Determining whether a software patch has to be applied today, next week, or next month is a major headache for information technology managers. While many software makers offer some system to rank the severity of security flaws, network administrators are still left to create their best estimate of how long they have before online miscreants start using a vulnerability to attack systems.

Security intelligence firm iDefense, for example, has a team of security experts who focus on researching online threats and figuring out which flaws will be targeted by the next attacks.

“I have six guys on my staff whose sole job is to find vulnerabilities in enterprise-level software,” says Rick Howard, director of intelligence for iDefense. “So when they see a piece of code, they have a sense about whether it is easy to exploit or not easy to exploit. They spend two days of work trying to figure that out.”

Microsoft is trying to make figuring it out a lot easier. Last year, the company launched a program to give IT managers more information by developing a three-level ranking system, known as the Exploitability Index. The program gauges whether a vulnerability is the equivalent of low-hanging fruit for online attackers or a much tougher nut to crack. The three levels are:

1. Consistent exploit code likely
2. Inconsistent exploit code likely
3. Functional exploit code unlikely

Microsoft only considers the question of exploitability for a 30-day period and does not try to forecast beyond that.

In a study released in July, iDefense found that Microsoft did a (relatively) respectable job of predicting whether an exploit would be released. Approximately one-third of all vulnerabilities assigned an Exploitability Index of one–“consistent exploit code likely”–were actually exploited in the 30 days following the release of the patch, while only one in five of the remaining vulnerabilities was exploited. Still, calling one-third correctly means that Microsoft thought it likely that the other two-thirds of vulnerabilities would be exploited and they were not.

“It is hard to figure out whether [researchers and attackers] will go public in 30 days,” says Howard. “It is not a bad indicator; it’s still not the best indicator.”

So far, there is little data on how the bad guys are using the Exploitability Index to focus their own efforts–an initial worry when Microsoft announced the program. The attackers could be focusing on quickly finding the easy-to-exploit vulnerabilities–those ranked first on the Exploitability Index–before companies plug the security holes, or they could focus on finding ways of reliably exploiting the harder flaws, expecting that companies might not patch those as quickly.

“Attackers know that companies and home users don’t patch their stuff very well,” Howard says, predicting, that “the harder stuff–that is the higher-end hackers–they will save those for bigger projects.”