Re: Buffer Over-runs, was Open source firewalls

[Arjan van de Ven]

On Fri, 2005-07-15 at 12:11 +0530, RVK wrote:

> Even in the presence of non-executable stack, Linux Torvalds explains 
> that "It's really easy. You do something like this: 1) overflow the 
> buffer on the stack, so that the return value is overwritten by a 
> pointer to the system() library function. 2) the next four bytes are 
> crap (a "return pointer" for the system call, which you don't care 
> about) 3) the next four bytes are a pointer to some random place in the 
> shared library again that contains the string "/bin/sh" (and yes, just 
> do a strings on the thing and you'll find it). Voila. You didn't have to 
> write any code, the only thing you needed to know was where the library 
> is loaded by default. And yes, it's library-specific, but hey, you just 
> select one specific commonly used version to crash. Suddenly you have a 
> root shell on the system. So it's not only doable, it's fairly trivial 
> to do. In short, anybody who thinks that the non-executable stack gives 
> them any real security is very very much living in a dream world. It may 
> catch a few attacks for old binaries that have security problems, but 
> the basic problem is that the binaries allow you to overwrite their 
> stacks. And if they allow that, then they allow the above exploit. It 
> probably takes all of five lines of changes to some existing exploit, 
> and some random program to find out where in the address space the 
> shared libraries tend to be loaded."

except this is no longer true really ;)

randomisation for example makes this a lot harder to do. 
gcc level tricks to prevent buffer overflows are widely in use nowadays
too (FORTIFY_SOURCE and -fstack-protector). The combination of this all
makes it a LOT harder to actually exploit a buffer overflow on, say, a
distribution like Fedora Core 4.


> 
> 
> rvk
> 
> >but your system is not compromised.
> >
> >One final point, in practice, you get lots of unwanted packets
> >off the internet, and in general you do not want them on your
> >internal net, both for performance and security reasons, if you
> >drop them on your router or firewall then you dont need to
> >worry if the remote app is mal-ware.
> >
> >--
> >mit freundlichen GrÃÃen, Brian.
> >
> >.
> >
> >  
> >
> 
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