An elite hacker group targeting defense industry sub-contractors has an inexhaustible supply of zero-days, or vulnerabilities that have yet to be publicized, much less patched, Symantec said today.

In a blog post, the security firm said, "The group seemingly has an unlimited supply of zero-day vulnerabilities."

Symantec also laid out its analysis of the gang, which it said was behind a slew of attacks dubbed the "Elderwood Project," after a source code variable used by the hackers.

Among the group's distinguishing characteristics, said Orla Cox, senior manager at Symantec's security response division, is its exploitation of at least eight zero-day vulnerabilities since late 2010, and four in a 16-week span this spring and summer.

"We've never see a group use so many zero-days," said Cox in an interview today. "We were amazed when Stuxnet used four zero-days, but this group has been able to discover eight zero-days. More, the fact that they have prepared [their attacks] and are ready to go as soon as they have a new zero-day, and the speed with which they use these zero-days, is something we've not seen before."

Stuxnet, first uncovered in 2010, relied on exploits of four different Windows zero-day vulnerabilities to infiltrate its targets, which most analysts now believe were Iranian nuclear fuel enrichment facilities.

Cox said that Symantec believes the hackers found the zero-days themselves, and did not purchase them from other sources.

According to Symantec's research, Elderwood exploited one zero-day in December 2010, three in 2011 and four this year during a stretch from April 24 through Aug. 15.

The 2010 zero-day attributed to the gang was notable: It was used by a Trojan horse called "Aurora" by most security experts, and pegged "Hydraq" by Symantec. Aurora was delivered using an Internet Explorer (IE) zero-day, and targeted a large number of Western companies, including Google.

Google accused Chinese hackers of breaking into its network using Aurora, a charge that prompted the search giant to threaten a shut-down of its Chinese operations.

Symantec found links between the Aurora/Hydraq attacks of late 2009 and early 2010 with the campaigns that exploited eight zero-days over the last 20+ months.

The security company connected the dots between the various attack campaigns by comparing elements ranging from the underlying command-and-control (C&C) infrastructure; the way the code in each Trojan was obfuscated, or masked; and the apparent sharing of a single custom-built malware development platform, said Cox.

The Elderwood campaign's targets also provided clues that the exploits of the eight zero-days were connected.

Elderwood focuses on defense sub-contractors, second-tier companies that manufacture electronic or mechanical components that are then sold to first-tier defense firms.

Symantec believes that the attacks are aimed at sub-contractors because the attackers find them easier to exploit. After infecting Windows PCs there, the hackers use them to forge a beachhead in companies further up the supply chain.

The Elderwood gang specializes in finding and exploiting zero-days in Microsoft's IE browser and Adobe's Flash Player.