"The use of behavioral targeting practices provides ad networks with the opportunity to tailor ads to the individual characteristics of users. As privacy concerns over behavioral targeting have been growing lately, an increasing number of ad networks offer ad preferences managers (APMs) that show collected and/or inferred information about users. The focus of our study is to investigate the accuracy and completeness of the information contained in such APMs. On the basis of our experimental results, we propose a structured methodology for APM validation. We also assess how third parties render ads based on users’ browsing behavior. Our findings reveal cases in which even sensitive information is leaked as part of an HTTP header and is used to serve ads on multiple sites. The third parties examined in this study include an intent-focused data exchange (BlueKai) and a social network (Facebook) along with the ad networks owned by AOL, Google, and Yahoo!."