privacy problem: text/html parts pull in network resources

[Austin Clements]

I have a fix for this on shr buried deep in an old patch series that I
never got back to: id:1398105468-14317-12-git-send-email-amdragon at mit.edu

For shr, the key is to set shr-blocked-images to ".".  However, IIRC,
in the current notmuch message rendering pipeline, mm overrides this
variable with something computed from gnus-blocked-images.  That said,
I'm not sure why gnus-blocked-images isn't *already* taking care of
this, but that's probably the place to start digging.

Quoth Daniel Kahn Gillmor on Jan 21 at  4:00 pm:
> If i send a message with a text/html part (either it's only text/html,
> or all parts are rendered, or it's multipart/alternative with only a
> text/html subpart) and that HTML has <img
> src="http://example.org/test.png"/> in it, then notmuch will make a
> network request for that image.
> 
> This is a privacy disaster, because it enables an e-mail sender to use
> "web bugs" to tell when a given notmuch user has opened their e-mail.
> 
> It's also a bit of a consistency/storage/indexing disaster because it
> means that what you see when you open a given message will change
> depending on the network environment you're in when you open it.
> 
> It's also potentially a security problem because it means that anyone in
> control of the remote server (or the network between you and the remote
> server if the image isn't sourced over https) can feed arbitrary data
> into whatever emacs image rendering library is being used.  (granted,
> this is not a unique problem because this can already be done by the
> original message sender with a multipart/mixed message, but it's an
> additional exposure of attack surface)
> 
> I just raised this on #notmuch, and i don't have the time or the
> knowledge to look into it now, but i think the defaults here need to be
> to avoid network access entirely unless the user explicitly requests it.
> 
>    --dkg



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