situationPros and cons of host- and network-based vulnerability assessment.Host-based toolsNetwork-based toolsProsProsCan provide rich security information, such as by checking user access logs. Can give a quick look at what weaknesses hackers and worms can exploit. Once deployed, have limited impact on network traffic.Available as software, appliances and managed services.ConsConsCosts can add up when deploying agents across many desktops and servers. Requires careful planning to avoid conflict with security systems. Deployment can be time-consuming.Generates considerable network traffic.Ken Saruwatari, product manager at Foundstone, which makes the\u00a0Foundstone Enterprise\u00a0scanner used mainly by large companies, says scans can be traffic-intensive and therefore most organizations do them during hours when the network is least in use for business purposes.Network-based vulnerability-assessment products are increasingly becoming available as appliances, which can make them easier to deploy than host-based software. Computer Associates, Foundstone and PredatorWatch are among the companies selling such boxes.Some observers say going with host- or network-based tools isn't necessarily an either\/or situation."In IT, everyone always wants one tool for everything," says Anthony Passaniti, head of IT security for the Armonk, N.Y., Americas division of global reinsurance firm Swiss Re, which uses a variety of network- and host-based tools. "But a skilled craftsman needs a tool kit with specialized tools."The Swiss Re division uses Application Security's network-based\u00a0AppDetective\u00a0for its databases because it focuses on database vulnerability checking, Passaniti says.Eric Pulaski, CEO of security products vendor BindView, says no one vendor offers it all in vulnerability assessment. His company makes host-based vulnerability-assessment agents for software from Check Point, Microsoft, Novell and others as well as a network-based tool that can be used to locate rogue computers, but the company doesn't have an agent for every popular application. The same is true of even bigger companies, such as CA and IBM, he says.