Debian bug report logs - #1674
fingerd allows recursion, -w forks two copies of the shell

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From server.et-inf.fho-emden.de!tobias Tue Oct 17 16:07:23 1995
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Subject: Re: Bug#1674: fingerd allows recursion, -w forks two copies of the shell
To: marekm@i17linuxb.ists.pwr.wroc.pl
Date: Tue, 17 Oct 1995 23:57:45 +0100 (MET)
From: "Peter Tobias" <tobias@server.et-inf.fho-emden.de>
Cc: debian-bugs-done@pixar.com
Reply-To: tobias@et-inf.fho-emden.de
In-Reply-To: <199510131617.RAA02299@i17linuxb.ists.pwr.wroc.pl> from "Marek Michalkiewicz" at Oct 13, 95 05:17:03 pm
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Marek Michalkiewicz wrote:
> Package: netstd
> Version: 1.17-1
> 
> It seems that the recursive finger problem has been attempted to solve
> by using the "finger.atbug" patch from sunsite.  This is wrong - this
> problem needs to be solved in fingerd, not finger.  Try to telnet to
> the finger port on Debian GNU/MIT/BSD/Linux system (I think that is
> the right name, to be fair :-), and type "user@host.some.domain" -
> and it will finger the requested address (this is only one level of
> recursion - but it is still not the right thing to do).
> 
> The right fix is to check for '@' characters in fingerd, not finger.
> 
> While we are at it, fingerd -w does system("/bin/sh -c /usr/bin/uptime")
> and system() forks yet another copy of the shell...  This only causes
> unnecessary system overhead for every incoming finger request.  At the
> very least, I suggest to change that to system("/usr/bin/uptime"), or
> (even better) use the classic fork/exec/wait piece of code to avoid
> running the shell at all (just run /usr/bin/uptime directly).
> 
> BTW, why does fingerd run as root?  If there is a user "nobody" listed
> in /etc/passwd, fingerd will change the uid to that user, but it would
> be a little safer to specify "nobody" as the user in /etc/inetd.conf -
> if getpwnam() fails (not necessarily because there is no user "nobody",
> another reason may be just not enough memory and malloc returning NULL),
> fingerd will still run as root...

I'll close this bug report with this message. All bugs you mentioned
have been fixed.


Peter

-- 
 Peter Tobias                                EMail:
 Fachhochschule Ostfriesland                 tobias@et-inf.fho-emden.de
 Fachbereich Elektrotechnik und Informatik   tobias@perseus.fho-emden.de
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Message received at debian-bugs:

From simons-rock.edu!jimr Mon Oct 16 18:34:29 1995
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To: Marek Michalkiewicz <marekm@i17linuxb.ists.pwr.wroc.pl>,
        debian-bugs@pixar.com
Subject: Re: Bug#1674: fingerd allows recursion, -w forks two copies of the shell 
In-Reply-To: Message from Marek Michalkiewicz <marekm@i17linuxb.ists.pwr.wroc.pl> 
   of "Fri, 13 Oct 1995 17:17:03 BST." <199510131617.RAA02299@i17linuxb.ists.pwr.wroc.pl> 
Date: Mon, 16 Oct 1995 21:33:10 -0400
From: "James A. Robinson" <jimr@simons-rock.edu>


These are a forward of two messages that got messed up in transit


Jim
-------------------------------------------------------------------------------
Date:    Fri, 13 Oct 1995 23:52:37 EDT
From:    "James A. Robinson" <jimr@simons-rock.edu>
cc:      Ian Jackson <iwj10@cus.cam.ac.uk>
Subject: Re: Bug#1674: fingerd allows recursion, -w forks two copies of the she
     ***ll 

> It seems that the recursive finger problem has been attempted to solve
> by using the "finger.atbug" patch from sunsite.  This is wrong - this
> problem needs to be solved in fingerd, not finger.  Try to telnet to

Perhaps people should look at kfingerd, I'm not sure how secure it is,
but it seems fairly nice -- can block site-wide queries, can allow the
user to log queries, can execute shell scripts on finger query, etc...

As far as I can tell, it does not allow recursive finger probes.


Jim
P.S. Ian J., you're the only security person I know of, so I 
     am cc'ing you. :)

-------------------------------------------------------------------------------
Date:    Tue, 17 Oct 1995 02:26:00 -0000
From:    Ian Jackson <ian@chiark.chu.cam.ac.uk>
To:      "James A. Robinson" <jimr@simons-rock.edu>
Subject: Re: Lost mail to iwj10@cus.cam.ac.uk 


Cheers.  

I'm not convinced that installing a new fingerd with more features
(esp. being able to run shell scripts) will improve security, but I do
think that having a range of software available is a good thing.

Do we have a GNU fingerd package ?  Obviously this is not the hottest
security thing since sliced bread.

The fingerd we have atm should be fixed (and reviewed to see if there
are any other obvious sillinesses).

Ian.

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From i17linuxb.ists.pwr.wroc.pl!marekm Fri Oct 13 09:17:19 1995
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From: Marek Michalkiewicz <marekm@i17linuxb.ists.pwr.wroc.pl>
Message-Id: <199510131617.RAA02299@i17linuxb.ists.pwr.wroc.pl>
Subject: fingerd allows recursion, -w forks two copies of the shell
To: debian-bugs@pixar.com
Date: Fri, 13 Oct 1995 17:17:03 +0100 (MET)
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Package: netstd
Version: 1.17-1

It seems that the recursive finger problem has been attempted to solve
by using the "finger.atbug" patch from sunsite.  This is wrong - this
problem needs to be solved in fingerd, not finger.  Try to telnet to
the finger port on Debian GNU/MIT/BSD/Linux system (I think that is
the right name, to be fair :-), and type "user@host.some.domain" -
and it will finger the requested address (this is only one level of
recursion - but it is still not the right thing to do).

The right fix is to check for '@' characters in fingerd, not finger.

While we are at it, fingerd -w does system("/bin/sh -c /usr/bin/uptime")
and system() forks yet another copy of the shell...  This only causes
unnecessary system overhead for every incoming finger request.  At the
very least, I suggest to change that to system("/usr/bin/uptime"), or
(even better) use the classic fork/exec/wait piece of code to avoid
running the shell at all (just run /usr/bin/uptime directly).

BTW, why does fingerd run as root?  If there is a user "nobody" listed
in /etc/passwd, fingerd will change the uid to that user, but it would
be a little safer to specify "nobody" as the user in /etc/inetd.conf -
if getpwnam() fails (not necessarily because there is no user "nobody",
another reason may be just not enough memory and malloc returning NULL),
fingerd will still run as root...

Marek