Alternatives

Browser Cache

The browser cache can also be used to store information that can be used to track individual users. This technique takes advantage of the fact that the web browser will use resources stored within the cache instead of downloading them from the website when it determines that the cache already has the most up-to-date version of the resource.

For example, a website could serve a JavaScript file that contains code which sets a unique identifier for the user (for example, var userId = 3243242;). After the user's initial visit, every time the user accesses the page, this file will be loaded from the cache instead of downloaded from the server. Thus, its content will never change.

Hidden Form Fields

Another form of session tracking is to use web forms with hidden fields. This technique is very similar to using URL query strings to hold the information and has many of the same advantages and drawbacks. In fact, if the form is handled with the HTTP GET method, then this technique is similar to using URL query strings, since the GET method adds the form fields to the URL as a query string. But most forms are handled with HTTP POST, which causes the form information, including the hidden fields, to be sent in the HTTP request body, which is neither part of the URL, nor of a cookie.

This approach presents two advantages from the point of view of the tracker. First, having the tracking information placed in the HTTP request body rather than in the URL means it will not be noticed by the average user. Second, the session information is not copied when the user copies the URL (to bookmark the page or send it via email, for example).

HTTP Authentication

The HTTP protocol includes the basic access authentication and the digest access authentication protocols, which allow access to a web page only when the user has provided the correct username and password. If the server requires such credentials for granting access to a web page, the browser requests them from the user and, once obtained, the browser stores and sends them in every subsequent page request. This information can be used to track the user.

IP Tracking

Some users may be tracked based on the IP address of the computer requesting the page. The server knows the IP address of the computer running the browser (or the proxy, if any is used) and could theoretically link a user's session to this IP address.

However, IP addresses are generally not a reliable way to track a session or identify a user. Many computers designed to be used by a single user, such as office PCs or home PCs, are behind a network address translator (NAT). This means that several PCs will share a public IP address. Furthermore, some systems, such as Tor, are designed to retain Internet anonymity, rendering tracking by IP address impractical, impossible, or a security risk.

JSON Web Tokens

JSON Web Token (JWT) is a self-contained packet of information that can be used to store user identity and authenticity information. This allows them to be used in place of session cookies. Unlike cookies, which are automatically attached to each HTTP request by the browser, JWTs must be explicitly attached to each HTTP request by the web application.

URL Query String

A more precise technique is based on embedding information into URLs. The query string part of the URL is the part that is typically used for this purpose, but other parts can be used as well. The Java Servlet and PHP session mechanisms both use this method if cookies are not enabled.

This method consists of the web server appending query strings containing a unique session identifier to all the links inside of a web page. When the user follows a link, the browser sends the query string to the server, allowing the server to identify the user and maintain state.

These kinds of query strings are very similar to cookies in that both contain arbitrary pieces of information chosen by the server and both are sent back to the server on every request. However, there are some differences. Since a query string is part of a URL, if that URL is later reused, the same attached piece of information will be sent to the server, which could lead to confusion. For example, if the preferences of a user are encoded in the query string of a URL and the user sends this URL to another user by e-mail, those preferences will be used for that other user as well.

Moreover, if the same user accesses the same page multiple times from different sources, there is no guarantee that the same query string will be used each time. For example, if a user visits a page by coming from a page internal to the site the first time, and then visits the same page by coming from an external search engine the second time, the query strings would likely be different. If cookies were used in this situation, the cookies would be the same.

Other drawbacks of query strings are related to security. Storing data that identifies a session in a query string enables session fixation attacks, referrer logging attacks and other security exploits. Transferring session identifiers as HTTP cookies is more secure.

"Window.Name" DOM Property

All current web browsers can store a fairly large amount of data (2–32 MB) via JavaScript using the DOM property window.name. This data can be used instead of session cookies and is also cross-domain. The technique can be coupled with JSON/JavaScript objects to store complex sets of session variables on the client side.

The downside is that every separate window or tab will initially have an empty window.name property when opened. Furthermore, the property can be used for tracking visitors across different websites, making it of concern for Internet privacy.

In some respects, this can be more secure than cookies due to the fact that its contents are not automatically sent to the server on every request like cookies are, so it is not vulnerable to network cookie sniffing attacks. However, if special measures are not taken to protect the data, it is vulnerable to other attacks because the data is available across different websites opened in the same window or tab.

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