Understanding RADIUS Response Forgery

Updated on June 30, 2025

Cybersecurity challenges are evolving rapidly, with attackers finding new ways to target even the smallest gaps in defense. One such threat is RADIUS response forgery (CVE-2024-3596), a critical vulnerability in the remote authentication dial-In user service (RADIUS) protocol. In this blog, we’ll break down how this attack works, its main features, and the steps IT professionals can take to safeguard their networks.

What is RADIUS Response Forgery?

RADIUS response forgery is a type of cyberattack where an attacker intercepts and manipulates RADIUS protocol responses to exploit weaknesses in its cryptographic checks. This attack targets vulnerabilities in the Message Digest Algorithm 5 (MD5) used by RADIUS, allowing the attacker to forge responses like changing an Access-Reject into an Access-Accept. As a result, the attacker tricks the RADIUS client, also known as the Network Access Device (NAD), into granting unauthorized access.

Core Concepts of the RADIUS Protocol 

To grasp the nature of RADIUS Response Forgery, it’s essential to understand the following components of the RADIUS protocol:

• RADIUS (Remote Authentication Dial-In User Service): Protocol for centralized Authentication, Authorization, and Accounting (AAA) in network devices like switches, routers, or VPNs. 
• Authentication Responses: Includes Access-Accept, Access-Reject, and Access-Challenge outcomes sent by the RADIUS server to approve, deny, or query access. 
• RADIUS Client (Network Access Device): Device requesting authentication verification from the RADIUS server to validate users attempting access. 
• Man-in-the-Middle (MitM) Attack: A network attack where the attacker intercepts and manipulates packets between communicating systems. 
• MD5 (Message Digest 5): A hashing function with known cryptographic weaknesses, including chosen-prefix collision attacks. 
• Response Authenticator Attribute: An attribute in RADIUS responses that ensures response integrity when properly calculated. 
• Network Access Bypass: The end goal of an attack to gain unauthorized access by tricking the RADIUS client.

How Does RADIUS Response Forgery Work?

This attack exploits the cryptographic limitations of the RADIUS protocol, specifically its use of MD5 for integrity checks. Here are the technical mechanisms behind the forgery process:

Attacker’s Positioning

The attack initiates with the hacker having “on-path” or Man-in-the-Middle (MitM) access to intercept traffic between the RADIUS client and server. This positioning is critical as it allows interception and manipulation of authentication data.

Intercepting the Access Request

The attacker intercepts the initial RADIUS Access-Request packet sent by the client. This packet contains predictable data components that the attacker can exploit.

Predicting the Response Authenticator

RADIUS uses MD5 to calculate a Response Authenticator hash based on the response packet’s attributes and a shared secret. However, the attacker can often predict parts of this calculation, even without knowing the shared secret, due to the deterministic nature of certain attributes in the protocol.

Chosen-Prefix Collision and Attribute Injection

Leveraging a chosen-prefix collision technique against MD5, the attacker crafts specific data (such as within the Proxy-State attribute) to append to a predictable prefix of the intercepted packet. This carefully crafted suffix ensures that the resulting MD5 hash (which forms the Response Authenticator) becomes predictable and controllable by the attacker. This allows the attacker to simultaneously modify key bytes of the packet (e.g., changing Access-Reject to Access-Accept) while ensuring the forged response carries a cryptographically valid authenticator for what the NAD expects.

Forged and Injected Response

The attacker forges the RADIUS response and sends it to the client before the legitimate response from the RADIUS server arrives. The NAD, verifying only the Response Authenticator’s validity, accepts this forged response.

Unauthorized Access Granted

The RADIUS client processes the forged response as legitimate and grants access to the attacker’s device or user.

Notably, this entire process does not require the attacker to know the shared secret, highlighting the vulnerability in the MD5-based integrity check.

Key Features and Components of This Attack

RADIUS Response Forgery has distinct characteristics that make it a critical vulnerability:

• Protocol-Level Vulnerability: Exploits flaws in RADIUS specifications (RFC 2865) rather than vendor-specific implementation bugs.
• MD5 Weakness: Relies on the inherent cryptographic weaknesses of MD5, particularly chosen-prefix collisions.
• On-Path Attacker Role: Requires an attacker in a MitM position to intercept and manipulate traffic.
• UDP Susceptibility: Primarily affects UDP-based RADIUS due to its connectionless nature.
• Password Independence: Does not require the attacker to crack or know passwords or shared secrets.
• Attribute Injection: Allows crafted attributes (e.g., VLANs or privileges) in forged responses.

Mitigation Measures to Prevent RADIUS Response Forgery

To defend against this attack, IT professionals can implement the following countermeasures:

Mandate Message-Authenticator Attribute 

What it does: Enforces the use of the Message-Authenticator attribute (RFC 2869), which employs HMAC-MD5 for packet validation. 

Why it works: Provides a significantly stronger integrity check by hashing the complete packet contents, including the authenticator field.

Implement RADIUS over TLS (RadSec) 

What it does: Encrypts the entire RADIUS communication channel using TLS over TCP. 

Why it works: Protects the integrity and confidentiality of RADIUS packets, preventing MitM attacks.

Deploy Strong EAP Methods (e.g., EAP-TLS) 

What it does: Utilizes EAP-TLS, which ensures mutual authentication and establishes encrypted tunnels. 

Why it works: Provides cryptographically secure mechanisms that render forgery attacks ineffective.

Network Segmentation and Isolation 

What it does: Implements secure VLANs and access control measures to restrict RADIUS traffic pathways. 

Why it works: Reduces the risk of attackers gaining on-path visibility.

Use Strong RADIUS Shared Secrets 

What it does: Encourages the use of long, unique secrets to secure RADIUS configurations. 

Why it works: While it doesn’t address MD5 weaknesses directly, it mitigates other attack vectors like brute-force attempts.

Regular Software Updates 

What it does: Applies patches that vendors provide to strengthen RADIUS protocol defenses (e.g., mandating stronger authenticator checks). 

Why it works: Keeps systems protected from the latest vulnerabilities, such as CVE-2024-3596.

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