Cylab Technical Report CMU-CyLab-10-014, September 10, 2010.
Pedro Giovanni Leon, Lorrie Faith Cranor, Aleecia M. McDonald, Robert McGuire
Electrical and Computer Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
Platform for Privacy Preferences (P3P) compact policies (CPs) are a collection of three-character and four-character tokens that summarize a website's privacy policy pertaining to cookies. User agents, including Microsoft's Internet Explorer (IE) web browser, use CPs to evaluate websites' data collection practices and allow, reject, or modify cookies based on sites' privacy practices. CPs can provide a technical means to enforce users' privacy preferences if CPs accurately reflect websites' practices. Through automated analysis we can identify CPs that are erroneous due to syntax errors or semantic conflicts. We collected CPs from 33,139 websites and detected errors in 11,176 of them, including 134 TRUSTe-certified websites and 21 of the top 100 most-visited sites. Our work identifies potentially misleading practices by web administrators, as well as common accidental mistakes. We found thousands of sites using identical invalid CPs that had been recommended as workarounds for IE cookie blocking. Other sites had CPs with typos in their tokens, or other errors. 98% of invalid CPs resulted in cookies remaining unblocked by IE under it's default cookie settings. It appears that large numbers of websites that use CPs are misrepresenting their privacy practices, thus misleading users and rendering privacy protection tools ineffective. Unless regulators use their authority to take action against companies that provide erroneous machine-readable policies, users will be unable to rely on these policies.
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