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# Sessions and security

External links: » Session fixation

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.

- session.cookie_lifetime=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.
- session.use_cookies=On and session.use_only_cookies=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.
- session.use_strict_mode=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.
- session.cookie_httponly=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.
- session.cookie_secure=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.
- session.gc_maxlifetime=[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 session.gc_probability and session.gc_divisor 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.
- session.use_trans_sid=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.
- session.referer_check=[your originating URL] When session.use_trans_sid 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.
- session.cache_limiter=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.
- session.hash_function="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.

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.

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 session.use_only_cookies. In that case, cookies must be enabled unconditionally on the user side, or sessions will not work.

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.

Since PHP 5.5.2, session.use_strict_mode 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 session.use_trans_sid is enabled, victim will start session using attacker provided session ID. session.use_strict_mode mitigates the risk.

Even though session.use_strict_mode 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.

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 session.use_only_cookies will not solve this issue. session.use_strict_mode mitigates this risk. With session.use_strict_mode=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.

session.use_strict_mode is good mitigation, but it is not enough mitigation for authenticated session. Developer must use session_regenerate_id() for authentication. session_regenerate_id() must be called prior to set authentication information to $_SESSION. session_regenerate_id() 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.

Calling session_regenerate_id() 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.

Note that session_regenerate_id() 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 `delete_old_session` 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.

session.use_only_cookies and proper use of session_regenerate_id() 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.

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.

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