Azure Application Gateway with its integrated Web Application Firewall (WAF) is the most appropriate solution. The WAF component can be configured with a managed bot protection rule set to identify and block malicious bots and scanners. Furthermore, the WAF supports custom rules for geo-filtering, which can restrict access to only users from Europe and the United States, directly minimizing the attack surface as required. This solution is specifically designed to protect web applications, such as Azure App Service, from common exploits, vulnerabilities, and automated threats.

A. Azure Firewall Premium: While it offers advanced threat protection, it is a network firewall, not a specialized web application firewall. A WAF is more suitable for protecting against web-specific attacks and bots.

C. network security groups (NSGs): NSGs operate at the network layer (L3/L4) and cannot inspect application traffic to identify or block malicious bots. They lack native geo-filtering capabilities.

D. Azure Traffic Manager and application security groups: Traffic Manager is a DNS-based load balancer and has no security inspection features. Application security groups are for simplifying NSG rules, which are insufficient for this task.

1. Microsoft Documentation

"What is Azure Web Application Firewall on Azure Application Gateway?": "Azure Web Application Firewall (WAF) on Azure Application Gateway provides centralized protection of your web applications from common exploits and vulnerabilities... This includes protection against things like malicious bot attacks."

Source: Microsoft Learn

learn.microsoft.com/en-us/azure/web-application-firewall/ag/ag-overview

2. Microsoft Documentation

"Bot protection overview": "Azure WAF on Application Gateway provides protection from bot attacks through the OWASP managed bot protection rule set."

Source: Microsoft Learn

learn.microsoft.com/en-us/azure/web-application-firewall/ag/bot-protection-overview

3. Microsoft Documentation

"Custom rules for Web Application Firewall v2 on Azure Application Gateway": This document details the creation of custom rules

including "Geomatch" as a match type. "You can create a custom rule to block or allow traffic from certain countries/regions... This helps you to block requests from geographic locations that are known to be malicious."

Source: Microsoft Learn

learn.microsoft.com/en-us/azure/web-application-firewall/ag/custom-rules-overview (See section on Match types and operators).

4. Microsoft Documentation

"Network security groups": "A network security group contains security rules that allow or deny inbound network traffic to

or outbound network traffic from

several types of Azure resources... You can filter traffic by source and destination IP address

port

and protocol." This confirms NSGs operate at L3/L4 and lack application-layer intelligence for bot detection.

Source: Microsoft Learn

learn.microsoft.com/en-us/azure/virtual-network/network-security-groups-overview