PostUp/PreUp/PostDown/PreDown Dangerous?

[Antonio Quartulli]

Hi,

On 22/06/18 09:41, Jason A. Donenfeld wrote:
> Hey list,
> 
> wg(8) is the main WireGuard configuration tool. It takes a fairly
> strict set of inputs, and is supposed to perform acceptable input
> validation on them.
> 
> https://git.zx2c4.com/WireGuard/about/src/tools/man/wg.8
> 
> wg-quick(8), on the other and, is a dinky bash script, that is useful
> for making some common limited use cases a bit easier.
> 
> https://git.zx2c4.com/WireGuard/about/src/tools/man/wg-quick.8
> 
> wg-quick(8) has the very handy feature of allowing
> PostUp/PostDown/PreUp/PreDown directives, to execute some helpers,
> such as iptables or whatever else you want in a custom setup. These
> have proven very useful to folks. And because these allow arbitrary
> execution anyway, wg-quick(8) doesn't try very hard to do proper input
> validation either.
> 
> I just saw this nice post pointing out a problem in OpenVPN:
> https://medium.com/tenable-techblog/reverse-shell-from-an-openvpn-configuration-file-73fd8b1d38da
> 
> The same thing applies to wg-quick(8) with
> PostUp/PostDown/PreUp/PreDown. The question is how seriously we should
> take the problem presented by this blog post. Namely, you can't trust
> configuration files given to you by outside parties. Maybe you
> shouldn't reconfigure your network without inspecting what those
> reconfigurations are first. However, one could argue that code
> execution is a bit beyond networking config.
> 
> So, the question we need to ask is whether this problem is important
> enough that these useful features should be _removed_? Or if there's a
> way to make them safer? Or if it just doesn't matter that much and we
> shouldn't do anything.
> 
> Thoughts?

In case this might be useful: in OpenVPN there is an additional
parameter called "--script-security" that requires to be set to a
certain level before allowing configured scripts to be executed.

Unfortunately there is no real protection against the clueless user, who
can and will blindly enable that setting if asked by a $random VPN provider.

However, I still believe (and hope) that forcing the user to enable a
specific knob may raise the level of attention.

Maybe something similar could be added as a command line parameter to
wg/wg-quick so that it will execute the various
PostUp/PreUp/PostDown/PreDown only if allowed to?


Just as a side note: this is not a VPN specific problem, this is
something users can end up with everytime they execute some binary with
a configuration they have not inspected. So, be careful out there ;-)


Cheers,


-- 
Antonio Quartulli