[Infowarrior] - Researchers look to predict software flaws

[Richard Forno]

 Researchers look to predict software flaws
Robert Lemos, SecurityFocus 2006-07-07
http://www.securityfocus.com/news/11399?ref=rss

Want to know how many flaws will be in the next version of a software
product? Using historical data, researchers at Colorado State University are
attempting to build models that predict the number of flaws in a particular
operating system or application.

In an analysis to be presented at a secure computing conference in
September, three researchers used monthly flaw tallies for the two most
popular Web servers--The Apache Foundation's Apache Web server and
Microsoft's Internet Information Services (IIS) server--to test their models
for predicting the number of vulnerabilities that will be found in a given
code base.

The goal is not to help software developers to create defect-free
software--which may be so unlikely as to be impossible--but to give them the
tools to determine where they need to concentrate their efforts, said
Yashwant Malaiya, professor of computer science at Colorado State University
and one of the authors of the paper on the analysis.

"The possible reasons that vulnerabilities arise are much smaller than the
reasons for the number of defects, so it should be possible to reduce the
number of vulnerabilities," Malaiya said. "It would never be possible to
reduce the issues to zero, but it should be possible to reduce it to a much
smaller number."

The research could be another tool for developers in the fight to improve
programmers' security savvy and reduce the number of flaws that open up
consumers and companies to attack. While the number of vulnerabilities found
in recent years leveled off, Web applications boosted the number of flaws
found in 2005. Moreover, the advent of data-breach notification laws has
forced companies, universities and government agencies to tell citizens when
a security incident has put their information in peril. The resulting
picture painted by numerous breach notifications has not been heartening.

The latest research focuses on fitting an S-shaped curve to monthly
vulnerability data, positing that a limited installed based and little
knowledge of new software limits the finding of vulnerabilities in a
just-released application, while exhaustion of the low-hanging fruit makes
finding vulnerabilities in older products more difficult.

The researchers found that the number of vulnerabilities found in Windows
95, Windows NT and Red Hat Linux 7.1 fit their model quite well as does
those found in the Apache and IIS Web servers between 1995 and the present.
The Web server analysis, which will be discussed in the September paper,
suggests that IIS has reached a saturation point, with a lower rate of
vulnerabilities discovered than Apache. Moreover, that analysis found that
the S-curve relationship holds for broad classes of vulnerabilities, such as
input validation errors, race conditions, and design errors.

Some software developers believe that such research could allow product
managers to make better decisions about when a software program is ready to
be shipped and how many vulnerabilities will likely be found.

"There isn't an engineering manager that wouldn't love to know the number of
vulnerabilities they should expect to have after pushing out a product,"
said Ben Chelf, chief technology officer for Coverity, a maker of
source-code analysis tools that can be used to detect potential software
flaws. "A VP of engineering can, on the release date, say, 'We expect to
find 50 more security issues in this code.' That helps mitigate cost and
risk."

Yet, the researchers' predictions have been hit or miss, even with a large
margin of error of 25 percent. A paper released in January 2006 predicted
that the number of flaws found in Windows 98 would saturate between 45 and
75; at the time, data from the National Vulnerability Database showed that
66 vulnerabilities had been found, but that number has continued to increase
to 91 as of July. However, the researchers' prediction for Windows 2000 has
apparently been accurate: The current number of vulnerabilities for the
operating system is 305, just within the 294-to-490 range given in the
computer scientists' paper.

Whether the models become more accurate may rely on getting better data on
the number of software flaws discovered after development. The models used
for prediction of future vulnerabilities assume that defect density--the
number of software flaws per 1,000 lines of code--remains the same between
software versions. It's not an unreasonable assumption: Historically, the
researchers found that a company's programming teams tend not to get better,
making the same number of mistakes in one version of software as the next,
said CSU's Malaiya.

However, such observations use data that predates the increasing use of
static code analysis software and initiatives among developers, such as
Microsoft, to improve the security of their products.

Some security experts have doubts whether the model will ever be able to
make better than a rough estimate of the number of vulnerabilities that will
likely to be found in a particular application.

The prediction of the number of vulnerabilities from general trend data may
gloss over too many important details to be of real value, said Gerhard
Eschelbeck, chief technology officer for anti-spyware firm Webroot Software.

"This is a little bit like predicting the next earthquake," Eschelbeck said.
"It's a valuable area of research but it may not, in the end, be practical."

Because vulnerability researchers' interest in finding flaws in a particular
product can be fickle, general trends could be swamped by other variables.
In July, for example, Microsoft's Internet Explorer browser will likely see
an uncharacteristically large number of vulnerabilities found because one
researcher has decided to release a bug each day of the month. Market forces
could also throw off the models, since a handful of companies now pay for
previously unknown flaws, a situation that could cause researchers to stay
interested in older operating systems.

Moreover, the discovery of less serious flaws is far less important than
critical vulnerabilities that could lead to remote compromises, Eschelbeck
said.

"It is not just about the number, but about the severity," he said. "Just
the pure number does not mean a lot without the context."

If such limitations could be overcome, the ability to predict the future
number of software flaws could have big benefits, said Brian Chess, chief
scientist with source-code analysis tool maker Fortify. For example, the
assumption that vulnerabilities will always be present in software may
suggest a better strategy for dealing with the issues. Developers can choose
to put their resources into finding the more serious issues, he said.

"If you accept that flaws can't be gotten rid of, you can decide which
mistakes you are going to make and which ones are not acceptable," Chess
said, "Even though you cannot predict which line of codes will have the
vulnerabilities, you can push the actual class of vulnerabilities one way or
another."

In the end, even if the research does not produce accurate predictions,
accepting that you will have security problems and learning to deal with the
aftermath of releasing a software product are important lessons, he said.

"The next thing you build will have security problems just like the last
thing you did, but let's make sure that when we have a vulnerability, we can
deal with it," Chess said. "I think that is an evolution in the way that
people think about building security into their software."