Linux and eduroam: RADIUS

RADIUSA service separate from, but tightly coupled to, eduroam is our RADIUS service. This is the service that authenticates a user, making sure that the username and password typed into the password dialog box (or WPA supplicant) is correct. Authorization is possible with RADIUS (where we can accept or reject a user based on a user’s roles) but for eduroam we do not make use of this; if you have a remote access account, and you know its password, you may connect to eduroam, both here and at other participating institutions.

This aims to be a post to set the scene for RADIUS, putting it into context, both in general, and our use of it. There have been generalizations and simplifications here so as not to cloud the main ideas of RADIUS authentication but if you feel something important has been omitted please add it as a comment.

What is RADIUS?

RADIUS is a centralized means of authenticating someone, traditionally by use of a username/password combination. What makes it stand out from other authentication protocols (e.g. LDAP) is how easy it is to create a federated environment (i.e. to be able to authenticate people from other organizations). For eduroam this is ideal: an institution will authenticate all users it knows about, and proxy authentication duties to another institution for the rest. For example, we authenticate all users within our own “realm” of ox.ac.uk, but because we do not know about external users (e.g. userX@eduroam.ac.uk), we hand the request off to janet who then hands it to the correct institution to authenticate. Similarly off-site users authenticating with a realm of ox.ac.uk will have their request proxied (eventually) to our RADIUS servers, who say yay or nay accordingly.

Anatomy of a RADIUS authentication request

WARNING: Simplifications ahead. Only take this as a flavour of what is going on.

Say I have a desktop PC that uses RADIUS to authenticate people that attempt to log in. At the login screen userX@ox.ac.uk types in a password “P4$$W0rd!” and hits enter. The computer then creates a RADIUS request in the following format and sends it to our RADIUS server.

Packet-Type = Access-Request
User-Name = userX@ox.ac.uk
Password = P4$$W0rd!

The RADIUS server receives this request and, depending on obvious criteria, accepts, denies or proxies the request. On a successful authentication, the RADIUS server sends the following which the desktop is free to use as required.

Packet-Type = Access-Accept

The Access-Deny packet is similar.

Packet-Type = Access-Reject

For proxied requests, the packet is received and forwarded to another RADIUS server whose reply is proxied back the other way. The possibilities to configure where to proxy packets are infinite, but traditionally it is based on something called a realm. For the example above, the realm is the part after the “@”, and for us here in Oxford University, this would mean that we do not proxy the request for userX@ox.ac.uk. If another realm had been provided, we could proxy that to another institution if we so wished.

That, at its heart, is RADIUS authentication.

Securing RADIUS

In many ways, RADIUS is a product of its time, and decisions that when made seemed sensible now make for a fairly frustrating protocol. For example in the beginning, as shown above, RADIUS sent the username and password in the clear (i.e. without any encryption.) Back when the primary use of RADIUS was to authenticate users of dial-up modems, this was deemed acceptable since phone conversations were (perhaps a little naively) considered secure. Now however, internet traffic can be sniffed easily and unencrypted passwords sent over the internet are very much frowned upon.

Step 1: Encrypting passwords

The first step to secure communications is obvious, you can encrypt the password. There are a number of protocols to choose from, MS-CHAPv2 and CHAP being but two that are available to standard RADIUS configurations. So long as the encryption is strong, then there’s little risk of a man in the middle (MITM) from intercepting the packets and reading the password. If we ignore the elephant in the room of how effective MS-CHAPv2 and CHAP actually are, this is a step in the right direction. The packet now looks something like the following:

Packet-Type = Access-Request
User-Name = userX@ox.ac.uk
Chap-Password = [Encrypted Password]

You can see that there is no mention of the password in the RADIUS request. As an aside, I will mention Access-Challenge packets here only insomuch as to acknowledge of their existence. Understanding how they slot into RADIUS would not greatly improve this post’s clarity and so I will deftly sidestep any issues introduced by them.

However, there’s a slight problem. RADIUS, as mentioned earlier, allows for request proxying. Encrypting the password is fine, but if the end point is not who is purports to be, then the process falls flat. Wearing my devious hat, I could set up my own RADIUS server, which accepts any request for the username “vice-chancellor@ox.ac.uk” regardless of password. I could then engineer it so that I could authenticate as this username at another institution (by re-routing RADIUS traffic), and wreck havoc with impunity, since the username is not traceable back to me. In a similar vein, I could create my own wifi at home, call it “eduroam” and have authentication data come in from passing phones as they try to connect to what they think is the centralized “eduroam” service. I’ll say more on this later.

Then there’s also the issue of the unencrypted parts of the request. The username is sent in the clear, because that part is used for proxying. This means that when at another institution, there is no way to authenticate yourself without divulging your username to anyone who looks at the request. With the benefit of hindsight, I’m sure that RADIUS would have three fields, username, password (or equivalent), and realm, where you can encrypt the username, but not the realm. The fact that the realm is bundled in with the username is the source of this problem.

Step 2: Encrypting usernames

The way RADIUS addresses the issue of privacy (i.e. how it allows for encrypted usernames) is fairly neat or fairly hackish, depending on your viewpoint. Assuming that the authentication side of RADIUS is all working smoothly, then you can encrypt the whole request and send it as an encrypted blob. That bit isn’t so surprising. The neat trick that RADIUS employs is that, having this encrypted blob, you now need to ensure that it reaches its correct destination, which isn’t necessarily the next hop. Since we’re using RADIUS already, which has all this infrastructure already to proxy requests, it makes sense to wrap the entire encrypted request as an attribute in another packet and send it.

Packet-Type Access-Request
User-Name = not_telling@ox.ac.uk
EAP-Message = [Encrypted message containing inner RADIUS request]

Here we can see that the User-Name does not identify the user. The only thing it does do (and in fact needs to do) is identify the realm of the user so that any RADIUS server can proxy the request to the correct institution. Now, we can decrypt the EAP-Message and retrieve back the actual request to be authenticated:

Packet-Type = Access-Request
User-Name = userX@ox.ac.uk
Chap-Password = [Encrypted Password]

This process is a two way street, with each inner packet, meant only for the eyes of the two endpoints, being wrapped up in outer packets which are readable by all points between them.

That solves the privacy issue of username divulgence, but it also solves the MITM problem identified earlier, by the encryption method chosen: SSL/TLS.

Step 3: Stopping man-in-the-middle

Supplementary warning: I did mention above that this post is a simplification, but this section is going to be more egregious than usual. Going into the intricacies of SSL/TLS is probably best left for another day.

When you, the client, want to send an SSL encrypted packet to a server, you encrypt the packet using a key that you downloaded from said server. The obvious question is “how do you know that the key downloaded is for the destination you want, and not some imposter?” The answer is “by use of certificates”. A collection of files reside on every computer called CA certificates (CA in this context means “certificate authority”). These files can be best reasoned as having a similar function to signatures on cheques. The key downloaded for encrypting packets is signed by one of the certificates on your computer and because of that, you “know” that the key is genuine.

A Certificate Authority is an organization whose sole job is to verify that a server host and its key are legitimate and valid for a domain (e.g. ox.ac.uk). Once it’s done that, the CA validates the key by signing it using its certificate. For our radius servers, the host is radius.oucs.ox.ac.uk and the CA that we use is currently AddTrust. In essence, we applied to AddTrust for permission to use its certificate to validate our key, and they agreed.

What would happen if I had applied for permission to use www.google.com? Well most likely AddTrust would have (after they’d finished laughing) told me to get lost, but hypothetically if they had signed a key I’d generated for www.google.com, then whole concept of security by SSL would fall like a stack of cards. This has happened before, with unsurprisingly dire immediate consequences.

How do CAs get this position of power? I could start up my own CA relatively easily, but it would count for nothing as nobody would trust my certificate. It all hinges on the fact that the certificates for all the CAs are installed on almost all computers by default.

Certificate validation error dialog on Windows 7

OK, who recognizes this, and more importantly who’s clicked “Connect” on this dialog box without reading the details?

What I have described is actually the behaviour of web browsers rather than WPA supplicants (or your wifi dialog box). By default browsers accept any key, so long as it’s signed by any certificate on your computer. Connecting to eduroam is more secure in that you have to specify which CA the key is signed with (“AddTrust External CA Root” in our case). It is crucial that you do not leave this blank. If you do, you’re basically saying you’ll accept any key including one from an imposter. Yes, it’s true you will get a warning, but I do wonder the number of people who connect to eduroam who click “Ignore” or “Connect” on that without reading it first. We have received reports of a rogue “eduroam” wireless network somewhere within Oxford city centre (you can name your wireless anything you like, after all). For anyone who configured the CA correctly on his or her device this is fine and it will not connect, but people ignoring the certificate’s provenance will be potentially divulging usernames and passwords to a malicious third party.

RADIUS passwords and SSO

Anyone who uses eduroam will know that it has a separate distinct password from the normal SSO password which is used for WebAuth and Nexus. The reasoning for that can be broadly split into three sections, technical, historical and political. I will only be covering the first two.

A History lesson and history’s legacy

RADIUS in Oxford came about from the need to authenticate dial-up users and predates all the EAP encryption above. Every authentication request was originally sent in the clear to the RADIUS servers. Thus, a separate password was felt to be needed so that any snooping would only grant access to dial-up, not to a user’s personal resources, like emails. Also at that time, there was no concept of a centralized password store like there is today, so the drive for unifying SSO and RADIUS would have been non-existent; there was no SSO!

Fast forward to today and you would think that to ease our security concerns we could turn off all requests that aren’t EAP. Unfortunately there are many tools, including those found in units around the university, that rely on traditional RADIUS behaviour (i.e. not using EAP) and we would not like to break anyone’s infrastructure without good reason. I will not point fingers, but we still receive authentication requests with Passwords sent in the clear. We strip this attribute from our logs so I would have to actively do something to generate usable statistics, but it was something that I noticed during the migration of our RADIUS servers in the second half of 2014.

Hooking into our Kerberos infrastructure

The first impulse for a unified password would be to use a common source. The Kerberos Domain Controllers [KDCs] should be considered the canonical location of authentication data. Could we just use that as our password store?

Short answer is “not easily”. You will probably find information on connecting a RADIUS server to a Kerberos server and think the job were easy. However, you will notice that it only supports one authentication protocol, PAP. PAP authentication is a technical way of saying “unencrypted password” and this protocol is unavailable in versions of Windows. To allow for a wider range of encryption methods, you would need to install something on the kerberos server itself to deal with them. The KDCs are run by a sister team here in IT Services and, while in and of itself not a hindrance, hooking into that infrastructure would require some planning before we could even considered this as a possibility.

Using our own infrastructure

There is a precedent for this: Nexus does not use the KDC, instead relying on its own authentication backend to store usernames and passwords. Could we not do the same for RADIUS?

Short answer is “yes”. Longer answer is “yes, but”. In order to accept the majority of password encryption methods that will be thrown at us, we have to currently store the passwords in a format that we believe to be suboptimal. Don’t think that we take security lightly; the servers themselves have been secured to the best of our ability and we have debated this topic for many years on whether to change the format. However, if you look at the compatibility matrix of compatible protocols to password storage, it wouldn’t take long to figure out the format we use to store it. As an extra precaution, a separate password would limit the scope of damage should it be divulged by a security breach and until we remove protocols that we know are in use around the university, we cannot change the storage format.

Wrapping up

I hope that this post gives a sense of some of the difficulty we face with creating a secure authentication mechanism for eduroam. Later blog posts will delve deeper into our relationship with FreeRADIUS, the RADIUS server software we use. In particular, logging accounting packets to a database will be covered next.

Posted in eduroam, Linux | Tagged , , , | 3 Comments

3 Responses to “Linux and eduroam: RADIUS”

  1. Mantas says:

    A bit more on the certificate issues. Many devices only allow selecting valid CAs, but not valid subject [alt] names. So even if your users mark AddTrust as acceptable, what prevents someone from getting a generic SSL certificate for random-domain.tld from AddTrust and using it in the rogue AP?

    (When setting up eduroam here at $WORK, I decided to use an internal CA for this reason. Really wish that clients could check certificates against the authentication @realm, though.)

    • Christopher Causer says:

      That’s a really good point, and yes this is something that I wish for as well. The janet documentation lists this as a risk of using a public CA, section 5 of the following URL.

      https://community.ja.net/library/janet-services-documentation/faqs-eduroam-system-administrators-and-implementation-techs

      I would say a few things first though:

      1) This is still (in my opinion) better than the standard web browser behaviour of accepting any signed certificate, regardless of who signed it.

      2) Since there are many different CAs in use around the world for eduroam, this MITM attack can only happen in the short term. Eventually someone will spot that the certificate is wrong for the client’s particular realm and raise the alarm (unless someone went to the trouble of obtaining multiple certificates).

      3) In theory because the certificate is still signed, the CA will have the necessary details of whose key it is and who requested it to be signed.

      However, don’t let these three points make anyone think I’m defending the behaviour. I still agree with you!

      • Chris Hills says:

        There is a browser extension called Perspectives that compares the fingerprint of SSL certificates on websites to look for potential MITM attacks (i.e. the lack of fingerprint consensus could indicate that a client is being attacked). It would be interesting to replicate this system on managed eduroam clients.

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