doc-en/security/database.xml

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  <chapter xml:id="security.database" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
   <title>Database Security</title>

   <simpara>
    Nowadays, databases are cardinal components of any web based application by
    enabling websites to provide varying dynamic content. Since very sensitive
    or secret information can be stored in a database, you should strongly
    consider protecting your databases.
   </simpara>
   <simpara>
    To retrieve or to store any information you need to connect to the database,
    send a legitimate query, fetch the result, and close the connection.
    Nowadays, the commonly used query language in this interaction is the
    Structured Query Language (SQL). See how an attacker can <link
    linkend="security.database.sql-injection">tamper with an SQL query</link>.
   </simpara>
   <simpara>
    As you can surmise, <acronym>PHP</acronym> cannot protect your database by itself. The
    following sections aim to be an introduction into the very basics of how to
    access and manipulate databases within <acronym>PHP</acronym> scripts.
   </simpara>
   <simpara>
    Keep in mind this simple rule: defense in depth. The more places you
    take action to increase the protection of your database, the less
    probability of an attacker succeeding in exposing or abusing any stored
    information. Good design of the database schema and the application
    deals with your greatest fears.
   </simpara>

   <sect1 xml:id="security.database.design">
    <title>Designing Databases</title>
     <simpara>
      The first step is always to create the database, unless you want to use
      one from a third party. When a database is created, it is
      assigned to an owner, who executed the creation statement. Usually, only
      the owner (or a superuser) can do anything with the objects in that
      database, and in order to allow other users to use it, privileges must be
      granted.
     </simpara>
     <simpara>
      Applications should never connect to the database as its owner or a
      superuser, because these users can execute any query at will, for
      example, modifying the schema (e.g. dropping tables) or deleting its
      entire content.
     </simpara>
     <simpara>
      You may create different database users for every aspect of your
      application with very limited rights to database objects. The most
      required privileges should be granted only, and avoid that the same user
      can interact with the database in different use cases. This means that if
      intruders gain access to your database using your applications credentials,
      they can only effect as many changes as your application can.
     </simpara>
   </sect1>

   <sect1 xml:id="security.database.connection">
    <title>Connecting to Database</title>
    <simpara>
     You may want to establish the connections over SSL to encrypt
     client/server communications for increased security, or you can use ssh
     to encrypt the network connection between clients and the database server.
     If either of these is used, then monitoring your traffic and gaining
     information about your database will be difficult for a would-be attacker.
    </simpara>
    <!--simpara>
     If your database server has native SSL support, consider using <link
     linkend="ref.openssl">OpenSSL functions</link> in communication between
     <acronym>PHP</acronym> and database via SSL.
    </simpara-->
   </sect1>

   <sect1 xml:id="security.database.storage">
    <title>Encrypted Storage Model</title>
    <simpara>
     SSL/SSH protects data travelling from the client to the server: SSL/SSH
     does not protect persistent data stored in a database. SSL is an
     on-the-wire protocol.
    </simpara>
    <simpara>
     Once an attacker gains access to your database directly (bypassing the
     webserver), stored sensitive data may be exposed or misused, unless
     the information is protected by the database itself. Encrypting the data
     is a good way to mitigate this threat, but very few databases offer this
     type of data encryption.
    </simpara>
    <simpara>
     The easiest way to work around this problem is to first create your own
     encryption package, and then use it from within your <acronym>PHP</acronym> scripts. <acronym>PHP</acronym>
     can assist you in this with several extensions, such as <link
     linkend="book.openssl">OpenSSL</link> and <link
     linkend="book.sodium">Sodium</link>, covering a wide variety of encryption
     algorithms. The script encrypts the data before inserting it into the database, and decrypts
     it when retrieving. See the references for further examples of how
     encryption works.
    </simpara>

    <sect2 xml:id="security.database.storage.hashing">
    <title>Hashing</title>
     <simpara>
      In the case of truly hidden data, if its raw representation is not needed
      (i.e. will not be displayed), hashing should be taken into consideration.
      The well-known example for hashing is storing the cryptographic hash of a
      password in a database, instead of the password itself.
     </simpara>
     <simpara>
      The <link linkend="ref.password">password</link> functions
      provide a convenient way to hash sensitive data and work with these hashes.
     </simpara>
     <simpara>
      <function>password_hash</function> is used to hash a given string using the
      strongest algorithm currently available and <function>password_verify</function>
      checks whether the given password matches the hash stored in database.
     </simpara>
     <example>
      <title>Hashing password field</title>
      <programlisting role="php">
<![CDATA[
<?php

// storing password hash
$query  = sprintf("INSERT INTO users(name,pwd) VALUES('%s','%s');",
            pg_escape_string($username),
            password_hash($password, PASSWORD_DEFAULT));
$result = pg_query($connection, $query);

// querying if user submitted the right password
$query = sprintf("SELECT pwd FROM users WHERE name='%s';",
            pg_escape_string($username));
$row = pg_fetch_assoc(pg_query($connection, $query));

if ($row && password_verify($password, $row['pwd'])) {
    echo 'Welcome, ' . htmlspecialchars($username) . '!';
} else {
    echo 'Authentication failed for ' . htmlspecialchars($username) . '.';
}

?>
]]>
      </programlisting>
     </example>
    </sect2>
   </sect1>

   <sect1 xml:id="security.database.sql-injection">
    <title>SQL Injection</title>
    <simpara>
     SQL injection is a technique where an attacker exploits flaws in
     application code responsible for building dynamic SQL queries.
     The attacker can gain access to privileged sections of the application,
     retrieve all information from the database, tamper with existing data,
     or even execute dangerous system-level commands on the database
     host. The vulnerability occurs when developers concatenate or
     interpolate arbitrary input in their SQL statements.
    </simpara>
    <para>
     <example>
      <title>
       Splitting the result set into pages ... and making superusers
       (PostgreSQL)
      </title>
      <simpara>
       In the following example, user input is directly interpolated into the
       SQL query allowing the attacker to gain a superuser account in the database.
      </simpara>
      <programlisting role="php">
<![CDATA[
<?php

$offset = $_GET['offset']; // beware, no input validation!
$query  = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;";
$result = pg_query($conn, $query);

?>
]]>
      </programlisting>
     </example>
      Normal users click on the 'next', 'prev' links where the <varname>$offset</varname>
      is encoded into the <acronym>URL</acronym>. The script expects that the incoming
      <varname>$offset</varname> is a number. However, what if someone tries to
      break in by appending the following to the <acronym>URL</acronym>
      <informalexample>
       <programlisting role="sql">
<![CDATA[
0;
insert into pg_shadow(usename,usesysid,usesuper,usecatupd,passwd)
    select 'crack', usesysid, 't','t','crack'
    from pg_shadow where usename='postgres';
--
]]>
       </programlisting>
      </informalexample>
      If it happened, the script would present a superuser access to the attacker.
      Note that <literal>0;</literal> is to supply a valid offset to the
      original query and to terminate it.
    </para>
    <note>
     <para>
      It is a common technique to force the SQL parser to ignore the rest of the
      query written by the developer with <literal>--</literal> which is the
      comment sign in SQL.
     </para>
    </note>
    <para>
     A feasible way to gain passwords is to circumvent your search result pages.
     The only thing the attacker needs to do is to see if there are any submitted variables
     used in SQL statements which are not handled properly. These filters can be set
     commonly in a preceding form to customize <literal>WHERE, ORDER BY,
     LIMIT</literal> and <literal>OFFSET</literal> clauses in <literal>SELECT</literal>
     statements. If your database supports the <literal>UNION</literal> construct,
     the attacker may try to append an entire query to the original one to list
     passwords from an arbitrary table. It is strongly recommended to store only
     secure hashes of passwords instead of the passwords themselves.
     <example>
      <title>
       Listing out articles ... and some passwords (any database server)
      </title>
      <programlisting role="php">
<![CDATA[
<?php

$query  = "SELECT id, name, inserted, size FROM products
           WHERE size = '$size'";
$result = odbc_exec($conn, $query);

?>
]]>
      </programlisting>
     </example>
     The static part of the query can be combined with another
     <literal>SELECT</literal> statement which reveals all passwords:
     <informalexample>
      <programlisting role="sql">
<![CDATA[
'
union select '1', concat(uname||'-'||passwd) as name, '1971-01-01', '0' from usertable;
--
]]>
      </programlisting>
     </informalexample>
    </para>
    <para>
     <literal>UPDATE</literal> and <literal>INSERT</literal> statements are also
     susceptible to such attacks.
     <example>
     <title>
      From resetting a password ... to gaining more privileges (any database server)
     </title>
      <programlisting role="php">
<![CDATA[
<?php
$query = "UPDATE usertable SET pwd='$pwd' WHERE uid='$uid';";
?>
]]>
      </programlisting>
     </example>
     If a malicious user submits the value
     <literal>' or uid like'%admin%</literal> to <varname>$uid</varname> to
     change the admin's password, or simply sets <varname>$pwd</varname> to
     <literal>hehehe', trusted=100, admin='yes</literal> to gain more 
     privileges, then the query will be twisted:
     <informalexample>
      <programlisting role="php">
<![CDATA[
<?php

// $uid: ' or uid like '%admin%
$query = "UPDATE usertable SET pwd='...' WHERE uid='' or uid like '%admin%';";

// $pwd: hehehe', trusted=100, admin='yes
$query = "UPDATE usertable SET pwd='hehehe', trusted=100, admin='yes' WHERE
...;";

?>
]]>
      </programlisting>
     </informalexample>
    </para>
    <simpara>
     While it remains obvious that an attacker must possess at least some
     knowledge of the database architecture to conduct a successful
     attack, obtaining this information is often very simple. For example,
     the code may be part of an open-source software and be publicly available.
     This information may also be divulged
     by closed-source code - even if it's encoded, obfuscated, or compiled -
     and even by your own code through the display of error messages.
     Other methods include the use of typical table and column names.  For
     example, a login form that uses a 'users' table with column names
     'id', 'username', and 'password'.
    </simpara>
    <para>
     <example>
     <title>Attacking the database host operating system (MSSQL Server)</title>
      <simpara>
       A frightening example of how operating system-level commands can be
       accessed on some database hosts.
      </simpara>
      <programlisting role="php">
<![CDATA[
<?php

$query  = "SELECT * FROM products WHERE id LIKE '%$prod%'";
$result = mssql_query($query);

?>
]]>
      </programlisting>
     </example>
     If attacker submits the value
     <literal>a%' exec master..xp_cmdshell 'net user test testpass /ADD' --</literal>
     to <varname>$prod</varname>, then the <varname>$query</varname> will be:
     <informalexample>
      <programlisting role="php">
<![CDATA[
<?php

$query  = "SELECT * FROM products
           WHERE id LIKE '%a%'
           exec master..xp_cmdshell 'net user test testpass /ADD' --%'";
$result = mssql_query($query);

?>
]]>
      </programlisting>
     </informalexample>
     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 <literal>sa</literal> and the MSSQLSERVER service was
     running with sufficient privileges, the attacker would now have an
     account with which to access this machine.
    </para>
    <note>
     <para>
      Some examples above are tied to a specific database server, but it
      does not mean that a similar attack is impossible against other products.
      Your database server may be similarly vulnerable in another manner.
     </para>
    </note>
    <para>
     <mediaobject>
      <alt>A funny example of the issues regarding SQL injection</alt>
      <imageobject>
       <imagedata fileref="en/security/figures/xkcd-bobby-tables.png" format="PNG"/>
      </imageobject>
      <caption>
       <simpara>
        Image courtesy of <link xlink:href="&url.xkcd;327">xkcd</link>
       </simpara>
      </caption>
     </mediaobject>
    </para>

    <sect2 xml:id="security.database.avoiding">
     <title>Avoidance Techniques</title>
     <para>
      The recommended way to avoid SQL injection is by binding all data via
      prepared statements. Using parameterized queries isn't enough to entirely
      avoid SQL injection, but it is the easiest and safest way to provide input
      to SQL statements. All dynamic data literals in <literal>WHERE</literal>,
      <literal>SET</literal>, and <literal>VALUES</literal> clauses must be
      replaced with placeholders. The actual data will be bound during the
      execution and sent separately from the SQL command.
     </para>
     <para>
      Parameter binding can only be used for data. Other dynamic parts of the
      SQL query must be filtered against a known list of allowed values.
     </para>
     <para>
      <example>
      <title>Avoiding SQL injection by using PDO prepared statements</title>
       <programlisting role="php">
<![CDATA[
<?php

// The dynamic SQL part is validated against expected values
$sortingOrder = $_GET['sortingOrder'] === 'DESC' ? 'DESC' : 'ASC';
$productId = $_GET['productId'];
// The SQL is prepared with a placeholder
$stmt = $pdo->prepare("SELECT * FROM products WHERE id LIKE ? ORDER BY price {$sortingOrder}");
// The value is provided with LIKE wildcards
$stmt->execute(["%{$productId}%"]);

?>
]]>
       </programlisting>
      </example>
     </para>

     <simpara>
      Prepared statements are provided
      <link linkend="pdo.prepared-statements">by PDO</link>,
      <link linkend="mysqli.quickstart.prepared-statements">by MySQLi</link>,
      and by other database libraries.
     </simpara>

     <simpara>
      SQL injection attacks are mainly based on exploiting the code not being written
      with security in mind. Never trust any input, especially
      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
      simple query can cause disasters.
     </simpara>

     <para>
      A defense-in-depth strategy involves several good coding practices:
      <itemizedlist>
       <listitem>
        <simpara>
         Never connect to the database as a superuser or as the database owner.
         Use always customized users with minimal privileges.
        </simpara>
       </listitem>
       <listitem>
        <simpara>
         Check if the given input has the expected data type. <acronym>PHP</acronym> has
         a wide range of input validating functions, from the simplest ones
         found in <link linkend="ref.var">Variable Functions</link> and
         in <link linkend="ref.ctype">Character Type Functions</link>
         (e.g. <function>is_numeric</function>, <function>ctype_digit</function>
         respectively) and onwards to the
         <link linkend="ref.pcre">Perl Compatible Regular Expressions</link>
         support.
        </simpara>
       </listitem>
       <listitem>
        <simpara>
         If the application expects numerical input, consider verifying data
         with <function>ctype_digit</function>, silently change its type
         using <function>settype</function>, or use its numeric representation
         by <function>sprintf</function>.
        </simpara>
       </listitem>
       <listitem>
        <simpara>
         If the database layer doesn't support binding variables then
         quote each non-numeric user-supplied value that is passed to the
         database with the database-specific string escape function (e.g.
         <function>mysql_real_escape_string</function>,
         <function>sqlite_escape_string</function>, etc.).
         Generic functions like <function>addslashes</function> are useful only
         in a very specific environment (e.g. MySQL in a single-byte character
         set with disabled <varname>NO_BACKSLASH_ESCAPES</varname>), so it is
         better to avoid them.
        </simpara>
       </listitem>
       <listitem>
        <simpara>
         Do not print out any database-specific information, especially
         about the schema, by fair means or foul. See also <link
         linkend="security.errors">Error Reporting</link> and <link
         linkend="ref.errorfunc">Error Handling and Logging Functions</link>.
        </simpara>
       </listitem>
      </itemizedlist>
     </para>

     <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.
     </simpara>
    </sect2>
   </sect1>
  </chapter>

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