Re: Why rewrite QP-encoded messages when the receiver cannot take 8BITMIME?

Hi Sam,
hi Alessandro,

thanks for trying to tackle this.

On Sun, 30 Jun 2024 19:41:09 -0400
Sam Varshavchik <[email protected]> wrote:

> I looked at this again and I agree. I just uploaded build 20240630
> that adds a separate indicator just for raw 8 bit content, that feeds
> into COMCTLFILE_8BIT, that drives reencoding. I can set up a rig to
> test it, but it'll take some time, perhaps someone can take it for a
> test drive.

I have installed the 20240630 version on one server and I can confirm
that it resolves the issue for me for now. It no longer rewrites
QP-mails that it shouldn't rewrite.
Now, of course I don't know if this breaks anything else, if you have
ideas what I should test, feel free to reach out.


A bit more context/info about the issue:

* As Alessandro mentioned, I discovered this via the aboutmy.email
  service. I should say that this was merely how I discovered it, but
  this caused problems before. Notably, there is a mail security
  service called mimecast that is relatively widely used and does not
  have 8BITMIME in its SMTP features. So this is not just some email
  testing service that has this issue. I previously got reports about
  broken DKIM signatures that I could not pinpoint why they happened,
  and I believe that was the reason.

* One can actually see the raw mail at aboutmy.email under "Delivery"
  "Payload". To reproduce this issue, one can send a mail with a header:
    Content-Type: text/plain; charset=UTF-8
  If the rewrite happens, this gets rewritten to lowercase:
   Content-Type: text/plain; charset=utf-8

It's possible to workaround this by not signing Content-Type (although
the DKIM RFC recommends doing this), but there's also rewriting
happening with MIME messages, so not signing Content-Type is not really
a solution.


-- 
Hanno Böck - Independent security researcher
https://itsec.hboeck.de/


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