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Re: Adgregate ShopAd widget validation is vulnerable to replay attack

As an update, since I submitted my first message, Adgregate changed
their validation mechanism.  The current method is still
intermittently vulnerable to replay attacks, but now there's actually
an expiration mechanism to deal with.

On Tue, Apr 7, 2009 at 5:42 PM, Matthew Dempsky <matthew@xxxxxxxxxxx> wrote:
> Adgregate is a "TechCrunch 50" startup that recently signed a
> distribution deal with Google/DoubleClick [1].  As a service, they
> offer a "viral widget" intended to be hosted on untrusted third-party
> sites through which consumers can enter their credit card information.
>  According to their website, they offer over 1.2 million products
> through this service.  More details can be found at [2].
> Consumers are able to validate that their data entry is secure by
> clicking on a "validate this widget" button within the widget.
> However, the widget proves itself by simply POSTing an (apparently)
> fixed (per ShopAd id) string to a fixed URL, which is trivially
> vulnerable to replay attacks.  For example, see the (clearly bogus)
> widget at [3].  Of course, a more serious attacker would spend more
> time reproducing the look-and-feel of the adgregate widget.
> Here's the relevant HTML that fakes the validation functionality:
> <form method="POST" action="https://secure.adgregate.com/AuthenticWidget.aspx";>
> <input type="hidden" name="widgetvalid" value="1w23e4r5-ijhyffrd">
> <input type="submit" value="Validate this ShopAd!">
> </form>
> (Bob Ippolito first alerted me to this service and provided the above
> HTML form snippet.)
> [1] http://www.techcrunch.com/2009/04/06/adgregate-markets-scores-distribution-deal-with-googles-doubleclick/
> [2] http://adgregate.com/web/demo/
> [3] http://shinobi.dempsky.org/~matthew/adgregate.html