One of the most widely used network protocols is vulnerable to a newly discovered attack that can allow adversaries to gain control over a range of environments, including industrial controllers, telecommunications services, ISPs, and all manner of enterprise networks.

Short for Remote Authentication Dial-In User Service, RADIUS harkens back to the days of dial-in Internet and network access through public switched telephone networks. It has remained the de facto standard for lightweight authentication ever since and is supported in virtually all switches, routers, access points, and VPN concentrators shipped in the past two decades. Despite its early origins, RADIUS remains an essential staple for managing client-server interactions for:

• VPN access
• DSL and Fiber to the Home connections offered by ISPs,
• Wi-Fi and 802.1X authentication
• 2G and 3G cellular roaming
• 5G Data Network Name authentication
• Mobile data offloading
• Authentication over private APNs for connecting mobile devices to enterprise networks
• Authentication to critical infrastructure management devices
• Eduroam and OpenRoaming Wi-Fi

RADIUS provides seamless interaction between clients—typically routers, switches, or other appliances providing network access—and a central RADIUS server, which acts as the gatekeeper for user authentication and access policies. The purpose of RADIUS is to provide centralized authentication, authorization, and accounting management for remote logins.

The protocol was developed in 1991 by a company known as Livingston Enterprises. In 1997 the Internet Engineering Task Force made it an official standard, which was updated three years later. Although there is a draft proposal for sending RADIUS traffic inside of a TLS-encrypted session that’s supported by some vendors, many devices using the protocol only send packets in clear text through UDP (User Datagram Protocol).

Roll-your-own authentication with MD5? For real?

Since 1994, RADIUS has relied on an improvised, home-grown use of the MD5 hash function. First created in 1991 and adopted by the IETF in 1992, MD5 was at the time a popular hash function for creating what are known as “message digests” that map an arbitrary input like a number, text, or binary file to a fixed-length 16-byte output.

For a cryptographic hash function, it should be computationally impossible for an attacker to find two inputs that map to the same output. Unfortunately, MD5 proved to be based on a weak design: Within a few years, there were signs that the function might be more susceptible than originally thought to attacker-induced collisions, a fatal flaw that allows the attacker to generate two distinct inputs that produce identical outputs. These suspicions were formally verified in a paper published in 2004 by researchers Xiaoyun Wang and Hongbo Yu and further refined in a research paper published three years later.

The latter paper—published in 2007 by researchers Marc Stevens, Arjen Lenstra, and Benne de Weger—described what’s known as a chosen-prefix collision, a type of collision that results from two messages chosen by an attacker that, when combined with two additional messages, create the same hash. That is, the adversary freely chooses two distinct input prefixes ? and ?′ of arbitrary content that, when combined with carefully corresponding suffixes ? and ?′ that resemble random gibberish, generate the same hash. In mathematical notation, such a chosen-prefix collision would be written as ?(?‖?)=?(?′‖?′). This type of collision attack is much more powerful because it allows the attacker the freedom to create highly customized forgeries.

To illustrate the practicality and devastating consequences of the attack, Stevens, Lenstra, and de Weger used it to create two cryptographic X.509 certificates that generated the same MD5 signature but different public keys and different Distinguished Name fields. Such a collision could induce a certificate authority intending to sign a certificate for one domain to unknowingly sign a certificate for an entirely different, malicious domain.

In 2008, a team of researchers that included Stevens, Lenstra, and de Weger demonstrated how a chosen prefix attack on MD5 allowed them to create a rogue certificate authority that could generate TLS certificates that would be trusted by all major browsers. A key ingredient for the attack is software named hashclash, developed by the researchers. Hashclash has since been made publicly available.

Despite the undisputed demise of MD5, the function remained in widespread use for years. Deprecation of MD5 didn’t start in earnest until 2012 after malware known as Flame, reportedly created jointly by the governments of Israel and the US, was found to have used a chosen prefix attack to spoof MD5-based code signing by Microsoft’s Windows update mechanism. Flame used the collision-enabled spoofing to hijack the update mechanism so the malware could spread from device to device inside an infected network.

More than 12 years after Flame’s devastating damage was discovered and two decades after collision susceptibility was confirmed, MD5 has felled yet another widely deployed technology that has resisted common wisdom to move away from the hashing scheme—the RADIUS protocol, which is supported in hardware or software provided by at least 86 distinct vendors. The result is “Blast RADIUS,” a complex attack that allows an attacker with an active adversary-in-the-middle position to gain administrator access to devices that use RADIUS to authenticate themselves to a server.

“Surprisingly, in the two decades since Wang et al. demonstrated an MD5 hash collision in 2004, RADIUS has not been updated to remove MD5,” the research team behind Blast RADIUS wrote in a paper published Tuesday and titled RADIUS/UDP Considered Harmful. “In fact, RADIUS appears to have received notably little security analysis given its ubiquity in modern networks.”

The paper’s publication is being coordinated with security bulletins from at least 90 vendors whose wares are vulnerable. Many of the bulletins are accompanied by patches implementing short-term fixes, while a working group of engineers across the industry drafts longer-term solutions. Anyone who uses hardware or software that incorporates RADIUS should read the technical details provided later in this post and check with the manufacturer for security guidance.