php-doc-en/reference/session/security.xml

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<chapter xml:id="session.security" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>Sessions and security</title>
 <para>
  External links: <link xlink:href="&url.session-fixation;">Session fixation</link>
 </para>
 <para>
  HTTP session management is core of web security. All of mitigation
  should be adopted to make sure session security. Developer should
  enable/use applicable settings appropriately.
 </para>

 <itemizedlist>
  <listitem>
   <simpara>
    <link linkend="ini.session.cookie-lifetime">session.cookie_lifetime</link>=0.
    0 have special meaning. It tells browsers not to store cookie to
    permanent storage. Therefore, when browser is terminated, session
    ID cookie is deleted immediately. If developer set other than 0, it may
    allow other users to use the session ID. Most applications should
    use "0" for this. If auto login feature is required, implement
    your own secure auto login feature. Do not use session ID for it.
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.use-cookies">session.use_cookies</link>=On and
    <link linkend="ini.session.use-only-cookies">session.use_only_cookies</link>=On.
    Although HTTP cookie has some problems, cookie is preferred way to
    manage session ID. Use only cookies for session ID management when
    it is possible. Most applications should use cookie for session
    ID.
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.use-strict-mode">session.use_strict_mode</link>=On.
    This prevents session module to use uninitialized session ID. In
    other word, session module only accepts valid session ID generated
    by session module. It rejects session ID supplied by
    users. Session ID injection could be done by cookie injection via
    JavaScript permanently or temporarily. When transparent session is
    enabled, session ID could be injected via query string or form
    parameter. There is no reason to accept user supplied session ID,
    most applications must not accept user supplied uninitialized
    session ID.
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.cookie-httponly">session.cookie_httponly</link>=On.
    Disallow access to session cookie by JavaScript. This setting
    prevents cookies stolen by JavaScript injection.  It is possible
    to use session ID as CSRF protection key, but this is not
    recommended. For example, HTML source may be saved and sent to
    other users. Developer should not write session ID in web pages for
    better security. Almost all applications must use httponly attribute for
    session ID cookie.
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.cookie-secure">session.cookie_secure</link>=On.
    Allows access to session ID cookie only when protocol is HTTPS. If
    your web site is HTTPS only web site, you must enable this
    setting. Use of HSTS should be considered for HTTPS only web site.
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.gc-maxlifetime">session.gc_maxlifetime</link>=[choose smallest possible].
    GC is performed by probability. This setting does not guarantee old
    session deletion. Some session save handler modules do not use
    this setting. Refer to session save handler documentation for
    details. Although developer cannot rely on this setting, setting this
    to smallest possible value is recommended. Adjust <link
    linkend="ini.session.gc-probability">session.gc_probability</link>
    and <link
    linkend="ini.session.gc-divisor">session.gc_divisor</link> so that
    obsolete sessions are deleted by appropriate frequency. If auto login
    feature is required, implement your own secure auto login feature.
    Do not use long life session ID for it.
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.use-trans-sid">session.use_trans_sid</link>=Off.
    Use of transparent session ID management is not prohibited. You
    may use it when it is needed. However, disabling transparent
    session ID management would improve general session ID security by
    removing possibility of session ID injection and session ID leak.
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.referer-check">session.referer_check</link>=[your originating URL]
    When <link
    linkend="ini.session.use-trans-sid">session.use_trans_sid</link>
    is enabled, use of this setting is recommended if it is
    possible. It reduces risk of session ID injection. If your site is
    http://example.com/, set http://example.com/ to it. Note that when
    HTTPS is used, browser will not send referrer header. Browser may
    not send referrer header by configuration. Therefore, this setting
    is not reliable security measure. 
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.cache-limiter">session.cache_limiter</link>=nocache.
    Make sure HTTP contents are not cached for authenticated
    session. Allow caching only when contents is not
    private. Otherwise, contents may be exposed. "private" may be used
    if HTTP content does not include security sensitive data. Note
    that "private" may leave private data cached by shared
    clients. "public" may be used only when HTTP content does not
    contain any private data at all.
   </simpara>
  </listitem>

  <listitem>
   <simpara>
    <link linkend="ini.session.hash-function">session.hash_function</link>="sha256".
    Stronger hash function will generates stronger session
    ID. Although hash collision is unlikely even with MD5 hash, developers
    should use SHA-2 or later hash functions for the task. Developers may
    use stronger hashes like sha384 and sha512.
   </simpara>
  </listitem>
 </itemizedlist>
 
 <para>
  The session module cannot guarantee that the information you store
  in a session is only viewed by the user who created the session. You
  need to take additional measures to actively protect the confidentiality
  of the session, depending on the value associated with it.
 </para>

 <para>
  Assess the importance of the data carried by your sessions and
  deploy additional protections -- this usually comes at a price,
  reduced convenience for the user.  For example, if you want to
  protect users from simple social engineering tactics, you need to
  enable <literal>session.use_only_cookies</literal>.  In that case,
  cookies must be enabled unconditionally on the user side, or
  sessions will not work.
 </para>

 <para>
  There are several ways to leak an existing session ID to third
  parties. A leaked session ID enables the third party to access all
  resources which are associated with a specific ID.  First, URLs
  carrying session IDs. If you link to an external site, the URL
  including the session id might be stored in the external site's
  referrer logs. Second, a more active attacker might listen to your
  network traffic. If it is not encrypted, session IDs will flow in
  plain text over the network. The solution here is to implement SSL
  on your server and make it mandatory for users.  HSTS should be used
  for this.
 </para>

 <para>
  Since PHP 5.5.2, <link
  linkend="ini.session.use-strict-mode">session.use_strict_mode</link>
  is available. When it is enabled and save handler module supports
  it, uninitialized session ID is rejected and new session ID is
  created. This protects attack that force users to use known session
  ID. Attacker may paste links or send mail that contains session
  ID. e.g. http://example.com/page.php?PHPSESSID=123456789 If <link
  linkend="ini.session.use-trans-sid">session.use_trans_sid</link> is
  enabled, victim will start session using attacker provided session
  ID.  <link
  linkend="ini.session.use-strict-mode">session.use_strict_mode</link>
  mitigates the risk.
 </para>

 <para>
  Even though <link
  linkend="ini.session.use-strict-mode">session.use_strict_mode</link>
  mitigates risk of adoptive session management, attacker can force
  users to use initialized session ID which is created by
  attacker. All attacker has to do is initialize session ID prior to
  attack and keep it alive.
 </para>

 <para>
  Session ID cookie could be set with domain, path, httponly, secure
  attributes. There is precedence defined by browsers. By using the
  precedence, attacker can set session ID that could be used
  permanently. Use of <link
  linkend="ini.session.use-only-cookies">session.use_only_cookies</link>
  will not solve this issue. <link
  linkend="ini.session.use-strict-mode">session.use_strict_mode</link>
  mitigates this risk. With <link
  linkend="ini.session.use-strict-mode">session.use_strict_mode</link>=On,
  uninitialized session ID will not be accepted. Session module
  creates new session ID always when session ID is not initialized by
  session module. This could result in DoS to victim, but DoS is
  better than compromised account.
 </para>

 <para>
  <link
  linkend="ini.session.use-strict-mode">session.use_strict_mode</link>
  is good mitigation, but it is not enough mitigation for
  authenticated session. Developer must use
  <function>session_regenerate_id</function> for authentication.
  <function>session_regenerate_id</function> must be called prior to
  set authentication information to
  $_SESSION. <function>session_regenerate_id</function> makes sure new
  session contains authenticated information stored only in new
  session. i.e. Errors during authentication process may save
  authenticated flag in old session.
 </para>

 <para>
  Calling <function>session_regenerate_id</function> function could
  result in personal DoS like use_strict_mode=On. However, DoS is
  better than compromised account. Session ID should be regenerated
  when user is authenticated at least. Session ID regeneration reduces
  risk of stolen session ID, thus is should be called periodically.
  Developer should not rely on session ID expiration. Attackers may
  access victim's session ID periodically to prevent expiration.
  Developers must implement their own expiration feature for old sessions.
 </para>

 <para>
  Note that <function>session_regenerate_id</function> does not delete
  old session by default. Old authenticated session may be available
  for use. If developer would like to prevent old authenticated session to
  be used by anyone, developer must destroy session by setting
  <parameter>delete_old_session</parameter> to &true;. However,
  immediate old session deletion has unwanted side effect. Session
  could be vanished when there are concurrent connections to web
  application and/or network is unstable. Instead of deleting old
  session immediately, you may set short term expiration time in
  $_SESSION by yourselves. If user accesses to obsolete
  session(expired session), deny access to it.
 </para>

 <para>
  <link
  linkend="ini.session.use-only-cookies">session.use_only_cookies</link>
  and proper use of <function>session_regenerate_id</function> could
  cause personal DoS. When this is the case, you may ask users to
  remove cookies and warn users that there could be possible security
  issues. Attackers may set malicious cookies via vulnerable web
  application (i.e. JavaScript injection), vulnerable/malicious
  browser plugins, etc.
 </para>

 <para>
  Developers must not use long life session ID for auto login as it
  increases risk of stolen session. Auto login should be implemented
  by developer. Use secure one time hash key as auto login key using
  cookie. Use secure hash stronger than SHA-2. e.g. SHA-256 or greater
  with random data from /dev/urandom or like. If user is not
  authenticated, check the one time auto login key is valid or not. If
  key is valid, authenticate user and set new secure one time hash
  key. Auto login key is long lived authentication key, this key should
  be protected as much as possible. Use path/httponly/secure
  attributes of cookie to protect. Developer must implement feature that
  disables auto login and removes unneeded auto login key cookie for
  users.
 </para>
</chapter>

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