RADIUS/UDP considered harmful

1 minute read


Blast-RADIUS is a vulnerability that affects the RADIUS protocol. RADIUS is a very common protocol used for authentication, authorization, and accounting (AAA) for networked devices on enterprise and telecommunication networks.


The RADIUS protocol is the de facto standard lightweight protocol for authentication, authorization, and accounting (AAA) for networked devices. It is used to support remote access for diverse use cases including network routers, industrial control systems, VPNs, enterprise Wi-Fi including the Eduroam network, Linux Pluggable Authentication Modules, and mobile roaming and Wi-Fi offload.

We have discovered a protocol vulnerability in RADIUS that has been present for decades. Our attack allows a man-in-the-middle attacker to authenticate itself to a device using RADIUS for user authentication, or to assign itself arbitrary network privileges. Our attack exploits an MD5 chosen-prefix collision on the ad hoc RADIUS packet authentication construction to produce Access-Accept and Access-Reject packets with identical Response Authenticators, allowing our attacker to transform a reject into an accept without knowledge of the shared secret between RADIUS client and server.

We optimize the MD5 chosen-prefix attack to produce collisions online in less than five minutes, and show how to fit the collision blocks within RADIUS attributes that will be echoed back from the server. We demonstrate our attack in a variety of settings against popular RADIUS implementations. Nearly all RADIUS/UDP implementations are vulnerable to our protocol attack when using non-EAP authentication methods. RADIUS Accounting and EAP authentication appear less practically exploitable, although a theoretical protocol vulnerability may exist. Our attack also requires man-in-the-middle network access. Not all RADIUS deployments are practically exploitable; organizations should independently verify. It is our hope that this attack will provide the impetus for vendors and the IETF to deprecate RADIUS over UDP, and to require RADIUS to run over secure channels with modern cryptographic privacy and integrity guarantees.

This paper is published at USENIX 2024.

Further Resources

More information on the paper, how the attack works, mitigations, and frequenlty asked questions is available on our website blastradius.fail.

Our paper is available here.