[cap-talk] Mac exploit a confused deputy attack

Jim Dennis jimd at starshine.org
Sat Mar 18 12:54:07 EST 2006

On Sat, Mar 04, 2006 at 07:44:14PM -0800, David Mercer wrote:
> On 2/28/06, Chris Hibbert <chris at pancrit.org> wrote:
>> The end result is that an application (Safari, the browser) can decide
>> that a file is safe to open, since it's only data (jpg, etc.) but once
>> passed to the open() call, it turns out that a different application is
>> used to open it, for instance Terminal, which treats it as a shell
>> script.  In that case, Safari was confused, but the user can also be
>> confused by a file that appears to be a jpg or pdf, but is an arbitrary
>> executable when opened.
> This reminds me of a unix trojan in the late 80s.  It replaced a
> binary executable owned by a user with a shell script that was padded
> out to the length of the original that it replaced with whitespace.
> -David Mercer
 That's really not the same.  A simple trojan is in a different category
 than a system that hands "data" off to a high level handler/dispatcher
 which then "executes it."

 When I read this it struck me how it's almost exactly like MS Windows
 ... where things that look like sound files or images are handed off
 to some sort of uncontained, non-validating "open/execute" dispatcher
 which then, in a blindly misguided effort to be "user friendly," finds
 out what executable to which it whould be passed.

 (Ooops, that's not a sound, it's a PIF or a .SCR or a VB script ---
 we'll just do what the "user" meant because he or she is obviously too 
 stupid to be have made any informed decision of how this file was named).

 It's clear to me that these bugs are from an absolute lack of
 containment for browsers and MUAs in particular ... and for data exchange
 resources in general.  (The issues with PDF handling over CUPS and
 other printing systems are a recent UNIX/Linux case in point ... and
 filesharing protocols such as NFS, and SMB have have weaknesses that
 are every bit as bad.

 It is an issue of both ambient authority (anything I run is running "as
 me" regardless of where it came from and whether it should be
 trustworthy).  We have almost no way in current tools of managing 
 policies about executable trust.  The best I've ever been able to come
 up with for myself on a personal level is a sort of dual UID restricted
 shell environment ... where my mail reading, web browsing account is
 restricted and my home directory account has to be separately invoked
 to migrate data into the restricted space (for outgoing attachments) or
 therefrom (for incoming).  Even that's pretty weak ... so I need to
 have separate sub accounts which have access to different, more
 privileged, online resources (such as a different account for each
 bank, broker, and retail business establishment with which I intend to
 do online business).

 The browser and the MUA have become the ultimate confused deputies
 in our personal lives. (At a minimum they should pass data to a
 type validation dispatcher that really understands MIME types and
 refuses to "play" anything that isn't "sounds, videos, pictures" ...
 or other "media" (okay --- let's not even discuss what sorts of
 "media" could be attached other than audio-visuals).  
 I personally also think that MUAs and browsers should be contained 
 (in something at the very least like a chroot for the filesystem 
 and some sort of constrained windowing decoration for their displays).

 My browser should not be able to access my financial records
 (forgetting the issue of my online, browser accessed Paypal and bank
 accounts for the moment) ... and it sure as heck shouldn't be able to
 write things into my ~/bin nor open up a dialog that isn't be glaringly
 distinguished from any system generated prompts or windows (think
 screen savers or MS Windows "shares" access dialogs).

 For now I'll stay a Luddite and stick with a "crippled" text mode
 mail client (mutt) ... but even as much as a curses curmudgeon as
 I've been I've been seduced by the GUI side for browsing.  I still
 use links/lynx or w3m

Jim Dennis

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