When an attacker promotes themselves as reputable third-party advertisers to distribute their malware through the web ads,
what type attack is being performed?

This type of attack is called Malvertising

Explanation:

Malvertising (Malware Advertising)  is a term in computer security that refers to the act of spreading computer malware through online advertisements. It is done by injecting malicious code into genuine online advertisement platforms or malicious webpages that appear as though they are genuine and from reputable third-party advertisers. While it is quite a new concept it is difficult to combact because content for advertisement can be placed in genuine and reputable websites that users trust.