Google Confirms Negative SEO Exists
For years, Google stood by the stance that there was little competitors could do to a website to negatively affect its search engine rankings. As natural links became more valuable and paid links were devalued or penalized, Google changed their stance slightly on the issue, but still maintained it wasn't a widespread issue, with Google's Matt Cutts admitting negative SEO isn't impossible, but it is difficult.
Then came the link disavow tool, which allows webmasters to disallow paid or low quality links and clean up their backlink profile, regardless of whether they or a competitor was responsible for pointing those backlinks at a website.
I guess everyone already knew that.