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Re: Issues with exported functions

From: David A. Wheeler
Subject: Re: Issues with exported functions
Date: Fri, 26 Sep 2014 17:47:47 -0400 (EDT)

I appreciate the effort made in patch bash43-026, but this patch doesn't even 
BEGIN to solve the underlying shellshock problem.  This patch just continues 
the "whack-a-mole" job of fixing parsing errors that began with the first 
patch.  Bash's parser is certain have many many many other vulnerabilities; it 
was never designed to be security-relevant!

I strongly recommend *TWO* changes which have been discussed here and on 
1. Add a prefix "BASH_FUNC..." (and maybe suffix) as proposed by Florian 
Weimer, per: http://www.openwall.com/lists/oss-security/2014/09/25/13
This is technically backwards-incompatible, but that will rarely matter.  The 
specific environment variable mechanism was never documented in the bash man 
page, after all, and it works just fine if both sending & receiving bashes are 
patched.  I would suggest NOT including the suffix "()", since some old systems 
might have trouble with such unusual environment variable names.
This change completely eliminates vulnerabilities from CGI and similar 
processing, where attacker data is being passed through environment variables 
to a receiving system.  It also eliminates the punning that comes when 
functions and regular environment variables have the same name, which isn't 
really POSIX-compliant anyway.

2. Import environment variables *ONLY* when they are requested; do *NOT* import 
them by default.  Christos Zoulas has proposed this.  This *IS* a real 
backwards-incompatible change.  But most users do *NOT* use this functionality, 
and increasingly downstream systems are *already* switching to this mode.  
E.G., FreeBSD has already switched to this; function imports require 
--import-functions or enabling the IMPORTFUNCTIONS option.   E.G., see: 
This change eliminates the entire class of problems.  It's still good to do #1, 
even with #2, because if someone DOES perform an import, it reduces the 
probability of accidentally importing the wrong thing.  People are ALREADY 
making this change, whether upstream does or not.

John Haxby recently posted that "A friend of mine said this could be a 
vulnerability gift that keeps on giving." 
(http://seclists.org/oss-sec/2014/q3/748).  Bash will be a continuous rich 
source of system vulnerabilities until it STOPS automatically parsing normal 
environment variables; all other shells just pass them through!   I've turned 
off several websites I control because I have *no* confidence that the current 
official bash patches actually stop anyone, and I am deliberately *not* buying 
products online today for the same reason.  I suspect others have done the 
same.  I think it's important that bash change its semantics so that it 
"obviously has absolutely no problems of this kind".

Thanks for listening!

--- David A. Wheeler

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