Attacks using previously unknown vulnerabilities are more numerous than once thought, often escape detection for more than a year, and lead to an explosion of malware once disclosed, according to an academic research paper released this week.

Using data from the computers of 11 million users who opted into security firm Symantec's antivirus telemetry and reputation services, two researchers looked for any malware that exploited vulnerabilities before the security issues had been publicly disclosed. Sifting through the large data set, they found 18 zero-day attacks, including 11 attacks that the industry had not previously discovered. The attacks lasted anywhere from 19 days to 30 months, with an average lifetime of 312 days before public disclosure of the vulnerability, the researchers stated in a paper presented at the ACM Conference on Computer and Communications Security this week.

"The thing that we were most excited about was being able to measure the duration of the zero-day attacks -- it has been an open question for a long time," said Tudor Dumitras, senior research engineer at Symantec Research Labs and a co-author of the paper.

Dumitras, along with his colleague Leyla Bilge, matched known attacks with the vulnerabilities, if any, exploited by the malicious programs used in the attacks between 2008 and 2011. They identified 18 zero-day attacks, including 11 attacks that had not previously been discovered, during the four-year time span.

Security professionals should be concerned that these attacks had gone unnoticed for such a long period of time, said Stefan Frei, research director at security intelligence firm NSS Labs.

"What this paper really shows is that there are many, many zero days that are known to just a few people," he says. "If you have invested a lot of time and resources into a zero-day, then you use it privately, you don't use it on a large-scale attack, which may result showing up on the radars of the antivirus industry."

Vulnerabilities used in the zero-day attacks were quickly used by other attackers and cybercriminals once they were publicly disclosed. The number of attacks increased hundreds and thousands of times in most cases, but at least doubled and in some cases increased by a factor of 100,000.

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The data is more fodder for the battle over full disclosure policies, which hold that vulnerability information should be publicly reported as soon as possible -- before a patch become available -- to spur development of patch and more secure software in general. The quick adoption of the exploit techniques once a vulnerability becomes known suggests that full disclosure could lead to widespread attacks.

Yet remaining quiet about vulnerabilities will only allow attackers to take advantage of zero-day attacks for longer periods, Frei says.

"You are doomed if you do, and you are doomed if you don't," he says.

While the public release of information about a vulnerability can prove harmful to the computers of many users, such attacks are like an inoculation using live pathogens, keeping the Internet healthier on the whole, Frei says.

"I would say that we have to disclose because, otherwise, we are much more vulnerable," he says.

The researchers' detection methods had some severe limitations, however, including the lack of telemetry on Web-based attacks, the inability to directly detect nonexecutable exploits, and, especially, a weakness to polymorphic attacks. In addition, targeted attacks were likely to escape detection if not broadly used, the researchers pointed out. While they detected seven previously known zero-day attacks, they did not find evidence of the other three dozen zero-day attacks previously discovered in the same four-year span.

Yet the research does suggest ways of improving detection, through analyzing more data, adding reputation to the mix, and using behavioral detection to shorten the life span of zero-day attacks, says Wolfgang Kandek, chief technology officer with cloud-security firm Qualys.

"It seems that this is a clear pointer for better defense," Kandek says. "Bring data from more sources, more of the security companies, and run that type of analysis on all of the incoming files, including PDFs and applets."

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