"It's not necessarily that plug-ins aren't or can't be secure, but that running a browser within a browser doubles the potential attack surface in a way that we don't see is particularly helpful," said Amy Bazdukas, Microsoft's general manager for IE.
Bazdukas also said that by running Chrome Frame, IE8 users were unwittingly discarding all the private browsing protections that Microsoft built into its newest browser.
"Chrome Frame breaks the privacy model of IE," she claimed. "Users are not going to be able to use IE's privacy features, something that's not made apparent to users. They're essentially circumvented."
Bazdukas also maintained that IE8's browser history deletion feature is crippled by Chrome Frame; users who decide to erase the history may think it's working when it's not.
In a statement earlier today, Microsoft said using Chrome Frame is not "a risk we would recommend our friends and families take." Bazdukas, however, got more specific.
"We're not saying that there's a specific security vulnerability in Chrome Frame, but the concern that plug-ins in general have had regarding security issues adds a new potential threat when Chrome Frame is used. Users have told us that they're looking for a better and safer browser, and we can't see how [using Chrome Frame] will deliver that."