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<TITLE>Tutorial - remote shell access</TITLE>
<LINK REV="made" HREF="mailto:satan@fish.com">
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<H1><IMG SRC="../../images/satan.gif">Remote shell access</H1>
<HR>
<H3>Summary</H3>
Remote shell/remote login access from arbitrary hosts.
<H3>Impact</H3>
The machine can be taken over by any malicious (super)user on the network.
<H3>The problem</H3>
When the remote login/remote shell service trusts every host on the
network, a malicious superuser on an arbitrary host can gain access as
any user (except perhaps <em>root</em>). Once inside, the intruder
can replace system programs or configuration files (such as the
password file) and take over the machine.
<p>
In addition, there are guest or administrative accounts that might not
have passwords protecting the account, which allows anyone to remotely
login as that user and gain access to the host.
<H3>Fix</H3>
Remove the wildcard (+) from the /etc/hosts.equiv file. Be careful with
the use of the <tt>-@group</tt> netgroup feature, as there are many
incorrect implementations.
<p>
Delete or disable any accounts without a password from the system or
NIS password file.
<H3>Other tips</H3>
<ul>
<li> Give system accounts such as <em>bin</em> and <em> daemon</em> a
non-functional shell (such as <em>/bin/false</em>) and put them in
the <em>/etc/ftpusers</em> file so they cannot use ftp.
<li>See the
<a href="../../docs/admin_guide_to_cracking.html#remote-shell-access">Admin
Guide to Cracking</a> for an example of why this is a problem.
</ul>
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