A cyberattack against the Veteran of Foreign Wars website, believed to have been initiated in China, may have sought to spy on U.S. military members, security company FireEye said Thursday.

FireEye said the attack, which has now been fixed on VFW.org, exploited a zero-day flaw in the Internet Explorer 10 browser. The company discovered the attack, and the flaw in the browser, on Tuesday.

A zero-day vulnerability is one for which a software fix is not ready, one of the most dangerous categories of security flaws. Microsoft, which occasionally issues emergency fixes for such problems, has said it is investigating.

FireEye, which scans the Internet for new attacks, has found two other lesser-trafficked websites have been rigged in the same attack as VFW.org, said Darien Kindlund, manager of threat intelligence for FireEye, in a phone interview. The company is calling the campaign "Operation SnowMan."

The group behind the attacks is a "fairly sophisticated threat actor," Kindlund said, but it tends to lean on finding new zero-day vulnerabilities rather than changing other aspects of their operations, such as their command-and-control systems.

"Once this operation subsides, they'll probably restart again," he said. "It still seems they're achieving mission success based on the slight tweaks they've done to their attack methodology."

The attackers compromised the VFW's website and placed a hidden iframe on the site that delivered an exploit from the attacker's server, FireEye wrote in a blog post. If successful, the attack deposits a backdoor called "ZxShell," which can steal files from a user's computer.

The style of attack is known as a drive-by download, and victims would not have an indication they've been compromised, Kindlund said.

The hackers may have been aiming to compromise service members visiting the site, he said. The VFW has 1.4 million members, including 75,000 still on active duty, according to its website.

The timing of the attack, ahead of the President's Day holiday on Monday, may have been intentional to try to maximize the exposure window, Kindlund said. The attack was active for about three days.

An analysis of the backdoor delivered to compromised computers showed that it contacts a website hosted on an IP address that has been linked to other hacking campaigns, "Operation DeputyDog" and "Operation Ephemeral Hydra," both suspected to originate in China, he said.

Those campaigns both used zero-day exploits to compromise targets. Operation Deputy Dog targeted organizations in Japan, according to FireEye.

Part of the attack code used on VFW.org incorporates a Flash object from Adobe's Flash program, a method similar to that used in the DeputyDog and Ephemeral Hydra attacks, FireEye said.

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