security.database.sql-injection.html

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     </div>
    </p>
    <p class="para">
     A frightening example how operating system level commands can be accessed
     on some database hosts.
     <div class="example">
     <p><b>Example #4 Attacking the database hosts operating system (MSSQL Server)</b></p>
      <div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br /><br />$query&nbsp;&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">"SELECT&nbsp;*&nbsp;FROM&nbsp;products&nbsp;WHERE&nbsp;id&nbsp;LIKE&nbsp;'%$prod%'"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$result&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #0000BB">mssql_query</span><span style="color: #007700">(</span><span style="color: #0000BB">$query</span><span style="color: #007700">);<br /><br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
      </div>

     </div>
     If attacker submits the value
     <i>a%&#039; exec master..xp_cmdshell &#039;net user test testpass /ADD&#039; --</i>
     to <var class="varname">$prod</var>, then the <var class="varname">$query</var> will be:
     <div class="informalexample">
      <div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br /><br />$query&nbsp;&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">"SELECT&nbsp;*&nbsp;FROM&nbsp;products<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;WHERE&nbsp;id&nbsp;LIKE&nbsp;'%a%'<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exec&nbsp;master..xp_cmdshell&nbsp;'net&nbsp;user&nbsp;test&nbsp;testpass&nbsp;/ADD'--"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$result&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #0000BB">mssql_query</span><span style="color: #007700">(</span><span style="color: #0000BB">$query</span><span style="color: #007700">);<br /><br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
      </div>

     </div>
     MSSQL Server executes the SQL statements in the batch including a command
     to add a new user to the local accounts database. If this application
     were running as <i>sa</i> and the MSSQLSERVER service is
     running with sufficient privileges, the attacker would now have an
     account with which to access this machine.
    </p>
    <blockquote><p><b class="note">Note</b>: 
     
      Some of the examples above is tied to a specific database server. This
      does not mean that a similar attack is impossible against other products.
      Your database server may be similarly vulnerable in another manner.
     <br />
    </p></blockquote>

    <div id="security.database.avoiding" class="sect2">
     <h3 class="title">Avoiding techniques</h3>
     <p class="simpara">
      You may plead that the attacker must possess a piece of information
      about the database schema in most examples. You are right, but you
      never know when and how it can be taken out, and if it happens,
      your database may be exposed. If you are using an open source, or
      publicly available database handling package, which may belong to a
      content management system or forum, the intruders easily produce
      a copy of a piece of your code. It may be also a security risk if it
      is a poorly designed one.
     </p>
     <p class="simpara">
      These attacks are mainly based on exploiting the code not being written
      with security in mind. Never trust any kind of input, especially that
      which comes from the client side, even though it comes from a select box,
      a hidden input field or a cookie. The first example shows that such a
      blameless query can cause disasters.
     </p>

     <ul class="itemizedlist">
      <li class="listitem">
       <span class="simpara">
        Never connect to the database as a superuser or as the database owner.
        Use always customized users with very limited privileges.
       </span>
      </li>
      <li class="listitem">
       <span class="simpara">
        Check if the given input has the expected data type. PHP has
        a wide range of input validating functions, from the simplest ones
        found in <a href="ref.var.html" class="link">Variable Functions</a> and
        in <a href="ref.ctype.html" class="link">Character Type Functions</a>
        (e.g. <a href="function.is-numeric.html" class="function">is_numeric()</a>, <a href="function.ctype-digit.html" class="function">ctype_digit()</a>
        respectively) and onwards to the
        <a href="ref.pcre.html" class="link">Perl compatible Regular Expressions</a>
        support.
       </span>
      </li>
      <li class="listitem">
       <p class="para">
        If the application waits for numerical input, consider verifying data
        with <a href="function.is-numeric.html" class="function">is_numeric()</a>, or silently change its type
        using <a href="function.settype.html" class="function">settype()</a>, or use its numeric representation
        by <a href="function.sprintf.html" class="function">sprintf()</a>.
        <div class="example">
         <p><b>Example #5 A more secure way to compose a query for paging</b></p>
         <div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br /><br />settype</span><span style="color: #007700">(</span><span style="color: #0000BB">$offset</span><span style="color: #007700">,&nbsp;</span><span style="color: #DD0000">'integer'</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">$query&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">"SELECT&nbsp;id,&nbsp;name&nbsp;FROM&nbsp;products&nbsp;ORDER&nbsp;BY&nbsp;name&nbsp;LIMIT&nbsp;20&nbsp;OFFSET&nbsp;$offset;"</span><span style="color: #007700">;<br /><br /></span><span style="color: #FF8000">//&nbsp;please&nbsp;note&nbsp;%d&nbsp;in&nbsp;the&nbsp;format&nbsp;string,&nbsp;using&nbsp;%s&nbsp;would&nbsp;be&nbsp;meaningless<br /></span><span style="color: #0000BB">$query&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #0000BB">sprintf</span><span style="color: #007700">(</span><span style="color: #DD0000">"SELECT&nbsp;id,&nbsp;name&nbsp;FROM&nbsp;products&nbsp;ORDER&nbsp;BY&nbsp;name&nbsp;LIMIT&nbsp;20&nbsp;OFFSET&nbsp;%d;"</span><span style="color: #007700">,<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #0000BB">$offset</span><span style="color: #007700">);<br /><br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
         </div>

        </div>
       </p>
      </li>
      <li class="listitem">
       <span class="simpara">
        Quote each non numeric user supplied value that is passed to the
        database with the database-specific string escape function (e.g.
        <a href="function.mysql-real-escape-string.html" class="function">mysql_real_escape_string()</a>,
        <b>sql_escape_string()</b>, etc.). If a database-specific
        string escape mechanism is not available, the
        <a href="function.addslashes.html" class="function">addslashes()</a> and <a href="function.str-replace.html" class="function">str_replace()</a>
        functions may be useful (depending on database type).
        See <a href="security.database.storage.html" class="link">the first example</a>.
        As the example shows, adding quotes to the static part of the query
        is not enough, making this query easily crackable.
       </span>
      </li>
      <li class="listitem">
       <span class="simpara">
        Do not print out any database specific information, especially
        about the schema, by fair means or foul. See also <a href="security.errors.html" class="link">Error Reporting</a> and <a href="ref.errorfunc.html" class="link">Error Handling and Logging Functions</a>.
       </span>
      </li>
      <li class="listitem">
       <span class="simpara">
        You may use stored procedures and previously defined cursors to abstract
        data access so that users do not directly access tables or views, but
        this solution has another impacts.
       </span>
      </li>
     </ul>

     <p class="simpara">
      Besides these, you benefit from logging queries either within your script
      or by the database itself, if it supports logging. Obviously, the logging is unable
      to prevent any harmful attempt, but it can be helpful to trace back which
      application has been circumvented. The log is not useful by itself, but
      through the information it contains. More detail is generally better than less.
     </p>
    </div>
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