[erlang-questions] Erlang SSL and Invalid Certificate Chains

[Ben Murphy]

Hi Ingela,

So the real server I'm having trouble with is not public. But I have a
made up server that is configured in a similar fashion.

https://test.fonix.io/

I've just added an extra cert to the end of the chain. This validates
correctly in chrome and with curl on the command line.

https://test.fonix.io:444/

Where this is an extra cert added to the end of the chain and the
order of the chain is broken. This validates correctly in chrome and
with curl on the command line.

I think some browsers/ssl implementations may also accept chains where
the first certificate in chain is not the client certificate but nginx
won't let me test this scenario :)

I don't think I would be able to use verify_fun to safely ignore
arbitrary extra certificates while not accepting untrusted chains. I
believe I would need a whitelist of extra certificates to ignore which
is problematic because things will explode when the server makes
changes that they think are 'safe'.

On Mon, Aug 11, 2014 at 3:59 PM, Ingela Andin <ingela.andin@REDACTED> wrote:
> HI!
>
> 2014-08-11 16:16 GMT+02:00 Ben Murphy <benmmurphy@REDACTED>:
>
>> (I originally sent this to erlang-bugs but it doesn't look like it
>> made it there)
>>
>> We have a problem where an SSL server sends back a certificate chain
>> that is invalid according to the TLS 1.2 specification and erlang
>> rejects this chain with an unknown ca error. However, openssl and
>> browsers will accept this chain because they are less strict about
>> validation.
>>
>> The chain looks something like:
>>
>> 0. Server Cert issued by Intermediate Cert
>> 1. Intermediate Cert issued by Root Cert
>> 2. Root Cert issued by Root Cert
>> 3. Unrelated certificate
>> 4. Unrelated certificate
>
>
>
> What server is sending such a chain? Could you give me a concrete
> example,maybe of list if you do not want to expose
> it to the world.
>
>
>
>>
>>
>> Which is invalid according to: http://www.ietf.org/rfc/rfc5246.txt
>>
>>    certificate_list
>>       This is a sequence (chain) of certificates.  The sender's
>>       certificate MUST come first in the list.  Each following
>>       certificate MUST directly certify the one preceding it.  Because
>>       certificate validation requires that root keys be distributed
>>       independently, the self-signed certificate that specifies the root
>>       certificate authority MAY be omitted from the chain, under the
>>       assumption that the remote end must already possess it in order to
>>       validate it in any case
>>
>> Looking at the openssl code they start at the beginning of the chain
>> then recursively find the issuer in order to build up a chain. While
>> the erlang ssl code assumes the last certificate in the chain is the
>> root CA (ssl_certificate:trusted_cert_and_path).
>>
>
> It assumes that the last certificate is the ROOT or the certificate  just
> before
> the ROOT (as the ROOT cert may be left out as you said above) and if
> the ROOT is left out it will try to find the ROOT in its database. If the
> ROOT is part
> of the chain it will still be looked up to make sure we already have it.
>
>
>> Maybe this is more of a feature request than a bug. But I was
>> wondering if it would be possible for erlang to either accept these
>> dodgy chains, provide an option when connecting to accept these dodgy
>> chains or allow users to supply a function to modify the certificate
>> chain before validation takes place.
>
>
>
> You can use the verify_fun to accept any path validation errors you want,
> at your own risk as you are lowering the security requirements by doing so.
>
> [...]
>
>
> Regards Ingela Erlang/OTP team - Ericsson AB