Understanding the Privacy Risks of Popular Search Engine Advertising Systems

TL;DR

This study measures the privacy risks of advertising systems in privacy-focused search engines, revealing that they often fail to protect user privacy and enable cross-site tracking similar to traditional search engines.

Contribution

First extensive measurement comparing privacy properties of privacy-focused and mainstream search engine advertising systems using automated analysis methods.

Findings

Privacy-focused search engines leak user requests via redirectors in most cases.

Advertising IDs enable cross-site user activity tracking across all tested search engines.

Both privacy-focused and traditional engines exhibit privacy-harming behaviors despite privacy claims.

Abstract

We present the first extensive measurement of the privacy properties of the advertising systems used by privacy-focused search engines. We propose an automated methodology to study the impact of clicking on search ads on three popular private search engines which have advertising-based business models: StartPage, Qwant, and DuckDuckGo, and we compare them to two dominant data-harvesting ones: Google and Bing. We investigate the possibility of third parties tracking users when clicking on ads by analyzing first-party storage, redirection domain paths, and requests sent before, when, and after the clicks. Our results show that privacy-focused search engines fail to protect users' privacy when clicking ads. Users' requests are sent through redirectors on 4% of ad clicks on Bing, 86% of ad clicks on Qwant, and 100% of ad clicks on Google, DuckDuckGo, and StartPage. Even worse, advertising systems collude with advertisers across all search engines by passing unique IDs to advertisers in most ad clicks. These IDs allow redirectors to aggregate users' activity on ads' destination websites in addition to the activity they record when users are redirected through them. Overall, we observe that both privacy-focused and traditional search engines engage in privacy-harming behaviors allowing cross-site tracking, even in privacy-enhanced browsers.