CVE-2024-3596

What it actually does

Permits a local attacker to forge any valid RADIUS Response (Access-Accept, Access-Reject, or Access-Challenge) by performing a chosen-prefix collision attack against the MD5-based Response Authenticator signature defined in RFC 2865.

Why it matters to the business

Attackers positioned on the network can impersonate RADIUS servers or clients, granting or denying access to network resources at will and undermining authentication controls for VPNs, Wi-Fi, and other services that rely on RADIUS.

How an attack unfolds

An attacker with network access observes a legitimate RADIUS exchange, crafts a chosen-prefix collision against the MD5 Response Authenticator, and substitutes a forged response packet that the client accepts as authentic, thereby altering the authentication outcome without knowledge of shared secrets.

How to fix it

Upgrade freeradius/freeradius-server to the commit f317c5b2668a4de7065df46b31267cd6ff32ddf1 or later; migrate to RADIUS over TLS (RadSec) or adopt stronger message authentication mechanisms such as those specified in RFC 6614 and RFC 7360.

How to confirm the fix worked

Confirm the running binary or source tree contains commit f317c5b2668a4de7065df46b31267cd6ff32ddf1; test that forged Response packets are rejected by replaying controlled collision attempts in a lab environment.

Typical attack path

1. Position on the path between RADIUS client and server to observe or intercept packets
2. Capture a valid Response packet containing the MD5 authenticator
3. Compute a chosen-prefix MD5 collision to produce a forged Response of the desired type
4. Inject the forged packet and suppress or replace the original response
5. Observe the client accepting the forged authentication decision

How to detect it

Inspect RADIUS server and client logs for unexpected authentication outcomes, repeated Access-Request/Response pairs with identical identifiers but differing results, or traffic anomalies on UDP port 1812; correlate with network flow data for signs of packet injection.

What your security team should do

• Immediately schedule upgrade of all FreeRADIUS instances to the fixed commit
• Prioritize migration to RadSec or another authenticated transport for high-value networks
• Review RADIUS deployments for exposure to untrusted network segments
• Monitor vendor and CISA guidance for additional protocol-level mitigations