A DigitalPoint Forums member points to a vulnerability within Google Reader that enables blog owners to add code to their blog that will allow any visitor to automatically subscribe to the RSS feed. Patrick Altoft wrote about this discovery and says that if you copy certain code to your site and a user has logged into his/her account, they will automatically be subscribed to your blog:
The problem is that unscrupulous websites can copy the links to Add to Google homepage or Add to Google Reader and open them up in an IFRAME for every visitor, meaning that anybody who visits their website while signed in to a Google account will suddenly have subscribed to the RSS feed on both Google Reader and the Google homepage automatically.
I tried Patrick's demo and it seems to have been fixed. In fact, Matt Cutts responded as well and it appears that the hole has been patched.
Forum discussion at DigitalPoint Forums.