Kerberos Bug Introduced in d777e2918a77?

Daniel Jones daniel.jones at engineerbetter.com
Thu Apr 23 11:25:32 UTC 2015


Hi all,

Thanks to everyone taking the time to look into this.

Before I get into the detail of the technical issue, can anyone postulate
as to *how quickly fixes tend to make it into releases of OpenJDK*? Are we
talking days, weeks, or months? I'm just trying to advise my client on the
best mitigation strategy until the issue is resolved.

The Spring code in question actually changed this morning to throw a more
useful error:
https://github.com/spring-projects/spring-security-kerberos/commit/f046bd7c69d6dad74eb06a7651cd68060b31ff6f

On the other hand, your program has
>    context.acceptSecContext(kerberosTicket, 0, kerberosTicket.length);
>    String user = context.getSrcName().toString();
> which is not standard. The acceptSecContext call should be in a loop until
> a context is established, and then you can call getSrcName().


I'm not sure that this would work. I'm not at all familiar with what is
best practice in handling Kerberos tickets, but let me explain what happens
at present in u40:


   1. Spring's SunJaasKerberosTicketValidator gets a GSSContextImpl, and
   calls acceptSecContext on it.
   2. The GSSContextImpl calls an overloaded accetSecContext method, which
   successfully creates a SpNegoContext and assigns it as the mechCtxt member.
   3. GSSContextImpl#acceptSecContext then calls
   SpNegoContext#acceptSecContext
   4. In SpNegoContext#acceptSecContext we have the new functionality that
   only looks at the top item of the list of OIDs from the service ticket.
   5. The inner mechContext of the SpNegoContext is not set.
   6. We return back to the Spring code, with a GSSContextImpl wrapping a
   SpNegoContext
   7. Spring calls GSSContextImpl#getSrcName(), which delegates to
   SpNegoContext#getSrcName(), which returns null as its mechContext member is
   null.


Spring passes the whole ticket into GSSContextImpl, and doesn't know about
OIDs and the list of acceptable mechanisms. It seems like it's a
responsibility of either SpNegoContext or GSSContextImpl to know about this
list and iterate over it.

If the Spring code were to attempt a repeat, how should it know that the
inner context was not set? What action should it perform next? It's passed
in the ticket it knows about, and got back a populated byte[], without any
exceptions. What would it use to determine that one of the side affects of
GSSContextImpl#acceptSecContext hasn't succeeded?

Does the above make sense? Please do get in touch if I an provide any other
assistance in helping with the issue, and thanks again to everyone looking
into it.



On Thu, Apr 23, 2015 at 1:58 AM, Weijun Wang <weijun.wang at oracle.com> wrote:

> Hi Daniel
>
> I've read more about your bug report and know what's happening.
>
> You are proposing 2 OIDs, [1.2.840.48018.1.2.2, 1.2.840.113554.1.2.2], 1st
> one being Microsoft's own krb5 OID and 2nd the standard one. Java only
> understands the 2nd one but before that changeset it blindly accepts the
> mechToken without looking at the OID. Since it's also krb5, the mechToken
> is processed correctly and everything goes on. After the changeset, it does
> not recognize the OID anymore and asks the client to send another mechToken
> with 1.2.840.113554.1.2.2.
>
> I believe a lot of people is using the Microsoft OID. I will see if it's
> possible to recognize both OIDs.
>
> On the other hand, your program has
>
>    context.acceptSecContext(kerberosTicket, 0, kerberosTicket.length);
>    String user = context.getSrcName().toString();
>
> which is not standard. The acceptSecContext call should be in a loop until
> a context is established, and then you can call getSrcName(). Can you try
> that? Hopefully after the client sees the server request for a
> 1.2.840.113554.1.2.2 mechToken it can send one and the server can go on.
>
> Thanks
> Max
>
>
> On 4/23/2015 7:22 AM, Weijun Wang wrote:
>
>> Hi Daniel
>>
>> Thanks for the report.
>>
>> In fact, the bug behind the changeset you mentioned -- 8048194 -- was
>> just meant to make your case work. Before that, the server blindly
>> accepts the mechToken and process it no matter if the OID is supported.
>> Now it first looks at the OID and accept the token if it supports the
>> OID; otherwise, only the negotiated result (its supported OID) is sent
>> back, and waits for the client sending the correct mechToken in the next
>> round.
>>
>> It seems the logic above is not implemented correctly, can you show me
>> the full stack of your NullPointerException? If it includes any
>> sensitive info you can write me privately.
>>
>> Thanks
>> Max
>>
>> On 4/23/2015 12:21 AM, Rob McKenna wrote:
>>
>>> Hi Daniel,
>>>
>>> Thanks for the report, I'm cc'ing the security-dev alias.
>>>
>>>      -Rob
>>>
>>> On 22/04/15 13:10, Daniel Jones wrote:
>>>
>>>> Hi all,
>>>>
>>>> Apologies if this is the wrong mailing list - please direct me to the
>>>> correct one if so.
>>>>
>>>> I believe I've found a bug in OpenJDK 1.8.0_40, introduced in commit
>>>> d777e2918a77:
>>>>
>>>> http://hg.openjdk.java.net/jdk8u/jdk8u40/jdk/file/d777e2918a77/src/share/classes/sun/security/jgss/spnego/SpNegoContext.java
>>>>
>>>>
>>>>
>>>> The change introduced on line 548 means that an authentication
>>>> mechanism is
>>>> only accepted if the OID of the mechanism desired is the *first* in the
>>>> list of mechanisms specified as acceptable in the incoming ticket.
>>>>
>>>> In the case of my current client their service tickets are specifying 4
>>>> acceptable mechanism OIDs, but the only available mechanism's OID
>>>> appears
>>>> second on that list. So whilst the server *can *satisfy the ticket, the
>>>> code change on line 548 prevents this from happening.
>>>>
>>>> Using the same server code, the same Kerberos KDC, and OpenJDK 1.8.0_31,
>>>> everything works. Changing only the JDK results in the mechContext not
>>>> being properly populated, which in turn causes a NullPointerException
>>>> from
>>>> some Spring Security Kerberos code.
>>>>
>>>> Has anyone else experienced this?
>>>>
>>>>
>>>>
>>>


-- 
Regards,

Daniel Jones
EngineerBetter.com
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