Implement MFA

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We are seeking an experienced full-stack developer or security-focused engineer to implement Multi-Factor Authentication (MFA/2FA) for an existing member login system.
This is not a standard WordPress login. Our architecture is layered and includes:
WordPress front-end (multiple sites)
Custom login forms embedded via iframe
Backend C# application (TeamWeb) that handles authentication logic
SQL Server database (user accounts stored in table: Web Registration)
Multiple domains that share similar login architecture
Cross-site redirects between a marketing site and legacy member portals
The login process:
Users access a Member Login page (WordPress template)
The login form loads via iframe
Authentication is handled by a C# application
Credentials are validated against a SQL database
On success, users are routed to protected member content
We need to add MFA at the authentication layer for all member login portals without breaking the existing ...