Preventing leakage of cookie data

Disclosed are various embodiments for preventing the unintended leakage of cookie data. In one embodiment, a browser application stores cookie data from a first network site having a high-level domain in a client computing device. The cookie data includes a sharing attribute. The cookie data is automatically made accessible to the first network site. A network service is queried to obtain data indicating a classification associated with the first network site. The cookie data is made accessible to a second network site having the same high-level domain based at least in part on the sharing attribute and the classification meeting at least one predetermined criterion.