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[zend.git] / documentation / manual / en / module_specs / Zend_Session-GlobalSessionManagement.xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- Reviewed: no -->
<sect1 id="zend.session.global_session_management">
    <title>Global Session Management</title>

    <para>
        The default behavior of sessions can be modified using the static methods of
        <classname>Zend_Session</classname>. All management and manipulation of global session
        management occurs using <classname>Zend_Session</classname>, including configuration of the
        <ulink url="http://www.php.net/session#session.configuration">usual options provided by
            ext/session</ulink>, using <methodname>Zend_Session::setOptions()</methodname>. For
        example, failure to insure the use of a safe <code>save_path</code> or a unique cookie name
        by ext/session using <methodname>Zend_Session::setOptions()</methodname> may result in
        security issues.
    </para>

    <sect2 id="zend.session.global_session_management.configuration_options">
        <title>Configuration Options</title>

        <para>
            When the first session namespace is requested, <classname>Zend_Session</classname> will
            automatically start the <acronym>PHP</acronym> session, unless already started with
            <link
                linkend="zend.session.advanced_usage.starting_a_session"><methodname>Zend_Session::start()</methodname></link>.
            The underlying <acronym>PHP</acronym> session will use defaults from
            <classname>Zend_Session</classname>, unless modified first by
            <methodname>Zend_Session::setOptions()</methodname>.
        </para>

        <para>
            To set a session configuration option, include the basename (the part of the name after
            "<code>session.</code>") as a key of an array passed to
            <methodname>Zend_Session::setOptions()</methodname>. The corresponding value in the
            array is used to set the session option value. If no options are set by the developer,
            <classname>Zend_Session</classname> will utilize recommended default options first, then
            the default php.ini settings. Community feedback about best practices for these options
            should be sent to <ulink
                url="mailto:fw-auth@lists.zend.com">fw-auth@lists.zend.com</ulink>.
        </para>

        <example id="zend.session.global_session_management.setoptions.example">
            <title>Using Zend_Config to Configure Zend_Session</title>

            <para>
                To configure this component using <link
                    linkend="zend.config.adapters.ini"><classname>Zend_Config_Ini</classname></link>,
                first add the configuration options to the <acronym>INI</acronym> file:
            </para>

            <programlisting language="ini"><![CDATA[
; Accept defaults for production
[production]
; bug_compat_42
; bug_compat_warn
; cache_expire
; cache_limiter
; cookie_domain
; cookie_lifetime
; cookie_path
; cookie_secure
; entropy_file
; entropy_length
; gc_divisor
; gc_maxlifetime
; gc_probability
; hash_bits_per_character
; hash_function
; name should be unique for each PHP application sharing the same
; domain name
name = UNIQUE_NAME
; referer_check
; save_handler
; save_path
; serialize_handler
; use_cookies
; use_only_cookies
; use_trans_sid

; remember_me_seconds = <integer seconds>
; strict = on|off

; Development inherits configuration from production, but overrides
; several values
[development : production]
; Don't forget to create this directory and make it rwx (readable and
; modifiable) by PHP.
save_path = /home/myaccount/zend_sessions/myapp
use_only_cookies = on
; When persisting session id cookies, request a TTL of 10 days
remember_me_seconds = 864000
]]></programlisting>

            <para>
                Next, load the configuration file and pass its array representation to
                <methodname>Zend_Session::setOptions()</methodname>:
            </para>

            <programlisting language="php"><![CDATA[
$config = new Zend_Config_Ini('myapp.ini', 'development');

Zend_Session::setOptions($config->toArray());
]]></programlisting>
        </example>

        <para>
            Most options shown above need no explanation beyond that found in the standard
            <acronym>PHP</acronym> documentation, but those of particular interest are noted below.

            <itemizedlist mark="opencircle">
                <listitem>
                    <para>
                        boolean <code>strict</code> - disables automatic starting of
                        <classname>Zend_Session</classname> when using
                        <code>new Zend_Session_Namespace()</code>.
                    </para>
                </listitem>

                <listitem>
                    <para>
                        integer <code>remember_me_seconds</code> - how long should session id cookie
                        persist, after user agent has ended (e.g., browser application terminated).
                    </para>
                </listitem>

                <listitem>
                    <para>
                        string <code>save_path</code> - The correct value is system dependent, and
                        should be provided by the developer using an
                        <emphasis>absolute path</emphasis> to a directory readable and writable by
                        the <acronym>PHP</acronym> process. If a writable path is not supplied, then
                        <classname>Zend_Session</classname> will throw an exception when started
                        (i.e., when <methodname>start()</methodname> is called).
                    </para>

                    <note>
                        <title>Security Risk</title>

                        <para>
                            If the path is readable by other applications, then session hijacking
                            might be possible. if the path is writable by other applications, then
                            <ulink url="http://en.wikipedia.org/wiki/Session_poisoning">session
                                poisoning</ulink> might be possible. If this path is shared with
                            other users or other <acronym>PHP</acronym> applications, various
                            security issues might occur, including theft of session content,
                            hijacking of sessions, and collision of garbage collection (e.g.,
                            another user's application might cause <acronym>PHP</acronym> to delete
                            your application's session files).
                        </para>

                        <para>
                            For example, an attacker can visit the victim's website to obtain a
                            session cookie. Then, he edits the cookie path to his own domain on the
                            same server, before visiting his own website to execute
                            <methodname>var_dump($_SESSION)</methodname>. Armed with detailed
                            knowledge of the victim's use of data in their sessions, the attacker
                            can then modify the session state (poisoning the session), alter the
                            cookie path back to the victim's website, and then make requests from
                            the victim's website using the poisoned session. Even if two
                            applications on the same server do not have read/write access to the
                            other application's <code>save_path</code>, if the
                            <code>save_path</code> is guessable, and the attacker has control over
                            one of these two websites, the attacker could alter their website's
                            <code>save_path</code> to use the other's save_path, and thus accomplish
                            session poisoning, under some common configurations of
                            <acronym>PHP</acronym>. Thus, the value for <code>save_path</code>
                            should not be made public knowledge and should be altered to a secure
                            location unique to each application.
                        </para>
                    </note>
                </listitem>

                <listitem>
                    <para>
                        string <code>name</code> - The correct value is system dependent and should
                        be provided by the developer using a value <emphasis>unique</emphasis> to
                        the application.
                    </para>

                    <note>
                        <title>Security Risk</title>

                        <para>
                            If the <code>php.ini</code> setting for <code>session.name</code> is the
                            same (e.g., the default "PHPSESSID"), and there are two or more
                            <acronym>PHP</acronym> applications accessible through the same domain
                            name then they will share the same session data for visitors to both
                            websites. Additionally, possible corruption of session data may result.
                        </para>
                    </note>
                </listitem>

                <listitem>
                    <para>
                        boolean <code>use_only_cookies</code> - In order to avoid introducing
                        additional security risks, do not alter the default value of this option.

                        <note>
                            <title>Security Risk</title>

                            <para>
                                If this setting is not enabled, an attacker can easily fix victim's
                                session ids, using links on the attacker's website, such as
                                <code>http://www.example.com/index.php?PHPSESSID=fixed_session_id</code>.
                                The fixation works, if the victim does not already have a session id
                                cookie for example.com. Once a victim is using a known session id,
                                the attacker can then attempt to hijack the session by pretending to
                                be the victim, and emulating the victim's user agent.
                            </para>
                        </note>
                    </para>
                </listitem>
            </itemizedlist>
        </para>
    </sect2>

    <sect2 id="zend.session.global_session_management.headers_sent">
        <title>Error: Headers Already Sent</title>

        <para>
            If you see the error message, "Cannot modify header information - headers already sent",
            or, "You must call ... before any output has been sent to the browser; output started in
            ...", then carefully examine the immediate cause (function or method) associated with
            the message. Any actions that require sending <acronym>HTTP</acronym> headers, such as
            sending a cookie, must be done before sending normal output (unbuffered output), except
            when using <acronym>PHP</acronym>'s output buffering.
        </para>

        <itemizedlist mark="opencircle">
            <listitem>
                <para>
                    Using <ulink url="http://php.net/outcontrol">output buffering</ulink> often is
                    sufficient to prevent this issue, and may help improve performance. For example,
                    in <code>php.ini</code>, "<code>output_buffering = 65535</code>" enables output
                    buffering with a 64K buffer. Even though output buffering might be a good tactic
                    on production servers to increase performance, relying only on buffering to
                    resolve the "headers already sent" problem is not sufficient. The application
                    must not exceed the buffer size, or the problem will occur whenever the output
                    sent (prior to the <acronym>HTTP</acronym> headers) exceeds the buffer size.
                </para>
            </listitem>

            <listitem>
                <para>
                    If a <classname>Zend_Session</classname> method is involved in causing the error
                    message, examine the method carefully, and make sure its use really is needed in
                    the application. For example, the default usage of
                    <methodname>destroy()</methodname> also sends an <acronym>HTTP</acronym> header
                    to expire the client-side session cookie. If this is not needed, then use
                    <methodname>destroy(false)</methodname>, since the instructions to set cookies
                    are sent with <acronym>HTTP</acronym> headers.
                </para>
            </listitem>

            <listitem>
                <para>
                    Alternatively, try rearranging the application logic so that all actions
                    manipulating headers are performed prior to sending any output whatsoever.
                </para>
            </listitem>

            <listitem>
                <para>
                    Remove any closing "<code>?&gt;</code>" tags, if they occur at the end of a
                    <acronym>PHP</acronym> source file. They are not needed, and newlines and other
                    nearly invisible whitespace following the closing tag can trigger output to the
                    client.
                </para>
            </listitem>
        </itemizedlist>
    </sect2>

    <sect2 id="zend.session.global_session_management.session_identifiers">
        <title>Session Identifiers</title>

        <para>
            Introduction: Best practice in relation to using sessions with Zend Framework calls for
            using a browser cookie (i.e. a normal cookie stored in your web browser), instead of
            embedding a unique session identifier in <acronym>URL</acronym>s as a means to track
            individual users. By default this component uses only cookies to maintain session
            identifiers. The cookie's value is the unique identifier of your browser's session.
            <acronym>PHP</acronym>'s ext/session uses this identifier to maintain a unique
            one-to-one relationship between website visitors, and persistent session data storage
            unique to each visitor. <classname>Zend_Session</classname>* wraps this storage
            mechanism (<varname>$_SESSION</varname>) with an object-oriented interface.
            Unfortunately, if an attacker gains access to the value of the cookie (the session id),
            an attacker might be able to hijack a visitor's session. This problem is not unique to
            <acronym>PHP</acronym>, or Zend Framework. The <methodname>regenerateId()</methodname>
            method allows an application to change the session id (stored in the visitor's cookie)
            to a new, random, unpredictable value. Note: Although not the same, to make this section
            easier to read, we use the terms "user agent" and "web browser" interchangeably.
        </para>

        <para>
            Why?: If an attacker obtains a valid session identifier, an attacker might be able to
            impersonate a valid user (the victim), and then obtain access to confidential
            information or otherwise manipulate the victim's data managed by your application.
            Changing session ids helps protect against session hijacking. If the session id is
            changed, and an attacker does not know the new value, the attacker can not use the new
            session id in their attempts to hijack the visitor's session. Even if an attacker gains
            access to an old session id, <methodname>regenerateId()</methodname> also moves the
            session data from the old session id "handle" to the new one, so no data remains
            accessible via the old session id.
        </para>

        <para>
            When to use regenerateId(): Adding <methodname>Zend_Session::regenerateId()</methodname>
            to your Zend Framework bootstrap yields one of the safest and most secure ways to
            regenerate session id's in user agent cookies. If there is no conditional logic to
            determine when to regenerate the session id, then there are no flaws in that logic.
            Although regenerating on every request prevents several possible avenues of attack, not
            everyone wants the associated small performance and bandwidth cost. Thus, applications
            commonly try to dynamically determine situations of greater risk, and only regenerate
            the session ids in those situations. Whenever a website visitor's session's privileges
            are "escalated" (e.g. a visitor re-authenticates their identity before editing their
            personal "profile"), or whenever a security "sensitive" session parameter change occurs,
            consider using <methodname>regenerateId()</methodname> to create a new session id. If
            you call the <methodname>rememberMe()</methodname> function, then don't use
            <methodname>regenerateId()</methodname>, since the former calls the latter. If a user
            has successfully logged into your website, use <methodname>rememberMe()</methodname>
            instead of <methodname>regenerateId()</methodname>.
        </para>

        <sect3
            id="zend.session.global_session_management.session_identifiers.hijacking_and_fixation">
            <title>Session Hijacking and Fixation</title>

            <para>
                Avoiding <ulink url="http://en.wikipedia.org/wiki/Cross_site_scripting">cross-site
                    script (XSS) vulnerabilities</ulink> helps preventing session hijacking.
                According to <ulink url="http://secunia.com/">Secunia's</ulink> statistics XSS
                    problems occur frequently, regardless of the languages used to create web
                applications. Rather than expecting to never have a XSS problem with an application,
                plan for it by following best practices to help minimize damage, if it occurs. With
                XSS, an attacker does not need direct access to a victim's network traffic. If the
                victim already has a session cookie, Javascript XSS might allow an attacker to read
                the cookie and steal the session. for victims with no session cookies, using XSS to
                inject Javascript, an attacker could create a session id cookie on the victim's
                browser with a known value, then set an identical cookie on the attacker's system,
                in order to hijack the victim's session. If the victim visited an attacker's
                website, then the attacker can also emulate most other identifiable characteristics
                of the victim's user agent. If your website has an XSS vulnerability, the attacker
                might be able to insert an <acronym>AJAX</acronym> Javascript that secretly "visits"
                the attacker's website, so that the attacker knows the victim's browser
                characteristics and becomes aware of a compromised session at the victim website.
                However, the attacker can not arbitrarily alter the server-side state of
                <acronym>PHP</acronym> sessions, provided the developer has correctly set the value
                for the <code>save_path</code> option.
            </para>

            <para>
                By itself, calling <methodname>Zend_Session::regenerateId()</methodname> when the
                user's session is first used, does not prevent session fixation attacks, unless you
                can distinguish between a session originated by an attacker emulating the victim. At
                first, this might sound contradictory to the previous statement above, until we
                consider an attacker who first initiates a real session on your website. The session
                is "first used" by the attacker, who then knows the result of the initialization
                (<methodname>regenerateId()</methodname>). The attacker then uses the new session id
                in combination with an XSS vulnerability, or injects the session id via a link on
                the attacker's website (works if <code>use_only_cookies = off</code>).
            </para>

            <para>
                If you can distinguish between an attacker and victim using the same session id,
                then session hijacking can be dealt with directly. However, such distinctions
                usually involve some form of usability tradeoffs, because the methods of distinction
                are often imprecise. For example, if a request is received from an IP in a different
                country than the IP of the request when the session was created, then the new
                request probably belongs to an attacker. Under the following conditions, there might
                not be any way for a website application to distinguish between a victim and an
                attacker:

                <itemizedlist mark='opencircle'>
                    <listitem>
                        <para>
                            attacker first initiates a session on your website to obtain a valid
                            session id
                        </para>
                    </listitem>

                    <listitem>
                        <para>
                            attacker uses XSS vulnerability on your website to create a cookie on
                            the victim's browser with the same, valid session id (i.e. session
                            fixation)
                        </para>
                    </listitem>

                    <listitem>
                        <para>
                            both the victim and attacker originate from the same proxy farm (e.g.
                            both are behind the same firewall at a large company, like AOL)
                        </para>
                    </listitem>
                </itemizedlist>

                The sample code below makes it much harder for an attacker to know the current
                victim's session id, unless the attacker has already performed the first two steps
                above.
            </para>

            <example
                id="zend.session.global_session_management.session_identifiers.hijacking_and_fixation.example">
                <title>Session Fixation</title>

                <programlisting language="php"><![CDATA[
$defaultNamespace = new Zend_Session_Namespace();

if (!isset($defaultNamespace->initialized)) {
    Zend_Session::regenerateId();
    $defaultNamespace->initialized = true;
}
]]></programlisting>
            </example>
        </sect3>
    </sect2>

    <sect2 id="zend.session.global_session_management.rememberme">
        <title>rememberMe(integer $seconds)</title>

        <para>
            Ordinarily, sessions end when the user agent terminates, such as when an end user exits
            a web browser program. However, your application may provide the ability to extend user
            sessions beyond the lifetime of the client program through the use of persistent
            cookies. Use <methodname>Zend_Session::rememberMe()</methodname> before a session is
            started to control the length of time before a persisted session cookie expires. If you
            do not specify a number of seconds, then the session cookie lifetime defaults to
            <code>remember_me_seconds</code>, which may be set using
            <methodname>Zend_Session::setOptions()</methodname>. To help thwart session
            fixation/hijacking, use this function when a user successfully authenticates with your
            application (e.g., from a "login" form).
        </para>
    </sect2>

    <sect2 id="zend.session.global_session_management.forgetme">
        <title>forgetMe()</title>

        <para>
            This function complements <methodname>rememberMe()</methodname> by writing a session
            cookie that has a lifetime ending when the user agent terminates.
        </para>
    </sect2>

    <sect2 id="zend.session.global_session_management.sessionexists">
        <title>sessionExists()</title>

        <para>
            Use this method to determine if a session already exists for the current user
            agent/request. It may be used before starting a session, and independently of all other
            <classname>Zend_Session</classname> and <classname>Zend_Session_Namespace</classname>
            methods.
        </para>
    </sect2>

    <sect2 id="zend.session.global_session_management.destroy">
        <title>destroy(bool $remove_cookie = true, bool $readonly = true)</title>

        <para>
            <methodname>Zend_Session::destroy()</methodname> destroys all of the persistent data
            associated with the current session. However, no variables in <acronym>PHP</acronym> are
            affected, so your namespaced sessions (instances of
            <classname>Zend_Session_Namespace</classname>) remain readable. To complete a "logout",
            set the optional parameter to <constant>TRUE</constant> (the default) to also delete the
            user agent's session id cookie. The optional <varname>$readonly</varname> parameter
            removes the ability to create new <classname>Zend_Session_Namespace</classname>
            instances and for <classname>Zend_Session</classname> methods to write to the session
            data store.
        </para>

        <para>
            If you see the error message, "Cannot modify header information - headers already sent",
            then either avoid using <constant>TRUE</constant> as the value for the first argument
            (requesting removal of the session cookie), or see <xref
                linkend="zend.session.global_session_management.headers_sent" />. Thus,
            <methodname>Zend_Session::destroy(true)</methodname> must either be called before
            <acronym>PHP</acronym> has sent <acronym>HTTP</acronym> headers, or output buffering
            must be enabled. Also, the total output sent must not exceed the set buffer size, in
            order to prevent triggering sending the output before the call to
            <methodname>destroy()</methodname>.
        </para>

        <note>
            <title>Throws</title>

            <para>
                By default, <varname>$readonly</varname> is enabled and further actions involving
                writing to the session data store will throw an exception.
            </para>
        </note>
    </sect2>

    <sect2 id="zend.session.global_session_management.stop">
        <title>stop()</title>

        <para>
            This method does absolutely nothing more than toggle a flag in
            <classname>Zend_Session</classname> to prevent further writing to the session data
            store. We are specifically requesting feedback on this feature. Potential uses/abuses
            might include temporarily disabling the use of
            <classname>Zend_Session_Namespace</classname> instances or
            <classname>Zend_Session</classname> methods to write to the session data store, while
            execution is transferred to view- related code. Attempts to perform actions involving
            writes via these instances or methods will throw an exception.
        </para>
    </sect2>

    <sect2 id="zend.session.global_session_management.writeclose">
        <title>writeClose($readonly = true)</title>

        <para>
            Shutdown the session, close writing and detach <varname>$_SESSION</varname> from the
            back-end storage mechanism. This will complete the internal data transformation on this
            request. The optional <varname>$readonly</varname> boolean parameter can remove write
            access by throwing an exception upon any attempt to write to the session via
            <classname>Zend_Session</classname> or <classname>Zend_Session_Namespace</classname>.
        </para>

        <note>
            <title>Throws</title>

            <para>
                By default, <varname>$readonly</varname> is enabled and further actions involving
                writing to the session data store will throw an exception. However, some legacy
                application might expect <varname>$_SESSION</varname> to remain writable after
                ending the session via <methodname>session_write_close()</methodname>. Although not
                considered "best practice", the <varname>$readonly</varname> option is available for
                those who need it.
            </para>
        </note>
    </sect2>

    <sect2 id="zend.session.global_session_management.expiresessioncookie">
        <title>expireSessionCookie()</title>

        <para>
            This method sends an expired session id cookie, causing the client to delete the session
            cookie. Sometimes this technique is used to perform a client-side logout.
        </para>
    </sect2>

    <sect2 id="zend.session.global_session_management.savehandler">
        <title>setSaveHandler(Zend_Session_SaveHandler_Interface $interface)</title>

        <para>
            Most developers will find the default save handler sufficient. This method provides an
            object-oriented wrapper for <ulink
                url="http://php.net/session_set_save_handler"><methodname>session_set_save_handler()</methodname></ulink>.
        </para>
    </sect2>

    <sect2 id="zend.session.global_session_management.namespaceisset">
        <title>namespaceIsset($namespace)</title>

        <para>
            Use this method to determine if a session namespace exists, or if a particular index
            exists in a particular namespace.
        </para>

        <note>
            <title>Throws</title>

            <para>
                An exception will be thrown if <classname>Zend_Session</classname> is not marked as
                readable (e.g., before <classname>Zend_Session</classname> has been started).
            </para>
        </note>
    </sect2>

    <sect2 id="zend.session.global_session_management.namespaceunset">
        <title>namespaceUnset($namespace)</title>

        <para>
            Use <methodname>Zend_Session::namespaceUnset($namespace)</methodname> to efficiently
            remove an entire namespace and its contents. As with all arrays in
            <acronym>PHP</acronym>, if a variable containing an array is unset, and the array
            contains other objects, those objects will remain available, if they were also stored by
            reference in other array/objects that remain accessible via other variables. So
            <methodname>namespaceUnset()</methodname> does not perform a "deep" unsetting/deleting
            of the contents of the entries in the namespace. For a more detailed explanation, please
            see <ulink url="http://php.net/references">References Explained</ulink> in the
            <acronym>PHP</acronym> manual.
        </para>

        <note>
            <title>Throws</title>

            <para>
                An exception will be thrown if the namespace is not writable (e.g., after
                <methodname>destroy()</methodname>).
            </para>
        </note>
    </sect2>

    <sect2 id="zend.session.global_session_management.namespaceget">
        <title>namespaceGet($namespace)</title>

        <para>
            DEPRECATED: Use <methodname>getIterator()</methodname> in
            <classname>Zend_Session_Namespace</classname>. This method returns an array of the
            contents of <varname>$namespace</varname>. If you have logical reasons to keep this
            method publicly accessible, please provide feedback to the <ulink
                url="mailto:fw-auth@lists.zend.com">fw-auth@lists.zend.com</ulink> mail list.
            Actually, all participation on any relevant topic is welcome :)
        </para>

        <note>
            <title>Throws</title>

            <para>
                An exception will be thrown if <classname>Zend_Session</classname> is not marked as
                readable (e.g., before <classname>Zend_Session</classname> has been started).
            </para>
        </note>
    </sect2>

    <sect2 id="zend.session.global_session_management.getiterator">
        <title>getIterator()</title>

        <para>
            Use <methodname>getIterator()</methodname> to obtain an array containing the names of
            all namespaces.
        </para>

        <note>
            <title>Throws</title>

            <para>
                An exception will be thrown if <classname>Zend_Session</classname> is not marked as
                readable (e.g., before <classname>Zend_Session</classname> has been started).
            </para>
        </note>
    </sect2>
</sect1>