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My university provides a VPN service to gain access to internal services to all members. They are apparently using an L2TP/IPsec solution by Cisco. I'm able to connect just fine, but I'm a bit worrying about the security:

In the configuration manual for various operating systems, they mention the "IPsec shared secret" in plaintext. These manuals, and this "secret", are publicly available on the internet. What significance does that secret have? Shouldn't it actually be secret?

Does that pose a security risk to the encryption of the VPN? Or is there an additional layer of security? I also have to provide my user credentials to connect (my user name and a password that is different from the shared secret).

So my question is: Would a (possibly active) attacker, for example the operator of an untrusted external network I'm visiting, be able to do a MITM attack on my VPN tunnel, and decrypt my traffic?

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2 Answers 2

L2PT protocol offers fabulous online security plus IPsec. It uses two means authentication procedure requiring computer-level authentication wherever digital certificates and alternative relevant info for initiating the IPSec session. Then, user-level authentication is additionally required requiring surgical procedure protocol for L2TP VPN tunnel.

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No, IPSEC doesn't require certificates. So the poster would have to 1) check that certificates are used 2) ensure his computer is configured to require the appropriate certificate. –  sourcejedi Oct 5 '12 at 11:19
    
IPSec is (usually) a dedicate box between users and the net that uses a shared secrete to encrypt/decrypt –  November Oct 5 '12 at 22:48

Good question. I'm not 100% sure about the answer, and of course it also depends on many aspects specific to the configuration of your L2TP/IPsec solution.

On the face of it, it does seem very likely that if the 'shared secret' is exposed in such a way, it would allow an attacker at the very least mount an active man-in-the-middle attack against you or your University. In such a case, the attacker should be able to observe / forward / modify any traffic passing through this tunnel. Depending on the type of traffic, this might include secure information such as your password. However, it might be that your personal password is protected via another layer(s) of security.

Depending on the IKE configuration, it might also be possible to mount a passive attack, whereas traffic is (either on-the-fly, or afterwards) analyzed and data is extracted from it without actually performing a MITM attack. This is obviously a more complex form of attack, and there are some IKE methods for perfect-forward-secrecy that might help mitigate it. However, I believe those methods won't be effective in case of a MITM attack.

That said, this might be an 'acceptable' risk from the University's perspective. In such an environment (typically involving wide physical campus area, with many network points which are almost impossible to control), it is usually assumed that gaining access to the network itself is easy/trivial. Therefore, the layers of protection for resouces are applied at a higher level, e.g. on the application stack, or via simple means such as SSL. It might be a legitimate decision therefore for the University to decide that the L2TP/IPSec connection is simply an extension to the (already open) network. As such, this connection itself is assumed to be insecure, but it should/does have additional layers of security to mitigate any such exposure.

In addition, in reference to publishing this 'shared secret': it might be technically difficult to keep a shared secret between many parties a secret for a long time, so it's best to not rely on its secrecy and assume it is already exposed. Or otherwise, technical or other problems prevent your University from applying a different (e.g. RSA keys) to protect such a network connection.

So whilst you have very good reasons for concern, you really need to see the bigger picture to know whether or not there's a real risk to yourself or your University. It's also good not to rely on an encrypted connection to provide all your security. I would recommend talking to the people in your University responsible for security and voicing this concern. You might have stumbled across a weakness and they should know about it. Otherwise, they might explain better why there is no reason for you to be concerned.

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