Yes, applications published through Application Proxy are protected against Distributed Denial of Service (DDoS) attacks. This protection is managed by Microsoft and is automatically enabled in all our datacenters. The Azure DDoS protection service provides always-on traffic monitoring and real-time mitigation of common network-level attacks.
You can check this in below article,
https://video2.skills-academy.com/en-us/azure/active-directory/app-proxy/application-proxy-security#azure-ddos-protection-service
You can also go through below article that talks about security considerations for accessing apps remotely with Azure Active Directory Application Proxy
https://video2.skills-academy.com/en-us/azure/active-directory/app-proxy/application-proxy-security#azure-ddos-protection-service
Hope this answers your question.
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