Security information provider Secunia has blown the whistle on some problems with the tabbed browsing feature found in several flavours of the Mozilla Foundation's browsers, the Opera browser, the Konqueror browser for Linux and two third-party plug-ins that add the feature to Microsoft's Internet Explorer.
One particularly nasty flaw allows a malicious Web site that's open in one tabbed window have access to the information typed into another tabbed window. So, one might be surfing a malicious site in one window, and logging into your on-line banking website in another. The possibilities are already frightening....
Other, more serious flaws identified by Secunia include a flaw which can be used to bypass the security mechanisms put in place by Microsoft's Windows XP Service Pack 2. When used with the "drag-and-drop" vulnerability found in IE, this could allow malicious Web sites to place and run code on a visitor's computer. More here