Re: Forwared to me

[medeiros () ENG USF EDU (Raymond Medeiros)]

I would have to only completely agree with you.  This fix which was
contained in the ISS security announcement was indeed very weak.  My
suggestion was to at the very least deny access to finger requests from
the outside.  This attack really isn't that bad however I have been able
to take out a machine on my own subnet using a simple perl script.  In
reality it doesn't appear to be more of a threat than a ping flood.  I
have also looked into using it as part of the beginning to a spoofing
attack (under controlled conditions of course) and it has no apparent
value.  Never the less it should be brought to everyones attention as it
is such a simple implementation and just one more reason to be suspicious
of the use of yp.

-------------------------------------------------------------------------------
Raymond R Medeiros II                   email: medeiros () eng usf edu
Junior Systems Administrator            www: http://www.eng.usf.edu/~medeiros
Engineering Computing
University of South Florida

On Fri, 10 Jul 1998, Solar Designer wrote:

> Hello,
>
> > # mv /usr/bin/finger /usr/bin/finger.exe
> > # cat > /usr/bin/finger
> > #!/bin/sh
> > exec /usr/bin/finger.exe -m $*
> > ^D
> > # chmod +x /usr/bin/finger
>
> Hmm, weird, this doesn't look safe to me. Why trust the extra parsing done
> by the shell? Look at this:
>
> sunny:~$ finger "a -b"
> finger: a -b: no such user.
>
> sunny:~$ finger a -b
> finger: illegal option -- b
> usage: finger [-lmps] [login ...]
>
> Now, many implementations of fingerd just run finger on data received from
> the remote, doing some sanity checks first, and splitting the arguments
> for execv(). These checks often include denying passing of some or all
> options to finger. If fingerd knows about less word separators than the
> shell does, then an attacker might be able to pass a forbidden option to
> finger. For example, if our fingerd didn't know about tabs (which isn't a
> security hole yet: our fingerd uses execv(), remember?), a remote attacker
> could send us "user\t-option".
>
> I admit that the problem isn't serious: not all fingerd's are done this
> way, forbidden finger options are likely to violate someone's privacy
> only, etc. Still, it's not a good idea to trust the shell, in general.
>
> Signed,
> Solar Designer