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Message-ID: <54273E5D.4040602@case.edu>
Date: Sat, 27 Sep 2014 18:46:53 -0400
From: Chet Ramey <chet.ramey@...e.edu>
To: John Haxby <john.haxby@...cle.com>, oss-security@...ts.openwall.com
CC: chet.ramey@...e.edu, christos@...las.com
Subject: Re: Re: CVE-2014-6271: remote code execution through
bash (3rd vulnerability)
On 9/26/14, 8:47 AM, John Haxby wrote:
> On 26/09/14 12:33, Florian Weimer wrote:
>> On 09/26/2014 10:54 AM, Mark R Bannister wrote:
>>> Testing patch 25 and 26 from Chet, it looks to me like this is still
>>> an incomplete fix. The third vulnerability I'd like to report is the
>>> feature itself in bash that allows functions to be passed in the
>>> environment, e.g.
>>> $ env ls='() { echo vulnerable; }' bash -c ls
>>>
>>> This allows an attacker to replace a command used by a bash script
>>> with arbitrary code. It is then down to an attacker to find a
>>> suitable command that the bash script (or any child shells) might call
>>> without a path component.
>>>
>>> I can't see this being a problem for Apache custom headers (the
>>> variable name is turned to uppercase and prefixed by HTTP_), nor sudo
>>> commands if env_reset is on (the default), but this continues to be a
>>> major vulnerability for setuid/setgid scripts (S_ISUID or S_ISGID)
>>> where the environment is preserved.
>>
>> I agree this looks scary at first glance, but we discussed this
>> previously, see for example:
>>
>> <http://www.openwall.com/lists/oss-security/2014/09/24/20>
>>
>> Shell scripts derive part of their power and flexibility from their
>> openness to the execution environment. You can tweak PATH, BASH_ENV (or
>> ENV for other Bourne-like shells), IFS, HOME, and many other variables
>> to change behavior. There are even more knobs to affect the behavior of
>> the external commands almost all shell scripts call when they run.
>>
>> This makes them not suitable at all for writing SUID programs or other
>> code that runs in untrusted environments. This is well-documented, and
>> given the amount of shell scripts out there which rely on these aspects
>> of the UNIX shell design, it's not something we can change, particularly
>> not as part of a security update which system administrators are more or
>> less forced to install.
>>
>> In your specific example, you can achieve the same effect by setting
>> PATH to a directory with a customer ls program, or by setting BASH_ENV
>> to a file which contains a definition of a function called ls.
>>
>> Overriding external programs with shell functions in such a way has to
>> be supported. Otherwise, scripts which define shell functions would
>> break if the system administrator installs new software which happens to
>> include a program of the same name of the shell function.
>>
>
>
> It's not so much the known attacks -- redefining ls, unset, command,
> typeset, declare, etc -- it's the future parser bugs that we don't yet
> know about.
>
> A friend of mine said this could be a vulnerability gift that keeps on
> giving.
>
> CVE-2014-7169 was discovered very quickly after CVE-2014-6271. Do you
> think that's the end of it? (Just in case: I'm not getting at anyone
> here, certainly not Chet, Florian or anyone else who has been working
> overtime on these.)
>
> Importing functions from the environment is relatively unusual. I'd
> probably go so far as to say very unusual.
>
> Sufficiently unusual, I'd venture, that it should not be done
> implicitly. Florian's "BASH_FUNC_x()" makes it easier to blacklist
> these environment variables and ensures that a web server's HTTP_ prefix
> will not just create an oddly named function ... is that enough? Should
> bash simply make importing functions something that one has to ask for
> explicitly as Christos Zoulas (and others) suggested[1]?
I think function exports are used more widely than you think, and I am not
willing to break backwards compatibility that much by disabling function
exports by default.
Chet
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU chet@...e.edu http://cnswww.cns.cwru.edu/~chet/
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