Re: Re: CVE-2014-6271: remote code execution through bash (3rd vulnerability)

[Kurt Seifried <kseifried () redhat com>]

On 26/09/14 11:55 AM, Rich Felker wrote:
> On Fri, Sep 26, 2014 at 01:41:45PM +0100, Mark R Bannister wrote:
> > Arguments that people shouldn't have setuid shell scripts don't
> > stack up, because even if you write your setuid program in some
> > other language, you might unwittingly exec something written in
> > bash.
> > As /bin/sh is symlinked to /bin/bash in RHEL, the moment you call
> > out to the system to do a piece of work for you, you're at risk of
> > invoking bash and thereby being vulnerable to a root exploit. For
> > example:
> >
> > $ env bzip2='() { echo vulnerable >&2; }' /usr/bin/bzdiff /tmp/file1.bz /tmp/file2.bz
> >
> > $ env test='() { echo vulnerable >&2; }' /usr/bin/ldd /usr/bin/gcc
> >
> > So this is not about whether or not someone has written a setuid
> > shell script. This has uncovered a potential new exploit for any
> > setuid program. Indeed the very first setuid program that I
> > discovered this exploit with was a binary (compiled C program) that
> > happened to exec ldd while it was running.
> >
> > I don't think this issue can be swept under the carpet.
>
> Any setuid program that's execing an external program that was not
> also designed to be run setuid, without scrubbing the environment and
> other environmental state (rlimits, inherited file descriptors, ...),
> or else fully dropping privileges to the original invoking user, is a
> gaping security hole already. This has nothing to do with bash.
>
> Rich

This is a classic case of "yes the correct thing to do is..." but the
reality is "we should fix this centrally rather than try to make
everyone do the right thing (aka boiling the ocean)". This is like tmp
vulns, it's 2014, the solution for tmp vulns is polyinstantiated /tmp
per user, and per application /tmp dirs in addition to this. Solve it
once centrally (e.g. in PAM/systemd) and boom, done.

We should always try to do the best/safest thing because most devs are
going to try to do the most insanely dangerous thing.

-- 
Kurt Seifried -- Red Hat -- Product Security -- Cloud
PGP A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993

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