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@suse.de> wrote:

Hi there,

> On 22 Apr 2021, at 03:52, Trevor Vaughan <tvaughan@onyxpoint.com> 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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