Trusted vs Unstrusted MIME types

Rodney Dawes dobey.pwns at
Mon Jul 9 06:57:16 PDT 2007

On Mon, 2007-07-09 at 11:57 +0200, Patryk Zawadzki wrote:
> On 7/9/07, Rodney Dawes <dobey.pwns at> wrote:
> > One very important design heuristic that should be followed here is
> > "Always let the user feel in control."
> I'd rather word that differently. For me "user in control" means
> ignoring any messages and blindly clicking "next." Been there, seen
> that.

I didn't say that the user should BE in control. I said the user should
FEEL in control. There's a big difference there. If the user keeps
blindly clicking "next" then they aren't in control. They are dismissing
the annoyances that make them feel as they aren't in control.

> > If the user doesn't feel like
> > she can control what's happening, the software is going to be an
> > annoyance more than an assistance.  Inciting fear with a pop-up stating
> > that a file might contain malicious code, for only a small subset of
> > the possible files that might do so, doesn't actively make the situation
> > any better.
> It will certainly teach users not to read any popup messages instead
> clicking "allow" as they would be unable to have any work done.

Exactly my point. All that adding a pop-up dialog will do, is delay the
agony that will come along with the malicious data.

> > Why not have magic matches for known malicious data in files, instead of
> > just blanketing whole mime types?
> It's not possible inside web browsers and generally any software
> dealing with remote files (and these are the major threat). Sniffing
> contents might be either impossible (a large file that is either not
> fully downloaded yet or not meant to be downloaded at all in case the
> opening application wants to do it by itself) or very expensive
> (samba, webdav over Internet).

Anything is possible if you write the software correctly. We (at least
in GNOME) already sniff files as they are being downloaded. I would
rather download only part of a large file that has bad data, than waste
time for the whole thing, to only later find out it's sending out
e-mails to everyone in my address book. If we sniff partial downloads,
we can then pause the download and inform the user of malicious files,
before everything gets down the pipe, and they trash their system. If
we just place the choice of fear on the user always at the start of
download, we will either prevent them from getting the data they want,
because of fear, or cause them to ignore fear, and just trash their
system. Either way, the software is to blame.

> > Doing that would take care of even
> > files we think we might trust, like JPEGs, without being overly
> > intrusive in the UI, when not necessary.
> The JPEG case needs to be fixed in the application and not in the
> sniffer. Both would take the same amount of work.

All cases need to be fixed. But they can't be guaranteed to be. There
is no way to guarantee that all users have all updates at all times.
It's just not possible.

> > Because, really, by definition,
> > any file not explicitly created by the user, should be considered as
> > potentially unsafe. And even some files created by the user, should be
> > considered unsafe, because we don't know if the software that created
> > it is safe.
> That's what I said earlier in the thread. A file that does not
> originate from my machine is considered not safe. If I use Firefox to
> save it locally I no longer get any warning about its contents.
> I think to solve this properly we'd need a new filesystem with more
> extended attributes that follow the file (think of "marked as safe")
> :)

There really is no way to solve this properly. You can't know for 100%
whether something is safe or not, until you run it. The absolute best
we can do to check for safety, is to sandbox, sniff, and run. If a file
marked as safe, is written to by a malicious program believed to be
safe, then the file is no longer safe, even though it is marked as such.
And presumably to mark something as safe, is something a user must do.
At that point, you might as well just do nothing. Malicious users will
mark malicious data as safe, if the meta-data is transferable with the
file. You then still end up with the problem of having malicious data.

-- dobey

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