php-doc-en/security/database.xml

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<!-- $Revision$ -->
  <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>
     <simpara>
      You are encouraged not to implement all the business logic in the web
      application (i.e. your script), instead do it in the database schema
      using views, triggers or rules. If the system evolves, new ports will be
      intended to open to the database, and you have to re-implement the logic
      in each separate database client. Over and above, triggers can be used
      to transparently and automatically handle fields, which often provides
      insight when debugging problems with your application or tracing back
      transactions.
     </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="ref.mcrypt">Mcrypt</link> and <link
     linkend="ref.mhash">Mhash</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>
      In PHP 5.5 or newer <link linkend="ref.password">password</link> functions
      provide a convenient way to hash sensitive data and work with these hashes.
      In PHP 5.3.7+ <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="&url.password.compat;">
      password_compat</link> library can also be used.
     </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>
     <simpara>
      In older versions of PHP this can be achieved using <function>crypt</function>
      function.
     </simpara>
     <example>
      <title>Hashing password using <function>crypt</function>s</title>
      <programlisting role="php">
<![CDATA[
<?php

// storing password hash
// $random_chars retrieved e.g. using /dev/random
$query  = sprintf("INSERT INTO users(name,pwd) VALUES('%s','%s');",
            pg_escape_string($username),
            pg_escape_string(crypt($password, '$2a$07$' . $random_chars . '$')));
$result = pg_query($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 ($row && crypt($password, $row['pwd']) == $row['pwd']) {
    echo 'Welcome, ' . htmlspecialchars($username) . '!';
} else {
    echo 'Authentication failed for ' . htmlspecialchars($username) . '.';
}

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

   <sect1 xml:id="security.database.sql-injection">
    <title>SQL Injection</title>
    <simpara>
     Many web developers are unaware of how SQL queries can be tampered with,
     and assume that an SQL query is a trusted command. It means that SQL
     queries are able to circumvent access controls, thereby bypassing standard
     authentication and authorization checks, and sometimes SQL queries even
     may allow access to host operating system level commands.
    </simpara>
    <simpara>
     Direct SQL Command Injection is a technique where an attacker creates or
     alters existing SQL commands to expose hidden data, or to override valuable
     ones, or even to execute dangerous system level commands on the database
     host. This is accomplished by the application taking user input and
     combining it with static parameters to build an SQL query. The following
     examples are based on true stories, unfortunately.
    </simpara>
    <para>
     Owing to the lack of input validation and connecting to the database on
     behalf of a superuser or the one who can create users, the attacker
     may create a superuser in your database.
     <example>
      <title>
       Splitting the result set into pages ... and making superusers
       (PostgreSQL)
      </title>
      <programlisting role="php">
<![CDATA[
<?php

$offset = $argv[0]; // 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 decimal number. However, what if someone tries to
      break in by appending a <function>urlencode</function>'d form of 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, then the script would present a superuser access to him.
      Note that <literal>0;</literal> is to supply a valid offset to the
      original query and to terminate it.
    </para>
    <note>
     <para>
      It is 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. Using encrypted password fields is 
     strongly encouraged.
     <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>
     If this query (playing with the <literal>'</literal> and
     <literal>--</literal>) were assigned to one of the variables used in
     <varname>$query</varname>, the query beast awakened.
    </para>
    <para>
     SQL UPDATE's are also susceptible to attack. These queries are
     also threatened by chopping and appending an entirely new query to it. But
     the attacker might fiddle with the <literal>SET</literal> clause. In this
     case some schema information must be possessed to manipulate the query
     successfully. This can be acquired by examining the form variable names, or
     just simply brute forcing. There are not so many naming conventions for
     fields storing passwords or usernames.
     <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='$uid';";
?>
]]>
      </programlisting>
     </example>
     But a malicious user sumbits the value
     <literal>' or uid like'%admin%</literal> to <varname>$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>
    <para>
     A frightening example how operating system level commands can be accessed
     on some database hosts.
     <example>
     <title>Attacking the database hosts operating system (MSSQL Server)</title>
      <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 is
     running with sufficient privileges, the attacker would now have an
     account with which to access this machine.
    </para>
    <note>
     <para>
      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.
     </para>
    </note>
    <para>
     <mediaobject>
      <alt>A worked example of the issues regarding SQL Injection</alt>
      <imageobject>
       <imagedata fileref="en/security/figures/xkcd-bobby-tables.png" format="PNG"/>
      </imageobject>
     </mediaobject>
     Image courtesy of <link xlink:href="&url.xkcd;327">xkcd</link>
    </para>

    <sect2 xml:id="security.database.avoiding">
     <title>Avoidance Techniques</title>
     <simpara>
      While it remains obvious that an attacker must possess at least some
      knowledge of the database architecture in order to conduct a successful
      attack, obtaining this information is often very simple.  For example,
      if the database is part of an open source or other publicly-available
      software package with a default installation, this information is
      completely open and available.  This information may also be divulged
      by closed-source code - even if it's encoded, obfuscated, or compiled -
      and even by your very own code through the display of error messages.
      Other methods include the user of common table and column names.  For
      example, a login form that uses a 'users' table with column names
      'id', 'username', and 'password'.
     </simpara>
     <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.
     </simpara>

     <itemizedlist>
      <listitem>
       <simpara>
        Never connect to the database as a superuser or as the database owner.
        Use always customized users with very limited privileges.
       </simpara>
      </listitem>
      <listitem>
       <simpara>
        Use prepared statements with bound variables. They are provided 
        <link linkend="pdo.prepared-statements">by PDO</link>,
        <link linkend="mysqli.quickstart.prepared-statements">by MySQLi</link>
        and by other libraries.
       </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>
       <para>
        If the application waits for numerical input, consider verifying data
        with <function>ctype_digit</function>, or silently change its type
        using <function>settype</function>, or use its numeric representation
        by <function>sprintf</function>.
        <example>
         <title>A more secure way to compose a query for paging</title>
         <programlisting role="php">
<![CDATA[
<?php

settype($offset, 'integer');
$query = "SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET $offset;";

// please note %d in the format string, using %s would be meaningless
$query = sprintf("SELECT id, name FROM products ORDER BY name LIMIT 20 OFFSET %d;",
                 $offset);

?>
]]>
         </programlisting>
        </example>
       </para>
      </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>
      <listitem>
       <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.
       </simpara>
      </listitem>
     </itemizedlist>

     <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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