Mashiki, while that is true I don't think that is the case here. Yahoo misrepresented nothing. I am assuming Yahoo did not know about the data breach when they signed the deal.
That being said, most merger agreements have a materiality clause allowing termination of the merger is something significance comes up. It is a bit of a weasel clause and is rarely used but it almost always in the merger agreement. I am not sure I would call this data breach material in the financial sense but it does give Verizon leverage.