Cookies provide a method for creating a stateful HTTP session and their recommended use is formally defined within RFC2965 and BCP44.
Although they are used for many purposes, they are often used to maintain a Session ID (SID), through which an individual user can be identified throughout their interaction with the site. For a site that requires authentication, this SID is typically passed to the user after they have authenticated and effectively maintains the authentication state. If an attacker can use a mechanism (such as sniffing or cross site scripting) to gain access to the SID, then potentially they can incorporate it within their own session to successfully assume the users identity. The cookie specifications provide arguments for restricting the domain and path for which the user agent (browser) will supply the cookie. Both of these should be matched by the request before the user agent sends the cookie data to the server.
It is common for the path argument to be specified as the root of the origin server; a practise that can expose the application cookies to unnecessary additional scrutiny. It is worth noting however, that whilst the various “same origin” security issues still afflict the browser vendors, the specification of the cookie path argument is somewhat of a moot point.
Download the paper in PDF format here.