Google’s position on negative SEO has been basically this:
Matt Cutts: “Because you can’t really control who links to you and how they link to you, that’s something that, being out of your control, we try to be very careful about to try to make sure it doesn’t affect your site’s reputation or hurt your site in some way.”
Even well known SEOs have defended Google’s position about negative SEO, including Rand Fishkin of SEOmoz. Fishkin issued a negative SEO challenge to take down SEOmoz or RandFishkin.com using negative SEO tactics. Fishkin issued the challenge interestingly close to but prior to the Penguin update in April. When asked in May about the challenge, Rand said, “It’s still ranking well, though!”
What most people missed with the results of SEOmoz’s negative SEO challenge is that negative SEO is a threat to smaller sites that do not have a solid link profile and reputation like SEOmoz. In fact, SEOmoz and big brands like them are largely immune to the effects of negative SEO.
But when Penguin hit in late April and sites with an unnatural link profile fell, it became clear that in addition to sites with unnatural links, some sites had unnatural links that were beyond the site owner’s control (no way to remove them at all), and indeed that affected the site’s reputation and ranking.
And although Google has not said anything officially about it… Google’s disavow link tool proves that negative SEO is real.