What are cookies for?
Cookies have different functions. They can enable the person who left them to recognise an Internet user, from one visit to another, by means of a unique identifier.
Some cookies can also be used to store the contents of a shopping cart, others can be used to record a site's language settings, and others can be used for targeted advertising.
Who can leave cookies and access them?
A website can only read and write cookies which belong to it.
But it must be borne in mind that a webpage often contains information from different sites, including via banner advertisements.
So cookies can be left and read by sites other than the one that you access directly…
For example, if you browse an information site, it is possible that other sites such as advertising networks will collect information about your browsing. To do that, all the advertising network needs to do is to leave a cookie on the information site that you look at.
How do these advertising cookies work?
Advertising cookies record details of Internet users' browsing and can use them to work out their interests or browsing path on the website. They then enable companies to personalise the advertising that the Internet user will see according to what they have done on the visited site and also other sites which use the same advertising network.
Social network share buttons ("Like" on Facebook, "+1" on Google, etc.) can be used to monitor you in the same way and suggest targeted advertising to you.
How do social network buttons such as the "Like" button on Facebook and Twitter or "+1" on Google work?
When you visit a webpage featuring one of these buttons, the social network can associate this viewing with your profile, even if you do not click on the button and are not connected to this network! So the social network can adapt its advertising to sites that you have visited and suggest "groups" which are appropriate to the interests deduced from your web browsing. It can also suggest that you become a fan of the webpage that you visit the most.
What does the law say about cookies?
The regulations state that websites must obtain your consent before leaving these cookies, tell you what they are used for and tell you how you can object to them. In practice, a message must appear when you connect to the site for the first time to tell you how to accept or reject cookies.
How can cookies be blocked?
You will find advice about installing tools which enable you to block certain cookies when you browse. For professionals, we offer factsheets so that they can fulfil their legal obligations.
How can I find out what cookies are left when I browse?
CNIL's experts have developed a tool called CookieViz which you can install with just a few clicks. It will give you a real-time view of the scale of the phenomenon of cookies and the considerable number of entities that are involved in analysing your browsing.
Source: CNIL website