A new attack targeting Internet Explorer 7 has been reported.

The attack is said to target an unpatched flaw in IE7 and is carried out by way of a specially-crafted XML file.

According to Sans researcher Bojan Zdrnja, the exploit was not addressed by Tuesday's monthly security update and is believed to affect both Windows XP and Server 2003.

The researcher explained that the flaw targets a component in IE7 that handles XML tags. When the page confirms that the user is running a vulnerable browser and operating system, a specially crafted tag is loaded.

Zdrnja said that the attack is not believed to be widespread, but public exploit code has been made available. He also noted that a special feature of the attack, waiting six seconds to launch, could make the exploit even more potent.

"This was probably added to thwart automatic crawlers by anti-virus vendors, " Zdrnja said of the feature.

A Microsoft spokesperson told vnunet.com that the company is investigating reports of an Internet Explorer vulnerability.

If confirmed the IE flaw would be the second unpatched vulnerability to emerge for a Microsoft product this month. Attached to yesterday's security release was a note from the company that a flaw in Word 97 had yet to be patched as well.

Though the company prefers to release patches on a monthly basis to lighten the maintenance burden on administrators, special out-of-cycle updates are sometimes released when a high-risk or widespread security issue is reported.