PHP RFC: Make session_regenerate_id() reliable


This RFC is renamed. Refer to the latest


Keeping HTTP session as secure as possible is what the session manager's task. Session manager can improve HTTP session security without user code modification while keeping compatibility with existing applications. Please note that this RFC is for session manager behavior.

session_regenerate_id() is used to generate new session ID. It's better to delete old session data to reduce risk of session hijack. However, session_regenerate_id() leave old session data by default currently. (i.e. session_regenerate_id(FALSE) is the default) Old session data is active and usable until GC.

Old session is left active for reliable session ID regeneration. There are many reasons why old session is left. Examples are:

  • Browsers connect to web server with multiple connections.
  • Mobile network may loose radio, may have hand over, etc.
  • Large network providers have multiple gateways for off loading traffic and packets may arrive out of order.

For reliable session ID regeneration, only short periods (few seconds for wired connections, few minutes for mobile connection) is enough.

Leaving old session opens window to attacker widely:

  1. Old session lives long term and never expires if there is access to it. i.e. Attacker may abuse stolen session forever.
  2. There is no mechanism to detect possibles attack even if session manager may detect attacks.

Counter measure for session hijack: Requirement - Session ID regeneration must be reliable.

  1. Make sure old session is deactivated/deleted after certain period.
  2. Raise error/exception for invalid access. (Raise error for should be deleted session access)

Problem of immediate old session deletion:

  1. Make session ID regeneration unreliable. (Unacceptable)
  2. Remove alarm for possible attacks. (No detection = Insecure)

“Make sure old session is deleted certain period” and “Raise error/exception for invalid access” provides much better security than current way or immediate deletion.

Errors may be raised for either legitimate user or attacker. If error is raised for legitimate user, legitimate user could know they are under attack. (Possibly network is dangerous or app has vulnerability) If error is raised for attacker, attacker could know they might be caught by illegal access.

Risk of stolen session

Stealing session ID is easy regardless of HTTPS. Attacker can set up fake router by ARP spoofing. Most networks do not have ARP spoofing prevention, even detection. For HTTP, attacker can view session ID simply. For HTTPS, attacker can set up transparent HTTPS stripping proxy and steal session ID. Most users do not care much if they are connecting via HTTPS or not.

Above attack can be done by existing tools. i.e. Script kiddies' task.

If you are curious, search YouTube or net.

This is known design issue for a long time

Even if there is only recent bug report for this, this bug is known more than 10 years since when session_regenerate_id() is introduced.



Add transparent __SESSION_DESTROY_TTL__ timestamp

Add 'session_destory_ttl' INI directive(INI_ALL, default 300 seconds) and “make sure old session is deleted certain period”.

This can be achieved by time stamp in session data. When session_regenerate_id()/session_destroy() is called without parameter, session manager sets

  $_SESSION['__SESSION_DESTORY_TTL__'] = time() + ini_get('session.regenerate_id_expire');

for old session data. This is pseudocode. User will never see $_SESSION['SESSION_DESTORY_TTL'] as it is removed/added upon session data serialization internally in session module.

$_SESSION['__SESSION_DESTORY_TTL__'] also stores new session ID when TTL is set by session_regenerate_id().


If browser accesses to be deleted session (old session), session module uses new session ID rather than old and try to set correct new ID. i.e. Send new session ID cookie to browser. This prevents lost session under unstable network.

If session module finds $_SESSION['__SESSION_DESTORY_TTL__'] and timestamp is old, delete old session data and create new session with new session ID. E_WARNING error is raised for this because it means either too short TTL or user is under attack.

When session_regenerate_id(true)/session_destroy(true) is called, session module destroy session data immediately.

Users may add $_SESSION['__SESSION_DESTORY_TTL__']. When this is happen, session module raise E_WARNING for this.

Why TTL default is 300 seconds and configurable

Session data may be lost when network connection is unstable. For example, when user ender elevator or subway connection can be lost in a way that session data is lost. 300 seconds would be enough for most elevators. However, it may not be enough for subways. PHP developer may require longer TTL for better stability.

Some PHP developers may want to be more strict and need shorter TTL even if it could result in lost session on occasions. They may set 30 seconds TTL which would be long enough for stable connection in most cases.

Why this is secure than now

Currently, users must call session_regenerate_id() without destroy flag to have stable session. Therefore, old session data is valid as long as it is accessed even if it should be discarded as invalid session. Attackers can take advantage of this behavior to keep stolen session forever.

Backward Incompatible Changes

  • If user script has __SESSION_DESTROY_TTL__ key in $_SESSION, it may break application.
  • Raised error may break application.

Proposed PHP Version(s)

PHP 7.0

SAPIs Impacted

  • None

Impact to Existing Extensions

  • Session

New Constants

  • None

php.ini Defaults

If there are any php.ini settings then list:

  • hardcoded default values
  • php.ini-development values
  • php.ini-production values
  • “session.destroy_ttl = 300” for all. (300 seconds, INI_ALL)

Open Issues

Make sure there are no open issues when the vote starts!

Unaffected PHP Functionality

Other than session management, there is no affected functionality.

Future Scope

Session expire and GC can be improved by time stamp also.


  • Add __SESSION_DESTORY_TTL__ time stamp Yes/No

Patches and Tests




  • 2015/03/21 - Added new session ID handling.
  • 2015/03/20 - Change INI directive name.
  • 2014/03/19 - Add exception option as Stas suggested.
  • 2014/03/18 - Change RFC to propose time stamping.
  • 2013/10/30 - Added details and message option.
  • 2013/10/29 - Created RFC
rfc/session_regenerate_id.txt · Last modified: 2017/09/22 13:28 by