# Blast-RADIUS, MD5 collision forges Access-Accept

Source: https://startwithidentity.com/cves/cve-2024-3596/
Last updated: 2026-08-29
License: content by Start with Identity. Cite the source URL.

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## What broke

RADIUS (RFC 2865) authenticates responses with an MD5-based Response Authenticator. A chosen-prefix collision lets an on-path attacker turn Access-Reject into Access-Accept. CHAP and PAP without `Message-Authenticator` are the easy cases. MFA that sits behind that RADIUS exchange is bypassed, because the NAS never sees the reject. CVSS 9.0. The research name is Blast-RADIUS. Vendor patching continued into 2026 (Schneider, among others, in April 2026).

## Why it matters

RADIUS is still how VPN, Wi-Fi, and a surprising amount of MFA talk to the [IdP](https://startwithidentity.com/glossary/identity-provider/). A protocol-level forge is an MFA bypass that does not care which vendor sold you the token. This is why we keep a 2024 CVE in a 2025-2026 catalog: the control (Message-Authenticator or RadSec) is what is still missing.

## What to do

- Require `Message-Authenticator` on every RADIUS client and server. Reject packets that omit it.
- Move to RadSec (RADIUS over TLS) where the NAS supports it.
- Stop sending PAP/CHAP across a network you do not trust. Prefer EAP methods that bind to TLS.
- Inventory MFA products that still front RADIUS. A "phishing-resistant" factor behind a forgeable Access-Accept is not phishing-resistant.

## After you patch

RADIUS carries authentication for network and VPN access, and its legacy cryptography is the recurring weakness rather than any single implementation.

- **Rotate every shared secret**, using long random values that differ per client rather than a single reused string.
- **Move RADIUS traffic inside an authenticated transport**, RADIUS over TLS or IPsec, so the protocol's own integrity weaknesses stop being reachable.
- **Prefer EAP methods with mutual authentication** over PAP or CHAP wherever the client supports it.
- **Audit which devices still authenticate over RADIUS** and what they grant, since network gear and VPN concentrators are common initial-access targets. See [MFA](https://startwithidentity.com/glossary/mfa/).

## Sources

- [NVD: CVE-2024-3596](https://nvd.nist.gov/vuln/detail/CVE-2024-3596)
- Blast-RADIUS research (2024) and follow-on vendor advisories through 2026