Persistent Tracking in Modern Browsers: Tales of Favicons and Caches
Explore the intriguing world of user tracking through favicons and caches in modern browsers, uncovering the potential for persistent tracking even after clearing cookies and incognito mode. Learn about the significance of favicons, their storage mechanisms, cache policies, and the threat models associated with them.
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Presentation Transcript
Tales of Favicons and Caches: Persistent Tracking in Modern Browsers NDSS 2021
What is user tracking? When not logged in: After clearing the cookies/ incognito mode :
What is a Favicon? Introduced in Internet Explorer 5, 1999 Favorite website icon Favicon Small icon associated with a webpage Different icon formats Supported by all browsers and devices Part of website s branding identity
Favicon Storage & Cache Automatically requested and fetched 1. Page URL 2. Favicon ID 3. Expiration Time (TTL) 4. Dimensions Dedicated Favicon Cache
Favicon Cache Policies 2 Subdomains and inner paths create different entries Access Control To clear the browser s cache/ cookies/ history do not affect the favicon cache Incognito mode can read and not write
Threat Model Victim visits website and the identifier is automatically stored Attacker.com/pathA Attacker.com/pathB ID=10101 Attacker.com/pathC Attacker.com/pathD Attacker.com/pathE
Write Identifier Victim Browser attacker.com GET / First visit GET favicon.ico 302 Redirect ID Generation
Write Identifier ID=10101 Victim Browser attacker.com/subpathX ID bit:1 2 3 K N GET /pathA GET favicon1.ico 302 Redirect GET /pathC GET favicon3.ico 302 Redirect GET /pathK
Read Identifier Victim Browser attacker.com No favicon request GET / 302 Redirect User Exisits
Read Identifier ID= 1 0 Victim Browser attacker.com/subpathX ID bit:1 2 3 K N GET /pathA 302 Redirect GET /pathB GET favicon2.ico 404 Not Found 302 Redirect
Affected Browsers and Modes Firefox *
Baseline Attack Performance Write Identifier 2.5s Read Identifier 5.5s How to do better?
Optimization: Fingerprints and Favicons Browser attributes that are immutable over time [Vastel et al., S&P18 ] Cookies Enabled, Local Storage, DNT, Ad Block, Platform, Encoding, Language, WebGL, Canvas Browser Fingerprints dataset amiunique.org [Lapperdrix et al., S&P16 ] Calculated Fingerprint Entropy of the available attributes
Optimization: Fingerprints and Favicons Lower: 16 bits Higher: 26 bits 50% desktop devices: 19-24 bits Combine Browser Fingerprints with Favicon Identifier
Optimization: Fingerprints and Favicons Fingerprint generation overhead 200ms Cookies, Local Storage, DNT 2ms Canvas, WebGL 100ms Reconstruct 32-bit ID with less redirections 20 bits Browser FP + 12 bits Favicon ID 2 sec Anti-Fingerprint tools Brave defense: Full ID reconstruct 3 sec
Network Effects Web server and client located in the same city 27% faster Id Generation 35% decreased read-ID time Optimal attack time with redirection overhead Write 1.5 sec Read 3 sec Large-Scale attack: dedicated CDNs and servers across locations
Proposed Countermeasures Incognito mode should use an isolated cache instance Default Clear browsing data should also clear Favicon Cache Notified Vulnerable browsers Confirmed and acknowledged Brave deployed countermeasure
Summary Demonstrated novel persistent favicon tracking technique Breaks incognito mode Robust against anti-tracking defenses Long-term identifier Browser FPs a powerful optimization mechanism for augmenting other tracking vectors Extensive experimental evaluation under different network/device/browser conditions Mitigation needs redesign of policies and browser architecture. Remediation under way.
