## 6 User Tracking and Profiling Techniques
Temporary cookies are stored in the browser cache and are expired as soon as the browser gets closed [32]. In contrast, permanent cookies have an expiration date and remain in the browser memory until that particular point in time (i.e., they can survive throughout multiple sessions). (For more information on other less common forms of storage-based tracking techniques, see [25].) 3. Cache-based Tracking. The cache-based tracking mechanisms identify a browser instance and its respective user by deploying various types of caches such as Web cache [24, 33], DNS cache [34], and Operational caches [24, 35]. Namely, by relying on some distinctive items stored in these caches (e.g., previously acquired images or DNS records), it is possible to determine whether this specific user has already visited a given website or not. 4. Fingerprinting. This approach to user tracking relies on different methods that can facilitate the extraction of unique identifiers associated with a user's device (e.g., IP address, operating system, browser version, system and user languages deployed, etc.) [36, 37, 38]. These identifiers can then assist in tracking the user across multiple websites. For example, JavaScript and Flash can distinguish between different versions and architectures of operating systems and are referred to as the Operating System Instance Fingerprinting technique [39, 40]. Furthermore, JavaScript can also facilitate the acquiring of information about the user's local time zone and local date in milliseconds. Over and above all, it should be noted that online tracking for advertising purposes has raised privacy concerns and is the subject of a number of emerging regulatory laws across the world. However, one of the most important pushes aimed at protecting user privacy seem to come from the major technology platforms, namely Google, Apple and Mozilla [41, 42]. Apple has started to fight against consumer tracking through the so-called Smart Tracking Prevention (\"TIP\") feature in the Safari browser (introduced in 2017) [43] to limit user tracking by blocking third-party cookies. Similarly, in 2019, Mozilla [44] has launched an anti-tracking mechanism called Enhanced Tracking Protection to block third-party cookies by default and incorporate features to address browser fingerprinting. Interestingly enough, Google (as one of the biggest names in the digital advertising industry), first, in May 2019, published a set of measures for users to delete third-party cookies without losing their log-in information, and with the intention of making user-fingerprinting much harder for trackers. They then introduced an open-source initiative--the Privacy Sandbox--a new set of web standards to enhance user privacy online and support a vibrant ad-funded web ecosystem [45]. The Privacy Sandbox included a series of browser Application Programming Interfaces (APIs) designed to satisfy advertising use cases without relying on third-party cookies. Concretely, a technology developed from Google's Privacy Sandbox--FLOCs (Federated Learning of Cohorts) [46]--has been developed to facilitate more anonymized advertising by targeting people only based on their membership in larger groups/cohorts of users with similar interests and behavior. Nevertheless, Google recently stated that it would not build alternate tracking identifiers with similar cross-site tracking capabilities after phasing out third-party cookies. Google will make this change in late 2023 [47].