Bug#939808: exim4: Very problematic default DKIM_SIGN_HEADERS

[Marc Haber]

tags #939808 upstream
thanks

Hi,

it would probably be a good idea to discuss this with upstream. I don't
think that Debian should do a local patch here.

Greetings
Marc

On Mon, Sep 09, 2019 at 04:10:33AM +0200, Guillem Jover wrote:
> The default DKIM_SIGN_HEADERS macro contains many headers that make
> sending mails to mailing lists or (re)sending mails on someone's
> behalf pretty much infeasible. This has big impact on systems with
> strict DKIM and DMARC policies.
> 
> There are several of the listed fields that are intended to be set by the
> system resending a mail, be that a mailing list or a third-party. If these
> fields are listed in the default set it means any mail going through those
> systems will contain a signature for an empty header, which will then be
> filled and fail signature validatation. Moreover the RFC4871 and RFC6376
> in their §5.4 section mentions that signing missing fields should be done
> carefully.
> 
> Mark Sender and all Resent-* and List-* fields to only be signed if
> present.
> 
> Add also duplicate entries for the From and Subject fields, to reject
> appended fields.
> 
> There's a related write up at
> <https://begriffs.com/posts/2018-09-18-dmarc-mailing-list.html>.
> 
> I'm attaching a patch that should fix this.
> 
> Thanks,
> Guillem

> Description: Fix default DKIM_SIGN_HEADERS macro.
>  There are several of the listed fields that are intended to be set by the
>  system resending a mail, be that a mailing list or a third-party. If these
>  fields are listed in the default set it means any mail going through those
>  systems will contain a signature for an empty header, which will then be
>  filled and fail signature validate. Moreover the RFC4871 and RFC6376 in
>  their §5.4 section mentions that signing missing fields should be done
>  carefully.
>  .
>  Add also duplicate entries for the From and Subject fields, to reject
>  appended fields.
> Author: Guillem Jover <guillem at hadrons.org>
> Last-Update: 2019-09-08
> 
> 
> ---
>  src/pdkim/pdkim.h |   16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> --- a/src/pdkim/pdkim.h
> +++ b/src/pdkim/pdkim.h
> @@ -26,14 +26,14 @@
>  #include "../blob.h"
>  #include "../hash.h"
>  
> -#define PDKIM_DEFAULT_SIGN_HEADERS "From:Sender:Reply-To:Subject:Date:"\
> -                             "Message-ID:To:Cc:MIME-Version:Content-Type:"\
> -                             "Content-Transfer-Encoding:Content-ID:"\
> -                             "Content-Description:Resent-Date:Resent-From:"\
> -                             "Resent-Sender:Resent-To:Resent-Cc:"\
> -                             "Resent-Message-ID:In-Reply-To:References:"\
> -                             "List-Id:List-Help:List-Unsubscribe:"\
> -                             "List-Subscribe:List-Post:List-Owner:List-Archive"
> +#define PDKIM_DEFAULT_SIGN_HEADERS \
> +  "From:From:=Sender:Reply-To:Subject:Subject:Date:To:Cc:"\
> +  "Message-ID:In-Reply-To:References:MIME-Version:"\
> +  "Content-Type:Content-Transfer-Encoding:Content-ID:Content-Description:"\
> +  "=Resent-Date:=Resent-From:=Resent-Sender:=Resent-To:=Resent-Cc:"\
> +  "=Resent-Message-ID:"\
> +  "=List-Id:=List-Help:=List-Unsubscribe:=List-Subscribe:=List-Post:"\
> +  "=List-Owner:=List-Archive"
>  
>  /* -------------------------------------------------------------------------- */
>  /* Length of the preallocated buffer for the "answer" from the dns/txt


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