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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML
><HEAD
><TITLE
>Security Issues</TITLE
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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
></TR
><TR
><TD
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VALIGN="bottom"
><A
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></TD
><TD
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>Chapter 36. Miscellany</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="SECURITYISSUES"
></A
>36.8. Security Issues</H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="INFECTEDSCRIPTS"
></A
>36.8.1. Infected Shell Scripts</H2
><P
><A
NAME="INFECTEDSCRIPTS1"
></A
></P
><P
>A brief warning about script security is indicated.
A shell script may contain a <I
CLASS="FIRSTTERM"
>worm</I
>,
<I
CLASS="FIRSTTERM"
>trojan</I
>, or even a
<I
CLASS="FIRSTTERM"
>virus</I
>. For that reason, never run
as <I
CLASS="FIRSTTERM"
>root</I
> a script (or permit it to
be inserted into the system startup scripts in <TT
CLASS="FILENAME"
>/etc/rc.d</TT
>) unless you have obtained
said script from a trusted source or you have carefully analyzed
it to make certain it does nothing harmful.</P
><P
>Various researchers at Bell Labs and other sites, including M.
Douglas McIlroy, Tom Duff, and Fred Cohen have investigated the
implications of shell script viruses. They conclude that it is
all too easy for even a novice, a <SPAN
CLASS="QUOTE"
>"script kiddie,"</SPAN
>
to write one.
<A
NAME="AEN20748"
HREF="#FTN.AEN20748"
>[1]</A
>
</P
><P
>Here is yet another reason to learn scripting. Being able to
look at and understand scripts may protect your system from
being compromised by a rogue script.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="HIDINGSOURCE"
></A
>36.8.2. Hiding Shell Script Source</H2
><P
>For security purposes, it may be necessary to render a script
unreadable. If only there were a utility to create a stripped
binary executable from a script. Francisco Rosales' <A
HREF="http://www.datsi.fi.upm.es/~frosal/sources/"
TARGET="_top"
>shc --
generic shell script compiler</A
> does exactly that.</P
><P
>Unfortunately, according to <A
HREF="http://www.linuxjournal.com/article/8256"
TARGET="_top"
>an article</A
> in
the October, 2005 <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Linux Journal</I
></SPAN
>,
the binary can, in at least some cases, be decrypted to recover
the original script source. Still, this could be a useful
method of keeping scripts secure from all but the most skilled
hackers.</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SECURITYTIPS"
></A
>36.8.3. Writing Secure Shell Scripts</H2
><P
><A
NAME="SECURITYTIPS1"
></A
></P
><P
><SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Dan Stromberg</I
></SPAN
> suggests the following
guidelines for writing (relatively) secure shell scripts.</P
><P
> <UL
><LI
><P
>Don't put secret data in <A
HREF="othertypesv.html#ENVREF"
>environment variables</A
>.</P
></LI
><LI
><P
>Don't pass secret data in an external
command's arguments (pass them in via a <A
HREF="special-chars.html#PIPEREF"
>pipe</A
> or <A
HREF="io-redirection.html#IOREDIRREF"
>redirection</A
> instead).</P
></LI
><LI
><P
>Set your <A
HREF="variables2.html#PATHREF"
>$PATH</A
>
carefully. Don't just trust whatever path you
inherit from the caller if your script is running as
<I
CLASS="FIRSTTERM"
>root</I
>. In fact, whenever you use
an environment variable inherited from the caller, think
about what could happen if the caller put something
misleading in the variable, e.g., if the caller set
<A
HREF="variables2.html#HOMEDIRREF"
>$HOME</A
> to <TT
CLASS="FILENAME"
>/etc</TT
>.</P
></LI
></UL
>
</P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
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><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN20748"
HREF="securityissues.html#AEN20748"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>See Marius van Oers' article, <A
HREF="http://www.virusbtn.com/magazine/archives/200204/malshell.xml"
TARGET="_top"
>Unix
Shell Scripting Malware</A
>, and also the
<A
HREF="biblio.html#DENNINGREF"
><SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Denning</I
></SPAN
>
reference</A
> in the
<I
CLASS="FIRSTTERM"
>bibliography</I
>.</P
></TD
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