Computers that have Adobe Flash installed which most computers do, small files may be stored on your computer by websites that contain Flash media, such as video clips. These files are known as Local Shared Objects (LSOs) or Flash cookies.
HttpOnly flag to the cookie.
Instead of expiring when the web browser is closed as session cookies do, a persistent cookie expires at a specific date or after a specific length of time. This means that, for the cookie's entire lifespan (which can be as long or as short as its creators want), its information will be transmitted to the server every time the user visits the website that it belongs to, or every time the user views a resource belonging to that website from another website (such as an advertisement).
For this reason, persistent cookies are sometimes referred to as tracking cookies because they can be used by advertisers to record information about a user's web browsing habits over an extended period of time. However, they are also used for "legitimate" reasons (such as keeping users logged into their accounts on websites, to avoid re-entering login credentials at every visit).
These cookies are however reset if the expiration time is reached or the user manually deletes the cookie.
In 2016 Google Chrome version 51 introduced a new kind of cookie which can only be sent in requests originating from the same origin as the target domain. This restriction mitigates attacks such as cross-site request forgery (XSRF). A cookie is given this characteristic by setting the
SameSite flag to
A secure cookie can only be transmitted over an encrypted connection (i.e. HTTPS). They cannot be transmitted over un-encrypted connections (i.e. HTTP). This makes the cookie less likely to be exposed to cookie theft via eavesdropping. A cookie is made secure by adding the
Secure flag to the cookie.
A session cookie, also known as an in-memory cookie or transient cookie, exists only in temporary memory while the user navigates the website. Web browsers normally delete session cookies when the user closes the browser. Unlike other cookies, session cookies do not have an expiration date assigned to them, which is how the browser knows to treat them as session cookies.
A supercookie is a cookie with an origin of a top-level domain (such as
.com) or a public suffix (such as
.co.uk). Ordinary cookies, by contrast, have an origin of a specific domain name, such as
Supercookies can be a potential security concern and are therefore often blocked by web browsers. If unblocked by the browser, an attacker in control of a malicious website could set a supercookie and potentially disrupt or impersonate legitimate user requests to another website that shares the same top-level domain or public suffix as the malicious website. For example, a supercookie with an origin of
.com, could maliciously affect a request made to
example.com, even if the cookie did not originate from
example.com. This can be used to fake logins or change user information.
The Public Suffix List helps to mitigate the risk that supercookies pose. The Public Suffix List is a cross-vendor initiative that aims to provide an accurate and up-to-date list of domain name suffixes. Older versions of browsers may not have an up-to-date list, and will therefore be vulnerable to supercookies from certain domains.
Normally, a cookie's domain attribute will match the domain that is shown in the web browser's address bar. This is called a first-party cookie. A third-party cookie, however, belongs to a domain different from the one shown in the address bar. This sort of cookie typically appears when web pages feature content from external websites, such as banner advertisements. This opens up the potential for tracking the user's browsing history, and is often used by advertisers in an effort to serve relevant advertisements to each user.
As an example, suppose a user visits
www.example.org. This web site contains an advertisement from
ad.foxytracking.com, which, when downloaded, sets a cookie belonging to the advertisement's domain (
ad.foxytracking.com). Then, the user visits another website,
www.foo.com, which also contains an advertisement from
ad.foxytracking.com, and which also sets a cookie belonging to that domain (
ad.foxytracking.com). Eventually, both of these cookies will be sent to the advertiser when loading their advertisements or visiting their website. The advertiser can then use these cookies to build up a browsing history of the user across all the websites that have ads from this advertiser.
As of 2014, some websites were setting cookies readable for over 100 third-party domains. On average, a single website was setting 10 cookies, with a maximum number of cookies (first- and third-party) reaching over 800.
Most modern web browsers contain privacy settings that can block third-party cookies.
A zombie cookie is a cookie that is automatically recreated after being deleted. This is accomplished by storing the cookie's content in multiple locations, such as Flash Local shared object, HTML5 Web storage, and other client-side and even server-side locations. When the cookie's absence is detected, the cookie is recreated using the data stored in these locations.