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Date: Sat, 27 Sep 2014 00:42:22 +0200
From: Ángel González <angel@...its.net>
To: oss-security@...ts.openwall.com
Subject: Re: Re: Non-upstream patches for bash
John Haxby wrote
> On 26/09/14 01:23, Ángel González wrote:
> > Forwarding to the oss-security thread the patch I sent to bug-bash
> > 1 hour ago.
> >
> > The trick here is to delay parsing of functions coming from the
> > environment until they are actually needed.
> >
> > Thus extra code (CVE-2014-6271) or even a parsing vulnerability like
> > CVE-2014-7169 won't be triggered unless you attempt to run the exported
> > function (or you use a builtin such as declare or type that must print
> > the code, things like type -t are safe to use).
>
> Even with this?
>
> type='() { echo hi there; }' bash
>
> (Or the added stuff from Florian's patch).
This patch doesn't remove the exportable functions feature. Thus, it is
a priori unsafe to run any command using an identifier which can be set
in the environment by an attacker*
I mentioned that `type -p' was safe meaning "not triggering the parsing
of the name passed as argument", as I expected that a careful script
might be running type -p on untrusted variables before doing something
with them. And indeed it is safe… if they use the real 'type' builtin.
> I got myself into a right old mess by redefining declare, typeset, unset
> and command.
Given that builtins have precedence over builtins, you can get trapped
very hard. You can't use builtin, unset, set, declare, eval…
IMHO, the best a script can do to protect itself (and only after patch
25 restricts the names of exported functions) is:
> if [ "$1" != "--environment-cleaned" ]; then
> /usr/bin/env -i "$0" --environment-cleaned "$@"
> else
> shift
> <do things>
> fi
Assuming that env(1) is on /usr/bin/env (it's /bin/env on some unix),
argv[0] is the real script name, and it has +x (instead of needing to be
passed as an argument to the shell).
Best regards
* Or conversely, it is unsafe to let an attacker set an environment
variable matching a command name later used without sanitization.
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