Local shared object

Local shared objects (LSOs), commonly called Flash cookies (due to their similarities with HTTP cookies), are pieces of data that websites which use Adobe Flash may store on a user's computer. Local shared objects have been used by all versions of Flash Player (developed by Macromedia, which was later acquired by Adobe Systems) since version 6.

Flash cookies, which can be stored or retrieved whenever a user accesses a page containing a Flash application, are a form of local storage. Similar to that of cookies, they can be used to store user preferences, save data from Flash games, or to track users' Internet activity. LSOs have been criticised as a breach of browser security, but there are now browser settings and addons to limit the duration of their storage.

Local shared objects contain data stored by individual websites. Data is stored in the Action Message Format. With the default settings, the Flash Player does not seek the user's permission to store local shared objects on the hard disk. By default, a SWF application running in Flash Player from version 9 to 11 (as of Sept 1, 2011) may store up to 100 kB of data to the user's hard drive. If the application attempts to store more, a dialog asks the user whether to allow or deny the request.

Adobe Flash Player does not allow third-party local shared objects to be shared across domains. For example, a local shared object from "www.example.com" cannot be read by the domain "www.example.net". However, the first party website can always pass data to a third party via some settings found in the dedicated XML file and passing the data in the request to the third party. Also, third party LSOs are allowed to store data by default. By default LSO data is shared across browsers on the same machine. As an example:

This is distinct from cookies which have directory isolated storage paths for saved cookies while LSOs use a common directory path for all browsers on a single machine.

Flash games may use LSO files to store the user's personal game data, such as user preferences and actual game progress. Backing up files such as these requires some technical understanding of software. However, both browser updates and programs designed to remove unused files may delete this data.

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