The issue here lies with the agility and creativity of a criminal advertising network. Cybercriminals are able to quickly and efficiently generate incredible quantities of unwanted ads, and this high rate of production and extensive reach outpace the effectiveness of even the best blocklist tools.
Malicious advertising URLs and snippets that aren’t present on the list of “known bad” offenders will be let through undetected. Additionally, domains can easily be rotated at scale, via automation, making it impossible to maintain an effective list. This means the malicious payload can be deployed as part of an exploit kit and the attacker ultimately gains access to the end-user.
Blocklists can also become stale and lead to large quantities of false positives tied to domains that are no longer malicious or never were. False positives result in lost revenue and extra operational overhead chasing down false leads.
A blocklist is built by catching a threat and subsequently creating that entry to block it. As such, blocklists are inherently reactive and have no way to proactively block novel threats.