Microsoft has bundled a Bing toolbar add-on along with its Patch Tuesday security update.
The download adds the Bing toolbar to the Internet Explorer and Mozilla Firefox web browsers of certain users without their permission.
Technology blog Ars Technica reported that last week's Patch Tuesday update contained a Microsoft
Search Enhancement Pack update that installed the Bing toolbar onto the IE and Firefox browsers of users that have installed the Windows Live Toolbar or MSN toolbar.
Microsoft told tech news site The Register that the stealth toolbar installation was the result of a bug in the update, and that the problem has since been taken care of.
The company said: "We fixed the update so that going forward folks who still have only the older Windows Live Toolbar or MSN Toolbar will not see this behavior any more."
The company explained that the updated was only supposed to install a Search Enhancement Pack for users of the Windows Live toolbar, MSN toolbar and Bing Bar.