Data leakage is, in digital marketing context, the process by which visitor data is unwittingly transferred from the visited website to third party servers.
Publishers are particularly exposed to data leakage through ad serving and ad tags. Stolen or leaked data can be used for selling the publisher audience on other websites or directly by competitors.
By unwittingly sharing user information with the advertiser, the advertiser will be able to target the same user elsewhere on cheaper publishers (on Google AdSense Network for instance). Data leakage is mainly a financial risk trough ad revenues for premium and niche publishers.
Merchant websites are also exposed due to all marketing partners and vendors who need third party tags implementation. The leaked data could be used by competition.
Data leakage can be prevented by:
effective tag management
data policy stating data collection and re-use rights
observing IAB guidelines "site tagging best practices"
bot monitoring of tag requests
See also search data leakage.