Extended Protection for Authnetication (EPA) was introduced in Windows 7/WS2008R2 to thwart reflection attacks. This blog describes the changes in the implementation of NTLM Authentication that are needed to successfully authenticate to servers that have EPA enabled. Windows 7/WS 2008R2 and Windows 8/ WS2012 have EPA enabled out of the box.

 You can read the details about EPA here http://technet.microsoft.com/en-us/security/advisory/973811

The concept in EPA is that authentication packets should be bound to the secure channel on which they are transmitted. This concept is not new and is known as channel binding (RFC 5056). RFC 5929 describes channel bindings for TLS that Winodws uses to bind the secure channel to authentication. Please note that EPA also uses Service Pricipal Name (SPN) but it is not used for TLS and we will not discuss it here.

Let's take an example of HTTPS, a protocol that uses TLS.  Once a secure channel is established and cipher change has happened, HTTP traffic starts flowing. In this example, we are only considering services that require authentication. NTLM or Kerberos will be used if you are using Windows authentication. You are most likely to use NTLM since the whole point of using HTTP and TLS is to allow clients to connect over internet (In Windows 8, Kerberos can be used on the internet but we will concentrate on NTLM here). In case of Windows client, these are the steps that are taken to incorporate channel binding in the authentication process after secure channel has been established:

1. The hashing algorithm for the signature in the certificate is identified, if present.

2. SSPI calculates a hash (almost always SHA256 hash, see below for details/exceptions) of the certificate, appends other data relevent to the type of channel bindings and returns it to the application.

3. The application, at the receipt of channel bindings, calls Initialize security context (ISC) and passes channel bindings as a parameter to the method call.

4. SSPI calculates the MD5 hash of channel bindings and uses it in  the calculation of NTLM version 2 response.

5. When server recives authenticate message, it queries SSPI for channel bindings. SSPI does exactly the same thing as on the client side and returns the data to the service. The service includes it in the call to method Accept Security Context (ASC)

6. In the process of verifying authenticate message, SSPI also takes into account the channel bindings. It calculates the MD5 hash of the channel bindings that were provided by the application (service) and compares it to the one sent by the client. If they match and rest of the authentication requirements are met, authentication is successful.

I'll now elaborate on each of the step listed above with a concrete example of RPC-over-HTTP traffic. The TLS network traffic is encrypted and I used Network Monitor expert Network Monitor Decryption Expert (NmDecrypt) to decrypt it. The decrypted network trace is attached to this blog.

If you open the network trace in Network Monitor, you'll see that in frame 16 server sends a certificate to client, as below (copied and pasted from the trace):

00000000`095c13e0  30 82 02 09 30 82 01 76-a0 03 02 01 02 02 10 cb  0...0..v........

00000000`095c13f0  69 79 cd 51 75 c5 b7 4b-67 30 83 6c 78 44 27 30  iy.Qu..Kg0.lxD'0

00000000`095c1400  09 06 05 2b 0e 03 02 1d-05 00 30 16 31 14 30 12  ...+......0.1.0.

00000000`095c1410  06 03 55 04 03 13 0b 44-43 2d 57 53 32 30 30 38  ..U....DC-WS2008

00000000`095c1420  52 32 30 1e 17 0d 31 32-31 31 31 37 30 30 35 39  R20...1211170059

00000000`095c1430  32 31 5a 17 0d 33 39 31-32 33 31 32 33 35 39 35  21Z..39123123595

00000000`095c1440  39 5a 30 16 31 14 30 12-06 03 55 04 03 13 0b 44  9Z0.1.0...U....D

00000000`095c1450  43 2d 57 53 32 30 30 38-52 32 30 81 9f 30 0d 06  C-WS2008R20..0..

00000000`095c1460  09 2a 86 48 86 f7 0d 01-01 01 05 00 03 81 8d 00  .*.H............

00000000`095c1470  30 81 89 02 81 81 00 9b-00 f8 1a 2d 37 c6 8d a1  0..........-7...

00000000`095c1480  39 91 46 f3 6a 1b f9 60-6c b3 6c a0 ac ed 85 e0  9.F.j..`l.l.....

00000000`095c1490  3f dc 92 86 36 bd 64 bf-36 51 db 57 3a 8a 82 6b  ?...6.d.6Q.W:..k

00000000`095c14a0  d8 94 17 7b d3 91 11 98-ef 19 06 52 30 03 73 67  ...{.......R0.sg

00000000`095c14b0  c8 ed 8e fa 0b 3d 4c c9-10 63 9f cf b4 cf 39 d8  .....=L..c....9.

00000000`095c14c0  fe 99 eb 5b 11 f2 fc fa-86 24 d9 ff d9 19 f5 69  ...[.....$.....i

00000000`095c14d0  b4 df 5a 5a c4 94 b4 b0-07 25 97 13 ad 7e 38 14  ..ZZ.....%...~8.

00000000`095c14e0  fb d6 33 65 6f e6 f7 48-4b 2d b3 51 2e 6d c7 ea  ..3eo..HK-.Q.m..

00000000`095c14f0  11 76 9a 2b f0 00 4d 02-03 01 00 01 a3 60 30 5e  .v.+..M......`0^

00000000`095c1500  30 13 06 03 55 1d 25 04-0c 30 0a 06 08 2b 06 01  0...U.%..0...+..

00000000`095c1510  05 05 07 03 01 30 47 06-03 55 1d 01 04 40 30 3e  .....0G..U...@0>

00000000`095c1520  80 10 eb 65 26 03 95 4b-d6 c0 54 75 78 7c b6 2a  ...e&..K..Tux|.*

00000000`095c1530  a1 bb a1 18 30 16 31 14-30 12 06 03 55 04 03 13  ....0.1.0...U...

00000000`095c1540  0b 44 43 2d 57 53 32 30-30 38 52 32 82 10 cb 69  .DC-WS2008R2...i

00000000`095c1550  79 cd 51 75 c5 b7 4b 67-30 83 6c 78 44 27 30 09  y.Qu..Kg0.lxD'0.

00000000`095c1560  06 05 2b 0e 03 02 1d 05-00 03 81 81 00 7b fa fe  ..+..........{..

00000000`095c1570  ee 74 05 ac bb 79 e9 da-ca 00 44 96 94 71 92 b1  .t...y....D..q..

00000000`095c1580  db c9 9b 71 29 c0 e4 28-5e 6a 50 99 cd a8 17 e4  ...q)..(^jP.....

00000000`095c1590  56 b9 ef 7f 02 7d 96 a3-48 14 72 75 2f b0 b5 87  V....}..H.ru/...

00000000`095c15a0  ee 55 e9 6a 6d 28 3c c1-fd 00 e4 76 e3 80 88 78  .U.jm(<....v...x

00000000`095c15b0  26 0d 6c 8c b8 64 61 63-b7 13 3a ab c7 dd 1d 0a  &.l..dac..:.....

00000000`095c15c0  d7 15 45 a1 d6 d9 34 c7-21 48 fb 43 87 38 da 1f  ..E...4.!H.C.8..

00000000`095c15d0  50 47 b1 a5 5c 47 ed 04-44 97 d3 ac 74 2d eb 09  PG..\G..D...t-..

00000000`095c15e0  77 59 bf a3 54 5b de 42-d5 23 5a 71 9f           wY..T[.B.#Zq..

 

After secure channel is established and cipher change has taken place, HTTP traffic starts flowing.

In this example, HTTP is being used as a transport for RPC and RPC server requires authentication. For authentication, the client application first calculates the channel binding by using the following process(in Windows this is done by SSPI but that is not important in this discussion ). This process is based on RFC 5929.

1. The channel binding type for this example is "tls-server-end-point" since a certificate is used in handshake (RFC5929).

2. The client calculates a hash of the certificate. The hashing algorithm is SHA-256, unless all of the following conditions are met, in which case the signature algorithm in the certificate will be used.

  • A certificate signature algorithm exist
  • The algorithm is only implemented in CNG (ALG_ID is CALG_OID_INFO_CNG_ONLY)
  • The algorithm has a corresponding CNG algorithm identifier string (pwszCNGAlgid)
  • The algorithm is not SHA1
  • The algorithm is not MD5

3. The SHA-256 hash of the above certificate is: ea 05 fe fe cc 6b 0b d5 71 db bc 5b aa 3e d4 53 86 d0 44 68 35 f7 b7 4c 85 62 1b 99 83 47 5f 95

4. The Channel binding unique prefix (RFC5929) "tls-server-end-point" is prefixed to the hash above (with a colon), resulting in  

74 6c 73 2d 73 65 72 76 65 72 2d 65 6e 64 2d 70 6f 69 6e 74 3a ea 05 fe fe cc 6b 0b d5 71 db bc 5b aa 3e d4 53 86 d0 44 68 35 f7 b7 4c 85 62 1b 99 83 47 5f 95

5. The above value is inserted as the value of application_data field of gss_channel_bindings_struct structure, as pointed out by MS-NLMP section 2.2.2.1

6. Windows always sets the other fields of gss_channel_bindings_struct as zeros (SEC_CHANNEL_BINDINGS
structure
). The resulting gss_channel_bindings_struct is as follows (little endian format):

00 00 00 00 //initiator_addtype
00 00 00 00 //initiator_address length
00 00 00 00 //acceptor_addrtype
00 00 00 00 //acceptor_address length
35 00 00 00 //application_data length (53 bytes)
74 6c 73 2d //application data, as calculated above
73 65 72 76
65 72 2d 65
6e 64 2d 70 

6f 69 6e 74
3a ea 05 fe
fe cc 6b 0b
d5 71 db bc 

5b aa 3e d4
53 86 d0 44
68 35 f7 b7
4c 85 62 1b 

99 83 47 5f
95

 

After calculating channel binding, the client application starts authentication and include channel binding as part of authentication. In case of NTLM, the gss_channel_bindings_struct  is called ClientChannelBindingUnhashed (MS-NLMP section 3.1.1.2). As explained in MS-NLMP section 3.1.5.1.2, the client adds an AV_PAIR structure and set the AvId field to MsvAvChannelBindings and the Value field to MD5(ClientChannelBindingsUnhashed). The MD5 hash of the above gss_channel_bindings_struct  turns out to be:

65 86 E9 9D 81 C2 FC 98 4E 47 17 2F D4 DD 03 10

This value is part of the AUTHENTICATE_MESSAGE in frame 27 in the network trace attached (in the network trace it is shown in Base64 encoding as 45 41 42 6C 68 75 6D 64 67 63 4C 38 6D 45 35 48 46 79 2F 55 33 51 4D 51 with AvLen) .

When server receives the AUTHENTICATE_MESSAGE, in addition to the regular authentication processing, it also verifies the channel binding hash by calculating it the same way the client did. If the channel binding hash does not match, the authentication will not be successful. The subsequent behavior is server dependent. In this example (IIS), the server will stop communication on unsuccessful authentication.

Please note that two step hashing is being employed here. First the application creates a hash of the certificate which becomes a part of gss_channel_bindings_struct structure. This structure is MD5 hashed again to be included in AUTHENTICATE_MESSAGE.

There are configurations on both Windows client and server side to disable the EPA. For the server side, please consult the server specific documentation. As for the server in this example, IIS, please consult  http://www.iis.net/configreference/system.webserver/security/authentication/windowsauthentication/extendedprotection.

On the client side, there is a registry setting that is described in KB976918 (http://support.microsoft.com/kb/976918) that can be used to configure EPA.