Vulnerability Development: Re: Thwarting /bin/bash, an anti-overflow concept ?

[security <security () kalamiteit nl>]

Hi,
well actually most exploits are calling /bin/sh .. but anyways .. you 
cannot escape that by removing bash or anything like that .. you "GOTTA" 
have a /bin/sh as there are so many programs that need that .. what you 
can do is to add the shells and users that have access to them to a 
special group! and restrict access to those users!
cheers,
Amine


Alex Schütz wrote:

> Dear Vuln-Dev's,
>
> Recently I had a simple idea about preventing hack attacks. Most 
> buffer overflows are pretty happy calling /bin/bash as a final means 
> to get an unauthorized root shell.
> However, if we do not have any shell, what is going to happen ? 
> There's no /bin/bash to call, thus, the exploit will surely crash some 
> application, but its final goal will be thwarted.
> Ofcourse we could rename /bin/bash to /bin/whatever_we_want, and thus 
> add some security by obscurity, but the next exploit is going to cat 
> /etc/shells or /etc/passwd, and then the attacker knows the name of 
> the shell.
> Anyhow, if we delete all shells... how safe are we, then ? (Ignoring 
> the case that crontab might not work anymore...)
> Thinking this farther, we are going to force the exploit developer to 
> bring along his own binary code of /bin/bash. This may not be possible 
> in every case, since the buffer overflow cannot hold so much data.
> Or we could code some kernel module that restricts any permission to 
> call /bin/bash by only a few selected trusted programs, i.e. /bin/login .
> What do you think ? Please let me know.
>
> Yours, Alex