Need help! Is this a negative SEO attack?

In the last 30 days, our well running website doubled the amount of backlinks it received without any of our efforts. We received about 500.000 new backlinks in this time frame.

When investigating it turned out most of these links come from ????.blogspot.com domains with a DA of 0. So my first question is: Is this a negative SEO attack?

The second question is: Will it have a negative impact and should I do something about it?

It doesn't feel natural just to let this happen and not do anything, but to disavow blogspot.com domains also seems a bit much to me.