[Bug 1793594] Re: IAKERB-HEADER "Realm" field incorrectly encoded as OCTET STRING

[ Christian Ehrhardt ]

Hi Wim,
this seems like a real quality bug report to me - thank you for that.
I checked other versions up to the latest but eventually found that even in latest upstream the same code exists [1].

To fully understand that and also to make the change that eventually helps all downstreams and not only Ubuntu it would be best to file an issue there.
In any case that should get the subject matter experts thoughts on it, and we can follow depending on what comes up in this upstream discussion.

Please can you verify this by building directly from the latest upstream
source? If this can be confirmed as an upstream bug, the best route to
getting it fixed in Ubuntu in this case would be to file an upstream bug
if you're able to do that. Otherwise, I'm not sure what we can do
directly in Ubuntu to fix the problem. You could IMHO as well try the
same on Ubuntu Bionic or Cosmis whihc has version 1.16 matching the
latest upstream releases (well not the 1.6.1 release, but 1.6 at least).

I'd highly appreciate if you file a bug upstream to mention it here to
we can link up the bug tracking with it.

[1]:
https://github.com/krb5/krb5/blob/master/src/lib/krb5/asn.1/asn1_k_encode.c#L1128

** Tags added: needs-upstream-report

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Title:
  IAKERB-HEADER "Realm" field incorrectly encoded as OCTET STRING

Status in krb5 package in Ubuntu:
  New

Bug description:
  Background:

  Under some circumstances, when the client/initiator has a TGT but no
  ticket for a particular principal, it needs to communicate with the
  KDC. The GSSAPI protocol includes a mechanism, a subprotocol named
  IAKERB, for the client to tunnel/proxy through the server/acceptor
  instead of directly communicating with the KDC. (This is useful if
  e.g. the GSSAPI initiator does not have full network access but the
  acceptor does.)

  Problem:

  The formatting of the IAKERB messages is incorrect. Every draft of the
  IAKERB protocol I have been able to find defines the IAKERB-HEADER
  structure to have a field "Realm" which is a UTF8String, like this:

             IAKERB-HEADER ::= SEQUENCE {
                 target-realm      [1] UTF8String,

  However, observed protocol exchanges tag the Realm field as an OCTET
  STRING.

  I believe the bug is in src/lib/krb5/asn.1/asn1_k_encode.c near line
  1146, where the DEFFIELD(iakerb_header_1,...) macro is invoked with
  "ostring_data". I think it should be invoked with "utf8_data" instead.

  Reproduction:

  I observed this using Firefox attempting to authenticate with a webserver using the "Negotiate" protocol. The first Negotiate message from the browser to the server contains:
    GSSAPI token (RFC2743 3.3); mechanism 1.3.6.1.5.5.2 (SPNEGO)
      innerToken is a NegTokenInit (RFC4178 4.2.1)
        mech = 1.3.6.1.5.2.5 (IAKERB)
        mechToken is a (wrapped) GSSAPI token (RFC2743 again) with mech = 1.3.6.1.5.2.5
          innerToken is the concatenation of:
            TOK_ID 05 01 (IAKERB)
            IAKERB-HEADER
            a Kerberos TGS-REQ

  Dumping the IAKERB-HEADER with `openssl asnparse` produces:

      0:d=0  hl=2 l=  12 cons: SEQUENCE          
      2:d=1  hl=2 l=  10 cons:  cont [ 1 ]        
      4:d=2  hl=2 l=   8 prim:   OCTET STRING      :HHHH.ORG

  As you can see the realm (HHHH.ORG) is tagged as OCTET STRING, rather
  than being tagged as UTF8String.

  Versions:

  Description:	Ubuntu 16.04.5 LTS
  Release:	16.04

  libgssapi-krb5-2:
    Installed: 1.13.2+dfsg-5ubuntu2
    Candidate: 1.13.2+dfsg-5ubuntu2
    Version table:
   *** 1.13.2+dfsg-5ubuntu2 500
          500 http://us.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
          100 /var/lib/dpkg/status
       1.13.2+dfsg-5 500
          500 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages

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