An enterprise uses Cloud Identity to govern administrative access to Google Cloud projects hosting mission-critical workloads. A recent internal red-team assessment demonstrated that threat actors could execute adversary-in-the-middle (AitM) reverse-proxy phishing attacks to intercept session cookies and bypass standard multi-factor authentication methods, such as SMS codes and mobile authenticator push prompts. The security architect must enforce an authentication policy for all privileged administrators that eliminates credential harvesting and AitM relay attacks. Which 2-Step Verification (2SV) configuration should the architect implement?

Assign privileged administrators to a dedicated Organizational Unit (OU) and configure the 2-Step Verification policy to enforce 'Only security key' using FIDO2/WebAuthn hardware keys.: What FIDO2/WebAuthn Security Key Enforcement Is FIDO2/WebAuthn hardware security keys represent a cryptographically bound authentication standard designed by the FIDO Alliance and W3C. In Cloud Identity and Google Workspace, administrators can create targeted security policies applied to specific Organizational Units (OUs) or access groups, configuring the 2-Step Verification (2SV) enforcement setting specifically to Only security key. This policy restricts the acceptable second-factor method exclusively to physical, hardware-based cryptographic keys (such as Google Titan Security Keys or YubiKeys) or platform authenticators supporting FIDO2/WebAuthn protocols. How It Meets the Requirements Eliminating Adversary-in-the-Middle (AitM) Attacks: FIDO2/WebAuthn relies on cryptographic origin binding. During the authentication handshake, the client browser passes the cryptographically verified origin domain (e.g., accounts.google.com) to the security key. The security key signs a challenge using the private key registered specifically for that domain. If an adversary proxies the traffic through a spoofed domain (such as login.attacker-domain.com), the origin signature fails, and the authentication request is instantly rejected. Preventing Credential Harvesting: Security keys do not produce human-readable one-time codes or shared secrets that can be typed into fake phishing landing pages or intercepted by reverse proxies. Protecting Privileged Accounts: Segmenting administrators into a dedicated OU allows security teams to mandate strict hardware-only enforcement for high-privilege roles without disrupting general workforce users who may rely on transitional 2SV methods. Key Advantages and Capabilities Cryptographic Domain Binding: Binds the private-key challenge-response directly to the legitimate Fully Qualified Domain Name (FQDN) in the browser. Elimination of Shared Secrets: Unlike TOTP or SMS, no shared symmetric secret key is stored on external verification servers or user mobile devices. Policy Enforcement Control: Cloud Identity prevents users under this policy from downgrading to fallback authentication factors like SMS or phone calls. Hardware Root of Trust: Secure cryptoprocessors resist physical tampering and extraction of private keys. Why This Is Optimal Enforcing Only security key at the OU level provides absolute phishing resistance against sophisticated AitM proxies and reverse-proxy credential harvesting kits. It represents the highest assurance level of multi-factor authentication supported in Google Cloud and Cloud Identity.