Another flaw in Internet Explorer has been uncovered by Danish security firm Secunia, which said that the gaffe left all PC users open to attack, even those who had updated Windows XP with the massive Service Pack 2 upgrade.

According to the alert that Secunia posted Thursday on its Web site, the vulnerability affects Internet Explorer 5.01, 5.5, and 6 on fully patched PCs running either Windows XP SP1 or the newer SP2.

Microsoft just began sending Service Pack 2 to Windows XP Home users this week, and although the update has been touted as a major security upgrade, the Secunia alert isn't the first problem that SP2 has faced. Microsoft has already issued a fix for SP2 that addresses problems some VPN users have encountered.

Grading the flaw "highly critical," Secunia says that proof-of-concept code has been published, and that the vulnerability--which stems from "insufficient validation of drag-and-drop events issued from the 'Internet' zone"--can be used by hackers to plant executable files in a Windows XP machine if the user is enticed to a malicious Web site.

"Even though the proof-of-concept depends on the user performing a drag-and-drop event, it may potentially be rewritten to use a single click as user interaction instead," Secunia warns.

It recommends either disabling Active Scripting within IE or using another browser until the problem is patched.

This flaw, says Secunia, is a close cousin of one discovered by a Chinese security researcher last September; those bugs have since been squashed.