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How Attackers Bypass RADIUS MFA (And How to Stop Them)

As organizations adopt Zero Trust architectures and hybrid infrastructure, RADIUS (Remote Authentication Dial-In User Service) remains a critical component for authenticating users into VPNs, network gear, and legacy applications. While adding Multi-Factor Authentication (MFA) to RADIUS improves security, it's not a silver bullet.

Sophisticated attackers are learning how to bypass RADIUS-based MFA—and in this blog, we'll break down the key attack vectors and outline strategies to harden your deployment against real-world threats.

1. Common Techniques Used to Bypass RADIUS MFA​

a. Pass-the-Request (Replay) Attacks​

Attackers with network access can replay intercepted RADIUS Access-Request packets to attempt MFA reuse. If the MFA provider accepts previously valid challenges, this could allow unauthorized access.

Mitigation:

  • Enforce strict one-time use of MFA tokens
  • Use short token expiration and nonce validation
  • Enable session binding where possible (e.g., tie challenge to originating IP/user/device)

b. Social Engineering & MFA Fatigue​

Once attackers obtain primary credentials, they bombard the user with repeated push-based MFA requests (often via mobile apps) hoping the user will approve out of fatigue or confusion.

Mitigation:

  • Enable rate limiting on MFA prompts
  • Use number matching or biometric confirmation
  • Implement user behavior analytics to detect anomalies

c. Man-in-the-Middle (MitM) Attacks​

Attackers intercept traffic between the RADIUS client (e.g., VPN appliance) and the RADIUS server. If encryption or mutual validation is weak, they may alter or replay packets to fool the MFA system.

Mitigation:

  • Always use RADIUS over TLS (RadSec)
  • Use certificate pinning and mutual TLS (mTLS) when available
  • Monitor for unexpected RADIUS clients or IPs

d. Legacy Protocol Exploits​

Legacy systems that rely on PAP or MS-CHAPv2 may expose MFA secrets or be more vulnerable to credential stuffing and brute-force attacks.

Mitigation:

  • Avoid legacy protocols; prefer EAP-TLS or PEAP
  • Block clear-text authentication methods
  • Enforce minimum encryption standards in VPN/SSO gear

2. Strengthening Your RADIUS MFA Posture​

Here's what security teams should consider to harden their RADIUS MFA environments:

a. Choose Phishing-Resistant MFA Factors​

  • FIDO2/WebAuthn, certificate-based authentication, or smart cards are resistant to phishing and replay.
  • Avoid SMS and basic TOTP as standalone methods.

b. Use Context-Aware Policies​

  • Evaluate context, device posture, location, time of access, and user behavior.
  • Block or challenge high-risk logins automatically.

c. Integrate with SIEM/XDR​

  • Correlate RADIUS logs with identity and endpoint telemetry.
  • Set alerts on suspicious login patterns, IPs, or failed challenges.

d. Keep Your MFA and RADIUS Components Updated​

  • Patch RADIUS servers, MFA plugins, and network appliances.
  • Perform regular pen testing and red teaming against your login flow.

Conclusion​

RADIUS is a foundational protocol in enterprise authentication, but attackers have evolved to target even MFA-backed setups. Security doesn't stop at enabling MFA—understanding attack vectors and layering defenses is key.

By using phishing-resistant factors, encryption best practices, and intelligent risk-based controls, you can ensure your RADIUS MFA is not just present—but resilient.

How Our RADIUS MFA Solution Stops MFA Fatigue Attacks​

Our solution integrates seamlessly into existing RADIUS authentication flows using a modular architecture: NPS → FreeRADIUS → Authnull. When a user initiates authentication, NPS first validates credentials with Active Directory. The request is then proxied to FreeRADIUS, which invokes our MFA. It enforces strict policies like per-user rate-limiting, single active challenge enforcement, and dynamic challenge expiry. This ensures that if multiple requests are made for the same user, only one MFA challenge is issued—and any repeated prompts within a short window are suppressed.

We've also implemented logic at the MFA layer to detect suspicious behavior patterns, such as repeated authentication attempts from the same IP or an unusual geographic location. Combined with our session-aware challenge tracking, this effectively blocks MFA fatigue attempts while maintaining a smooth experience for legitimate users. Our solution supports both push-based and phishing-resistant MFA methods (e.g., FIDO2, number matching), giving organizations flexibility without compromising security. By positioning the intelligence in the custom MFA layer, we maintain full compatibility with legacy systems while delivering modern Zero Trust-grade protection.