wireless PEAP with EAP-MSCHAP v2 authentication - certificate spoof possible?

Discussion in 'Wireless Internet' started by Todd H., Oct 23, 2006.

  1. Todd H.

    Todd H. Guest


    I have a Cisco/Microsoft Wireless security question that's pretty
    in-depth. I'm hoping there's someone out there who's been down this
    road. Specifically I'm curious how strong this setup is in deflecting
    a targeted evil-twin man-in-the-middle access point attack against our
    employees (i.e. bad guy in airport or starbucks setting up an access
    point trying to get an employee machine to associate to it and get the
    machine to volunteer the active directory username/password).

    Our networking vendor is spec'ing a wireless system for a large
    facility, and intends to use Cisco Aironet infrastructure, and
    leveraging PEAP authentication against our Microsoft Active directory
    suing username/passwords. All our client machines are Windows XP

    Their technical guy showed in a demo that Windows would be configured
    for WPA/TKIP using 802.1x authentication using PEAP, and EAP-MSCHAP v2
    as the authentication mechanism which'll go against our active
    directory infrastructure.

    Client systems (for our wireless workstations) will be set up to
    validate the access point's certificate which they intend to use an
    [name of a listed certificate authority] authority certificate for
    this. Perhaps this picture helps, where only one trusted root CA
    would be checked in our configuration:

    The benefit they say is that we won't have to install any client side
    certificates which will simplify management quite a bit but, here's
    where the big question of security comes...

    In this setup, would it be possible for an attacker to set up
    an evil twin access point if they take the time to purchase
    their own certificate from that same certificate authority?

    My understanding may be flawed, but I don't see that the client checks
    anything except that the access point has a matching BSSID and
    posesses a valid certificate from that certificate authority. I don't
    see where it does any checking to make sure that it's actually our
    company's individual certificate.

    Can anyone confirm or deny?

    I guess I'd hate for someone with a laptop sitting at an airport being
    able to coax one of our employees' machines into joining his network
    automatically if he just knew out BSSID and had a certificate from the
    same CA. It may be an acceptable level of risk to balance out the
    management headache or client side certificates, but I just want to be
    sure we know what the exposure is so we can do a proper risk

    Best Regards,
    Todd H., Oct 23, 2006
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  2. Hi Todd,

    When dealing with digital certificates it is all about established

    There is a chain linked to all certificates that leads back to the main
    certificate authority.

    If you "trust" that authority as having done their job then you
    assume they have verified the certificate owner because there are
    digital certificate links that lead back to the top authority.

    If you do not trust them then you can get a certificate from another
    authority, perhaps even setup your own root authority that is 802.1x

    Actually, even Microsoft provides this type of solution.

    With 802.1x you cannot only authenticate the access point, but you can
    authenticate the end points and users as well.

    In addition, the 802.1x leads into network access control (NAC)
    solutions and this is the true direction mature organizations should be

    Courtesy of CompuCom Systems Network Security Expert, Andrew R. Reese:



    Hope this helps.

    Brad Reese
    BradReese.Com - Cisco Repair
    1293 Hendersonville Road, Suite 17
    Asheville, North Carolina USA 28803
    USA & Canada: 877-549-2680
    International: 828-277-7272
    Fax: 775-254-3558
    AIM: R2MGrant
    BradReese.Com - Cisco Power Supply Headquarters
    www.BradReese.Com, Oct 24, 2006
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  3. Todd H.

    Todd H. Guest

    Hi Brad, thanks for your response. Let's assume we trust the
    certificate authority in question to verify that someone is who they
    say they are.

    I guess my question boils down to this: Does PEAP (using only server
    side certificates) really give a client any assurance that they're
    connecting to their company's access point, or does it only guarantee
    that they're connecting to an access point where the owner has
    purchased a certificate from a given CA?

    As a recap, we're talking about the sitation where the client is
    configured like this, with exactly one of the Microsoft listed Trusted
    Root CA's checked in this dialog box:

    I'm thinking a bad guy, knowing only what CA a Big Company uses, could
    cheerfully purchase a certificate in his own name or his own company's
    name, the CA would do their job and verify who he is and all, and then
    head down to the airport with his laptop and see what laptops from Big
    Company he could get to autojoin his access point because I suspect
    that this method isn't checking the content of the certificate other
    than for validity and for a trusted CA.

    Or am I all wet?
    Todd H., Oct 24, 2006
  4. Hi Todd,

    Recommend that you check out Verisign's FAQ section and review what
    kinds of digital certificates are available for purchase, and then
    review what the requirements are to get these digital certificates

    As stated earlier, it is about trusting the certificate authorities.

    If you do not trust the root authorities to properly check the
    credentials of someone before handing out a digital certificate in some
    companies name, then setup your own root server and architect it from

    Am sure the root authorities post their policies and procedures for
    ensuring the integrity of their root servers.

    The client has the responsibility for accepting the validity of what
    ever certificates are presented to them, they need to verify the chain
    of trust back to the root server and they need to make sure the digital
    certificate they have for the root server is valid.


    Brad Reese
    BradReese.Com - Cisco Network Engineer Directory
    www.BradReese.Com, Oct 24, 2006
  5. Todd H.

    Todd H. Guest

    That's true, but not applicable to the scenario I'm posing.

    I'm trying to explore suppose a completely legitimate, non-forged
    certificate is purchased in the attacker's name, and is associated the
    the rogue access point. The real and rogue AP's will have different
    certificates, but both certs are from teh same CA. Will PEAP
    EAP/MSCHAP v2 as implemented in Windows XP sp2's built in PEAP
    supplicant ever tell the user about the certitificate or not, or will
    it quietly and happily connect to the rogue access point since it has
    a cert from the trusted CA?

    For the purposes of this, assume the "Connect to these servers" field of
    this dialog is blank, and one CA of the trust list is selected to trust:

    Right. I couldn't agree more.

    But, my question is "Will the microsoft PEAP supplicant even ask the
    user to okay the certificate that is presented, or will it quietly
    accept it because it came from the trusted CA?"

    Thanks again for your input!

    Best Regards,
    Todd H., Oct 25, 2006
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