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NEgative SEO damn near impossible.
Those links won't do anything so long as you have a natural, quality link profile.
@Carlos then your entire link profile must be terrible....nofollow blog comments got you a penalty...I highly doubt that.
As for who did this - I haven't looked into it really because I can't see how knowing the culprit's identity would be of any help to me - the links have been built and that's that in my mind. Or am I missing something?
Your best course of action is to conduct a thorough audit and submit a proper detailed report to Google going right down to every single suspect link. Then you just hope that Google a) reacts and b) gets it right.
not a bunch of links thrown at every Tom Dick and Harry website to see what might stick.
But I think a lot more people assume they're the victim of it than actually are. In hundreds and hundreds of site profiles I have reviewed over the years, I've only seen one profile where I seriously thought it *might* be happening.