Font Detection

How Font-Based Browser Fingerprinting Works

What is Font Detection?

Font detection is a sophisticated browser fingerprinting technique that identifies users by analyzing the fonts installed on their system. This method creates a unique fingerprint based on the combination of available fonts, helping websites track users across different sessions and browsers.

By measuring how text renders with different font families, websites can create a unique profile of your system's font collection, which is often distinctive enough to identify you across different websites, even when cookies are disabled.

How Font Detection Works

Detection Process

  1. 1Test hundreds of font families by measuring text dimensions
  2. 2Compare text dimensions to determine available fonts
  3. 3Create unique fingerprint from font combination
  4. 4Use fingerprint for cross-site tracking

Detection Methods

  • Canvas-based text measurement
  • CSS font-family fallback testing
  • JavaScript dimension analysis
  • Flash-based detection (legacy)

Technical Implementation

How Font Detection Works

Font detection uses sophisticated techniques to identify which fonts are installed on your system. Understanding these methods helps you recognize when websites are trying to fingerprint your browser.

Canvas-Based Detection

The most common and accurate method

  • • Creates invisible canvas elements
  • • Measures text with different fonts
  • • Compares dimensions to detect available fonts
  • • Very accurate and hard to detect

CSS Font-Family Testing

Uses CSS fallback mechanisms

  • • Tests font-family fallbacks
  • • Measures element dimensions
  • • Less accurate than canvas method
  • • Easier to detect and block

JavaScript API Detection

Uses modern browser APIs

  • • FontFace API for font loading
  • • getComputedStyle() for style analysis
  • • Performance timing measurements
  • • Advanced detection techniques

Canvas Measurement

The most common method uses HTML5 canvas elements to measure text dimensions with different font families, comparing results to detect available fonts.

CSS Fallback Testing

CSS font-family fallbacks are tested by measuring text dimensions and comparing them to known default font measurements.

Font Detection Methods

Canvas-Based Detection

Uses HTML5 canvas elements to measure text dimensions with different font families. This is the most accurate and widely used method for font detection.

Advantages:

  • • High accuracy
  • • Works across all browsers
  • • Fast execution
  • • Hard to detect

Detection:

  • • Monitor canvas operations
  • • Check for font measurements
  • • Look for repeated text rendering
  • • Analyze JavaScript patterns

DOM-Based Detection

Uses CSS font-family fallbacks and DOM manipulation to detect available fonts by measuring text dimensions and comparing them to known defaults.

Techniques:

  • • CSS font-family testing
  • • Element dimension measurement
  • • Font loading detection
  • • Style computation analysis

Limitations:

  • • Less accurate than canvas
  • • Affected by CSS styling
  • • Browser-specific behavior
  • • Easier to detect

JavaScript-Based Detection

Uses various JavaScript techniques including font loading APIs, style computation, and element measurement to detect available fonts on the system.

APIs Used:

  • • FontFace API
  • • getComputedStyle()
  • • offsetWidth/Height
  • • scrollWidth/Height

Modern Methods:

  • • Font loading detection
  • • CSS font-display testing
  • • Web font API usage
  • • Performance timing analysis

Privacy Risks and Implications

Persistent Tracking

Font fingerprints remain consistent across different websites and sessions, enabling persistent tracking even when cookies are disabled or cleared.

Unique Identification

The combination of installed fonts is often unique enough to identify individual users, creating detailed profiles without explicit consent.

Difficult Detection

Font detection is hard to detect and prevent, working even with privacy-focused browsers and extensions.

Cross-Platform Tracking

Font fingerprints can be used to link users across different devices and platforms, creating comprehensive tracking profiles.

Protection Methods

Browser Extensions

  • • Canvas Defender
  • • Privacy Badger
  • • uBlock Origin
  • • Font Fingerprint Defender
  • • NoScript
  • • Ghostery

Browser Settings

  • • Disable web fonts
  • • Use privacy-focused browsers
  • • Enable strict tracking protection
  • • Use Tor Browser
  • • Disable JavaScript
  • • Use incognito mode

Detection and Testing

Font Fingerprint Testing

Use online tools to test your browser's font fingerprint and see what information websites can collect about your installed fonts.

Browser Developer Tools

Monitor network requests, JavaScript execution, and canvas operations to detect font fingerprinting attempts on websites.

Privacy Analysis Tools

Use privacy analysis tools and browser extensions to identify and block font fingerprinting attempts in real-time.

Best Practices for Protection

•Use privacy-focused browsers like Firefox with strict privacy settings and tracking protection
•Install browser extensions that specifically block font fingerprinting and canvas-based tracking
•Consider using Tor Browser for maximum anonymity and protection against all forms of fingerprinting
•Regularly clear browser data and use incognito/private browsing mode for sensitive activities
•Disable JavaScript on untrusted websites to prevent font-based tracking attempts
•Stay informed about new fingerprinting techniques and protection methods

Check Your Font Fingerprint

Now that you understand font detection, check your browser's font fingerprint and see what information websites can collect about your system.

Check My Font Fingerprint Now