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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>4.2. Choice of a service name</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="Linux-PAM_ADG.html" title="The Linux-PAM Application Developers' Guide"><link rel="up" href="adg-security.html" title="Chapter 4. Security issues of Linux-PAM"><link rel="prev" href="adg-security-library-calls.html" title="4.1. Care about standard library calls"><link rel="next" href="adg-security-conv-function.html" title="4.3. The conversation function"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">4.2. Choice of a service name</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="adg-security-library-calls.html">Prev</a> </td><th width="60%" align="center">Chapter 4.
Security issues of <span class="emphasis"><em>Linux-PAM</em></span>
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When picking the <span class="emphasis"><em>service-name</em></span> that
corresponds to the first entry in the
<span class="emphasis"><em>Linux-PAM</em></span> configuration file,
the application programmer should <span class="emphasis"><em>avoid</em></span>
the temptation of choosing something related to
<code class="varname">argv[0]</code>. It is a trivial matter for any user
to invoke any application on a system under a different name and
this should not be permitted to cause a security breach.
</p><p>
In general, this is always the right advice if the program is
setuid, or otherwise more privileged than the user that invokes
it. In some cases, avoiding this advice is convenient, but as an
author of such an application, you should consider well the ways
in which your program will be installed and used. (Its often the
case that programs are not intended to be setuid, but end up
being installed that way for convenience. If your program falls
into this category, don't fall into the trap of making this mistake.)
</p><p>
To invoke some <span class="emphasis"><em>target</em></span> application by
another name, the user may symbolically link the target application
with the desired name. To be precise all the user need do is,
<span class="command"><strong>ln -s /target/application ./preferred_name</strong></span>
and then run <span class="command"><strong>./preferred_name</strong></span>.
</p><p>
By studying the <span class="emphasis"><em>Linux-PAM</em></span>
configuration file(s), an attacker can choose the
<span class="command"><strong>preferred_name</strong></span> to be that of a service enjoying
minimal protection; for example a game which uses
<span class="emphasis"><em>Linux-PAM</em></span> to restrict access to
certain hours of the day. If the service-name were to be linked
to the filename under which the service was invoked, it
is clear that the user is effectively in the position of
dictating which authentication scheme the service uses. Needless
to say, this is not a secure situation.
</p><p>
The conclusion is that the application developer should carefully
define the service-name of an application. The safest thing is to
make it a single hard-wired name.
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