Hi All,
UPDATE :
So, apparently one of our third party ad vendor was serving malicious ads, but since we served their tags via our Ad server, google flagged our domain as Malicious.
We had to contact the DFP support and explain them the server is fine and also we are not directly associated with the malicious advertiser, after a bunch of mails and convincing we were able to whitelist our domain.
I would suggest you to contact DFP support and explain the situation. If you use third party tags to server ads it could be one of them that's injecting the malicious ads.