browser.user agent sniffing

browser.user-agent-sniffing

Rule ID: browser.user-agent-sniffing Severity: WARN Category: browser Target Standards: W3C HTML Design Principles (Avoid Browser Sniffing), Chromium Client Hints & User-Agent Reduction Guidelines, MDN Web Docs (Browser Detection Using the User Agent - Best Practices)

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1. Overview & Core Invariant

Flags conditional branching based on navigator.userAgent string sniffing and enforces W3C capability/feature detection

Core Invariant:

"Application logic and responsive branching must not rely on substring or regex matching of 'navigator.userAgent'. Use W3C capability detection instead."

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2. Technical Grounding & Engine Realities

User-Agent strings are historically fragile, frequently spoofed, and currently frozen across major browsers (Chrome, Safari, Edge).

For example, Chrome contains 'Safari' and 'WebKit', Edge contains 'Chrome', and iPadOS reports as macOS 'Macintosh'. Branching on User-Agent strings leads to silent feature failures and broken responsive layouts on newer devices.

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3. Vulnerability & Risk Taxonomy

Risk Vector	Severity	Impact
Frozen & Spoofed UA Strings	MEDIUM	Browsers freeze version numbers or disguise platform tokens, causing browser sniffing logic to misclassify modern mobile devices as desktop or vice versa.
Cross-Browser Engine Breakage	MEDIUM	Alternative browsers (Brave, Vivaldi, Arc, Firefox Focus) or tablets (iPadOS) receive crippled mobile views or desktop-only controls that cannot be touched.

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4. Non-Compliant Code Patterns (Bad Examples)

JAVASCRIPT (Branching layout or feature based on navigator.userAgent regex):

if (/android|iphone|ipad/i.test(navigator.userAgent)) {
  initMobileLayout();
}

TYPESCRIPT (Checking browser brand via userAgent.includes):

if (navigator.userAgent.includes("Chrome")) {
  enableChromeFeature();
}

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5. Compliant Implementation Patterns (Good Examples)

JAVASCRIPT (Using W3C CSS Media Queries for pointer capability detection):

if (window.matchMedia("(pointer: coarse)").matches) {
  initMobileLayout();
}

TYPESCRIPT (Using feature detection instead of browser sniffing):

if ("visualViewport" in window) {
  enableViewportFeature();
}

TYPESCRIPT (Telemetry logging is allowed and not flagged):

logger.sendMetrics({
  userAgent: navigator.userAgent,
  timestamp: Date.now(),
});

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6. How to Suppress (Ignore Directives)

If this pattern is required for an intentional exception, suppress the diagnostic using the canonical Charites Rule ID:

<!-- charites:ignore browser.user-agent-sniffing intentional exception -->

// charites:ignore browser.user-agent-sniffing intentional exception

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7. Configuration Reference (charites.yaml)

rules:
  browser.user-agent-sniffing:
    severity: warn # error | warn | info | off

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8. Architectural Domain & Verification Reference

• Domain Architecture & Analysis Pipeline: See the browser Category Guide.
• 1-SSOT Golden Tri-Corpus Test Matrix: See the Verification Harness on the Master Home page.
• Rule Catalog Index: See Master Wiki Home.