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CRV2 SessionHandling

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Session Management

A web session is a sequence of network HTTP request and response transactions associated to the same user. Session management or state is needed by web applications that require the retaining of information or status about each user for the duration of multiple requests. Therefore, sessions provide the ability to establish variables – such as access rights and localization settings – which will apply to each and every interaction a user has with the web application for the duration of the session.

Code reviewer needs to understand what session techniques the developers used, and how to spot vulnerabilities that may create potential security risks.

Web applications can create sessions to keep track of anonymous users after the very first user request. An example would be maintaining the user language preference. Additionally, web applications will make use of sessions once the user has authenticated. This ensures the ability to identify the user on any subsequent requests as well as being able to apply security access controls, authorized access to the user private data, and to increase the usability of the application. Therefore, current web applications can provide session capabilities both pre and post authentication.

Session-Management-Diagram Cheat-Sheet.png

The session ID or token binds the user authentication credentials (in the form of a user session) to the user HTTP traffic and the appropriate access controls enforced by the web application. The complexity of these three components (authentication, session management, and access control) in modern web applications, plus the fact that its implementation and binding resides on the web developer’s hands (as web development framework do not provide strict relationships between these modules), makes the implementation of a secure session management module very challenging.

The disclosure, capture, prediction, brute force, or fixation of the session ID will lead to session hijacking (or sidejacking) attacks, where an attacker is able to fully impersonate a victim user in the web application. Attackers can perform two types of session hijacking attacks, targeted or generic. In a targeted attack, the attacker’s goal is to impersonate a specific (or privileged) web application victim user. For generic attacks, the attacker’s goal is to impersonate (or get access as) any valid or legitimate user in the web application.

With the goal of implementing secure session IDs, the generation of identifiers (IDs or tokens) must meet the following properties:

  1. The name used by the session ID should not be extremely descriptive nor offer unnecessary details about the purpose and meaning of the ID.
  2. It is recommended to change the default session ID name of the web development framework to a generic name, such as “id”.
  3. The session ID length must be at least 128 bits (16 bytes) (The session ID value must provide at least 64 bits of entropy).
  4. The session ID content (or value) must be meaningless to prevent information disclosure attacks, where an attacker is able to decode the contents of the ID and extract details of the user, the session, or the inner workings of the web application.
  5. It is recommended to create cryptographically strong session IDs through the usage of cryptographic hash functions such as SHA1 (160 bits).

Recommendation: Require cookies.

Require cookies when your application includes authentication. Code reviewer needs to understand what information is stored in the application cookies. Risk management is needed to address if sensitive information is stored in the cookie requiring SSL for the cookie


The following example shows how to specify in the Web.config file that Forms Authentication requires a cookie that is transmitted over SSL.

<authentication mode="Forms">
 <forms loginUrl="member_login.aspx"
   path="/MyApplication" />

The ASP.NET session identifier is a randomly generated number encoded into a 24-character string consisting of lowercase characters from a to z and numbers from 0 to 5.

The SessionID value by default is sent in a cookie with each request to the ASP.NET application.

Custom Session frameworks

Web development frameworks, such as J2EE, ASP .NET, PHP, and others, provide their own session management features and associated implementation. It is recommended to use these built-in frameworks versus building a home made one from scratch, as they are used worldwide on multiple web environments and have been tested by the web application security and development communities over time.

This code shows a developer who is replacing the default session management in .Net with his own custom class. The secure code reviewer needs to understand why a custom session class is being used and if if a risk assessment has been done on the custom session management class.


 <remove name="SessionID" />
 <add name="SessionID"
      type="Samples.AspNet.Session.GuidSessionIDManager" />


In PHP, a session handled by cookies is created when session_start() is called. All security related options for session cookies are in PHP's configuration file php.ini and can be also be set with session_set_cookie_params().

session.use_cookies = 1
session.use_only_cookies = 1
session.cookie_httponly = 1
session.cookie_secure = 1

See for more information.

Session Expiration

In review session handling code the reviewer needs to understand what expiration timeouts are needed by the web application or if default session timeout are being used. Insufficient session expiration by the web application increases the exposure of other session-based attacks, as for the attacker to be able to reuse a valid session ID and hijack the associated session, it must still be active. Remember for secure coding one of our goals is to reduce the attack surface of our application.


ASPX the developer can change the default time out for a session. This code in the web.config file sets the timeout session to 15 minutes. The default timeout for a aspx session is 30 minutes.

     timeout="15" />

Session Logout/Ending.

Web applications should provide mechanisms that allow security aware users to actively close their session once they have finished using the web application.

.Net ASPX Session.Abandon() method destroys all the objects stored in a Session object and releases their resources. If you do not call the Abandon method explicitly, the server destroys these objects when the session times out. You should use it when the user logs out. Session.Clear() Removes all keys and values from the session. Does not change session ID. Use this command if you if you don't want the user to relogin and reset all the session specific data.

Session Attacks

Generally three sorts of session attacks are possible:

  1. Session Hijacking: stealing someone's session-id, and using it to impersonate that user.
  2. Session Fixation: setting someone's session-id to a predefined value, and impersonating them using that known value
  3. Session Elevation: when the importance of a session is changed, but its ID is not.

Session Hijacking

  1. Mostly done via XSS attacks, mostly can be prevented by HTTP-Only session cookies (unless Javascript code requires access to them).
  2. (charly proposes to eliminate this...) It's generally a good idea for Javascript not to need access to session cookies, as preventing all flavors of XSS is usually the toughest part of hardening a system.
  3. Session-ids should be placed inside cookies, and not in URLs. URL informations are stored in browser's history, and HTTP Referrers, and can be accessed by attackers.
  4. (...and add this) As cookies can be accessed by default from javascript and preventing all flavors of XSS is usually the toughest part of hardening a system, there is an attribute called "HTTPOnly", that forbids this access. The session cookie should has this attribute set. Anyway, as there is no need to access a session cookie from the client, you should get suspicious about client side code that depends on this access.
  5. Geographical location checking can help detect simple hijacking scenarios. Advanced hijackers use the same IP (or range) of the victim.
  6. An active session should be warned when it is accessed from another location.
  7. An active users should be warned when s/he has an active session somewhere else (if the policy allows multiple sessions for a single user).

Session Fixation

  1. If the application sees a new session-id that is not present in the pool, it should be rejected and a new session-id should be advertised. This is the sole method to prevent fixation.
  2. All the session-ids should be generated by the application, and then stored in a pool to be checked later for. Application is the sole authority for session generation.

Session Elevation

  1. Whenever a session is elevated (login, logout, certain authorization), it should be rolled.
  2. Many applications create sessions for visitors as well (and not just authenticated users). They should definitely roll the session on elevation, because the user expects the application to treat them securely after they login.
  3. When a down-elevation occurs, the session information regarding the higher level should be flushed.
  4. Sessions should be rolled when they are elevated. Rolling means that the session-id should be changed, and the session information should be transferred to the new id.

Server-Side Defenses for Session Management.


Generate a new session ID

Generating new session Id's helps prevent, session rolling, fixation, hijacking.

public class GuidSessionIDManager : SessionIDManager {
   public override string CreateSessionID(HttpContext context){
return Guid.NewGuid().ToString();
   public override bool Validate(string id) {
       Guid testGuid = new Guid(id);
       if (id == testGuid.ToString())
         return true;
     }catch(Exception e) { throw e }
     return false;