Most of the cookies that resides in our browser has an expiry period. It will automatically deleted over a certain period of time or when there is no activity. Even with the help of some tools we can also remove cookies.
But Flash cookies will even exist for years until unless we clear it manually. This is a security vulnerability which can be exploited by malware programs. Without our permission they can access and manipulate. This may create some severe security issues later. Moreover 98% of the browser are installed with Macromedia Flash player. The data store use 100KB of data by default to unlimited maximum.
You can clear the cookies manually. Check out this link. You can see flash cookies resides in your system. Delete flash cookies you feel no more useful.