They don&#x27;t have a system that makes malicious uploads impossible (outside of hacking, of course).  They could implement such a system, without much trouble.  They choose not to.<p>Using a third party ad network is not inherently risky.  But most <i>specific</i> third party ad networks are risky, because of bad practices.<p>That most ads are harmless is enforced through social norms and after-the-fact takedowns.  They could do better, but don&#x27;t.  Negligence.