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The U.S. Department of Labor website was hacked Tuesday evening to launch drive-by attacks at visitors' Web browsers.

That warning was sounded Wednesday morning by Jaime Blasco, director of AlientVault Labs, as well as Anup Ghosh, CEO of Invincea, both of whom reported that the Department of Labor servers had been infected by malicious code.

A Department of Labor spokeswoman, reached by phone, declined to comment on the attack reports. But Blasco said via email: "Several people within the U.S. government have been contacted so they should be working on it right now. We published this information because the exploit is still there and we are tying to warn people not to visit the website."

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By late Wednesday morning, the malware campaign appeared to have been stopped. "The site has since been fixed and law enforcement is investigating," said Invincea's Ghosh in a blog entry posted late Wednesday morning.

How did the attack work? If a system was successfully compromised by the malicious code running on the Department of Labor's website, it would "phone home" to a command-and-control (C&C) server that's disguised as a Microsoft update server. "The C&C protocol matches with a backdoor used by a known Chinese actor called DeepPanda," Blasco said in a blog post.

In addition, Blasco said the attack code used strongly resembled a previous exploit seen against a Thai nongovernmental organization that focuses on human rights under the auspices of the Association of Southeast Asian Nations.

Security intelligence firm CrowdStrike has tied DeepPanda to a number of advanced persistent threat (APT) attacks, noting that the group's attacks "target various strategic interests of the United States including high tech/heavy industry, non-governmental organizations (NGOs), state/federal government, defense industrial base (DIB), and organizations with vast economic interests."

The malware served by the Department of Labor website targeted a vulnerability that's been patched by Microsoft. According to Blasco, "after a quick analysis it seems the malicious server is exploiting CVE-2012-4792 that was fixed earlier this year." According to a related vulnerability summary from NIST, the flaw involves a "use-after-free vulnerability in Microsoft Internet Explorer 6 through 8" which attackers can use to remotely execute arbitrary code in a vulnerable browser. The vulnerability was first discovered in December 2012, when it was seen in zero-day attacks.

The malware loaded onto the Department of Labor server also attempted to execute JavaScript code in a browser, with the code being served up directly from the Department of Labor website. The malware also attempted to execute a malicious PHP script that's downloaded from an external server that's currently hosted by OC3 Networks & Web Solutions in Los Angeles, and which also received information about compromised systems.

If the malware was successfully able to exploit the IE vulnerability, it downloaded an attack payload from a remote server. Blasco said that as of early Wednesday morning, according to VirusTotal, the downloaded code was being flagged as malicious by only two out of 46 antivirus scanners. But by later that morning, 13 antivirus scanners had been updated to identify the attack.

The PHP script used in the attack "will collect a lot of information from the system and then it will upload the information collected to the malicious server," said Blasco. In particular, the script checks to see if Flash or Java browser plug-ins are installed on the system, and if so, which versions. Other routines, meanwhile, look for the presence of BitDefender security software, and if they find it, attempt to deactivate it. The script also searches for the presence of other information security software, including AVG, Avira, Dr.Web, ESET, F-Secure, Kaspersky Lab, McAfee, Microsoft Security Essentials and Sophos. The script also looks for the Google Chrome plug-ins for the Avast or Avira antivirus, and checks to see if Microsoft Office is installed.