US-CERT, the government's cybersecurity arm, is warning users of Microsoft Internet Explorer 7 about a Trend Micro report claiming that cybercriminals are attacking Internet Explorer 7 through a vulnerability disclosed and patched by Microsoft last week.

"Cybercriminals are actively exploiting a critical vulnerability in Internet Explorer 7, which arises from the browser's improper handling of errors when attempting to access deleted objects," said Trend Micro's Jake Soriano in a blog post on Tuesday. "This vulnerability allows remote attackers to execute arbitrary codes on a vulnerable machine."

Microsoft designated Security Bulletin MS09-002 to be "critical" and gave the vulnerability a score of 1 on its Exploitability Index, indicating that functioning exploit code was expected.

According to Trend Micro, the attack begins with a malicious Microsoft Word (.doc) file distributed through spam. It contains an ActiveX control that reaches out to a malicious site that exploits the vulnerability patched by MS09-002.

If the system attacked is unpatched, the ActiveX control, identified as HTML_DLOADER.AS, will download a secret back door, identified as BKDR_AGENT.XYMS, designed to steal information from the compromised system. It also takes screenshots of the affected system and sends these to a remote location, and it will listen for commands through a hidden Internet Explorer window.

"Although the install base of the IE family is slowly eaten up by stiff competition such as Firefox and Chrome, IE7 is used by about one in every four Web users, a much larger share than previous versions of IE," observes Soriano. "This could explain why cybercriminals seem to be eagerly searching for more bugs."

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