Re: [nfsv4] Last call start for "RPCSEC_GSS Version 2"

On Fri, June 13, 2008 12:00 pm, Nicolas Williams wrote:
> On Mon, Jun 02, 2008 at 11:30:43PM -0500, Spencer Shepler wrote:
>> http://www.ietf.org/internet-drafts/draft-ietf-nfsv4-rpcsec-gss-v2-03.txt
>
> There's a rather minor, but easy to fix attack on RPCSEC_GSSv2 as
> described in -03.
>
> I started wondering what kind of games an attacker could play with the
> RPCSEC_GSS version number.  I think an MITM could modify RPCs from a v1
> client, talking to a v2 server, to set the RPCSEC_GSS version to 2, and
> in the process obtain a few MICs that it might, by sheer luck, cause the
> server to accept in the channel binding portion of the protocol.

I'm not seeing it. The MIC in the verifier of both v1 and v2
of RPCSEC_GSS is compute from the RPC header "up to and including
      the credential"

Since the credential contains the RPCSEC_GSS version number (field
rgc_version below) the RPCSEC_GSS target will reject the MITM's
modified RPCs.

      union rpc_gss_cred_t switch (unsigned int rgc_version) {
        case RPCSEC_GSS_VERS_1:
        case RPCSEC_GSS_VERS_2: /* new */
          rpc_gss_cred_vers_1_t rgc_cred_v1;
      };

-- 
Mike Eisler, Senior Technical Director, NetApp, 719 599 9026,
http://blogs.netapp.com/eislers_nfs_blog/

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