Re: minssf and TLS cipher ordering

[Pierre Rogier <progier@xxxxxxxxxx>]

Hi Trevor,

I do not think it is possible to specify the cypher order negotiation:
     I am not sure whether TLS protocol allow to specify an order when negotiating the cypher,
     but at 389 level there is no way to specify an order: 

The NSS security layer provides the list of supported cypher and 389 use nsSSL3Ciphers config parameter to enable/disable theses cyphers in the list (without changing the order)     So my feeling is that if there is an order it is up to the different
     security layer implementations and may differs between the applications,

Regards,
   Pierre

On Thu, Apr 22, 2021 at 7:28 PM Trevor Vaughan <tvaughan@xxxxxxxxxxxxx> wrote:

Hi William,

In terms of the STARTTLS bits (in theory) properly configuring your client software mitigates the password leak risk. But this also happens with pure (non-RFC) LDAPS connections.

The docs note that minssf applies to the crypto required bits as well as the SASL layer.

Ignoring most of that, my issue is that I don't understand why I have to nail my client software to ciphers explicitly known by 389-DS instead of the two negotiating the strongest things possible out of the gate.

For instance, if I use AES256 with a minssf=256, everything works just fine.

But, if I use AES128:AES256:@STRENGTH (which should sort strongest to weakest) then access is denied.

How do I get 389-DS to negotiate the strongest ciphers first (regardless of the method)?

Thanks,

Trevor

On Wed, Apr 21, 2021 at 7:34 PM William Brown <wbrown@xxxxxxx> wrote:

Hi there,

> On 22 Apr 2021, at 03:52, Trevor Vaughan <tvaughan@xxxxxxxxxxxxx> wrote:
>
> Hi All,
>
> OS Version: CentOS 8
> 389-DS Version: 1.4.3.22 from EPEL
>
> I have a server set up with minssf=256 and have been surprised that either 389-DS, or openssl, does not appear to be doing what I would consider a logical TLS negotiation.
>
> I had thought that the system would start with the strongest cipher and then negotiate down to something that was acceptable.
>
> Instead, I'm finding that I have to nail up the ciphers to something that the 389-DS server both recognizes and is within the expected SSF.
>
> Is this expected behavior or do I have something configured incorrectly?

That's not what minssf does.

minssf says "during a bind operation, reject if the encryption strength used is less than 256 bits or equivalent".

The "bit strength" is arbitrary though, because it's a concept from sasl, and generally is very broken.

Remember, minssf does NOT do what you think though! Because bind is the *first* message on the wire, the series of operations is


   client                   server
open plain text conn  ->   
                      <-   accept connection
send bind on conn     ->   
                      <-   reject due to minsff too weak.


So you have already leaked the password!


The only way to ensure this does not occur is to set "nsslapd-port: 0" which disables plaintext. Then you *only* use ldaps on port 636, which is guarantee encrypted from the start.

It is worth noting that the use of starttls over ldap, does *NOT* mitigate this issue, for a similar reason.


Caveat: If you are using kerberos/gssapi you can NOT disable plaintext ldap due to these protocols attempting to install their own encryption layers.


Hope that helps,


>
> Thanks,
>
> Trevor
>
> --
> Trevor Vaughan
> Vice President, Onyx Point, Inc
> (410) 541-6699 x788
>
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—
Sincerely,

William Brown

Senior Software Engineer, 389 Directory Server
SUSE Labs, Australia
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--

Trevor Vaughan
Vice President, Onyx Point, Inc

(410) 541-6699 x788

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