Using PEAP and WinXP

Michael Griego mgriego at
Thu May 25 04:18:43 CEST 2006

What Michael said is correct.  By default, the Windows XP supplicant  
will verify the certificate against its list of known trusted root  
CAs.  Without specifying both a trusted CA and the certificate CN  
(usually a hostname), then an attacker could get a cert from another  
trusted CA or one from the same CA with a different CN.

Say for instance I have a network that uses a certificate from  
MyRootCA with CN using SSID example_ssid.  An  
attacker could set up their own AP somewhere in the vacinity of your  
users using the same ESSID (example_ssid) causing users to associate  
with their AP.  802.1x is designed to keep users from further  
connecting that network by verifying the authenticity of that  
network.  If the rogue AP uses a cert from OtherCA with a CN of and both MyRootCA and OtherCA are known roots  
already existing in the default Windows cert list, then a user not  
checking the cert signer against the trusted signer, all known  
signers are trusted meaning that OtherCA is trusted as a signer and  
the rogue AP now has your user's connection.  In addition, if you're  
checking the CA list but not the CN, then the attacker could obtain a  
cert from MyRootCA with the CN of  Since you're  
not checking the CN and MyRootCA is the trusted signer, then the cert  
is trusted and, again, the attacker has your user's connection.

So, for truly secure mutual authentication, you must specify both the  
trusted CA and the CN in the supplicant.


On May 24, 2006, at 3:34 PM, King, Michael wrote:

>> -----Original Message-----
>> From:
>> at lists.freeradius.or
>> g
>> [ at lists.freer
>>] On Behalf Of simon at
>> Sent: Wednesday, May 24, 2006 3:02 PM
>> To: freeradius-users at
>> Subject: Using PEAP and WinXP
>> Hi,
>> I have a question regarding the setup for the WinXP client
>> when using PEAP.  Does one always need to go into the
>> properties for the AP and configure which servers to connect
>> to or which root certification authorities are trusted?  What
>> I mean is, whether you produced a server certificate yourself
>> and imported that CA onto the client machine, or whether you
>> had a certificate signed by someone like Verisign, you would
>> need to check the corresponding CA within the list.
> It's my understanding that this is to prevent a man in the middle
> attack.  Someone could easily setup a rouge AP, with a RADIUS Server.
> Since your requiring the server to identify itself (Via the Cert) you
> could detect this, and prevent it.
> -
> List info/subscribe/unsubscribe? See 
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