Chrome Extension Permissions Explained: The Complete Security Guide

Every Chrome extension you install asks for permissions. Some want to "Read and change all your data on all websites." Others request access to your tabs, cookies, or downloads. But what do these permissions actually allow an extension to do?

This guide breaks down every Chrome extension permission, explains the real-world security implications, and gives you a framework for deciding which extensions are safe to install.

How Chrome Extension Permissions Work

Chrome uses a permission system to control what extensions can access. When you install an extension, Chrome shows you what permissions it's requesting. Once you approve, the extension can use those capabilities until you uninstall it.

There are three categories of permissions:

1. Declared Permissions: Listed in the extension's manifest.json, granted at install time
2. Optional Permissions: Can be requested later, require user approval
3. Host Permissions: Control which websites the extension can access

The problem? Most users click "Add extension" without understanding what they're approving. Security teams often have no visibility into what permissions are active across their organization.

Permission Risk Tiers

We categorize Chrome extension permissions into four risk tiers based on their potential for abuse:

🔴 Critical Risk Permissions

These permissions can access your most sensitive data and should only be granted to highly trusted extensions.

🟠 High Risk Permissions

These permissions have significant security implications but are commonly needed for legitimate functionality.

🟡 Medium Risk Permissions

These permissions have limited but notable security implications.

🟢 Low Risk Permissions

These permissions have minimal security implications when used alone.

Deep Dive: The Most Dangerous Permissions

<all_urls> — Access to All Websites

This is the most powerful and most abused permission. When an extension has <all_urls> or *://*/* host permission, it can:

• Read the content of every webpage you visit
• Modify any webpage's content (inject scripts, change text, add elements)
• Intercept form submissions, including login forms
• Access data on banking sites, email, social media, corporate applications

When it's legitimate: Ad blockers, password managers, and some productivity tools genuinely need broad access.

Red flag: A simple utility (calculator, color picker, screenshot tool) requesting access to all websites.

How to evaluate: Ask "Does this extension need to work on every website, or just specific ones?" Many extensions could function with narrower host permissions.

webRequest + webRequestBlocking — Network Interception

This combination allows an extension to intercept, block, redirect, or modify any HTTP request before it's sent or after a response is received.

What attackers can do with it:

• Redirect you to phishing sites
• Inject malicious content into responses
• Steal authentication headers
• Bypass HTTPS by modifying requests

When it's legitimate: VPNs, ad blockers, privacy tools, and developer tools.

Red flag: Extensions that don't obviously need network-level access requesting these permissions.

cookies — Session Hijacking Made Easy

The cookies permission lets an extension read, create, and delete cookies for any website.

What attackers can do with it:

• Steal session cookies and hijack your authenticated sessions
• Access accounts without knowing passwords
• Persist access even after password changes

This permission is what made the Cyberhaven breach so damaging—the malicious code could steal authentication tokens for Facebook Ads and other services.

When it's legitimate: Cookie managers, privacy extensions, some authentication tools.

Red flag: Any extension requesting cookie access that doesn't explicitly manage cookies as its primary function.

nativeMessaging — Escape the Sandbox

Chrome extensions run in a sandboxed environment. The nativeMessaging permission allows an extension to communicate with native applications installed on your computer.

What attackers can do with it:

• Execute code outside the browser
• Install additional malware
• Access files on your system
• Keylog or screen capture

When it's legitimate: Password managers (for desktop app integration), development tools.

Red flag: This permission is rarely necessary. Question any extension requesting it.

Host Permissions: The Hidden Risk

Beyond API permissions, extensions request "host permissions" that control which websites they can access:

// Examples of host permissions
"host_permissions": [
  "https://mail.google.com/*",     // Specific site
  "https://*.example.com/*",        // All subdomains
  "*://*/*",                         // All websites (HTTP and HTTPS)
  "<all_urls>"                       // All URLs including file:// and chrome://
]

Understanding Host Permission Patterns

The Principle of Least Privilege

A well-designed extension requests only the specific sites it needs. Compare:

Good: A Gmail productivity extension requesting https://mail.google.com/*

Concerning: A Gmail productivity extension requesting <all_urls>

If an extension works only with specific services, it shouldn't need access to all websites.

Permission Combinations That Signal Risk

Some permissions are dangerous on their own. Others become dangerous in combination:

Data Exfiltration Combo

cookies + <all_urls> + webRequest

Can steal any cookie and send it anywhere.

Credential Theft Combo

<all_urls> + tabs + clipboardRead

Can track what sites you visit, read page content, and capture clipboard.

Malware Delivery Combo

downloads + nativeMessaging + <all_urls>

Can download files, execute native code, and operate on any site.

Complete Surveillance Combo

history + tabs + webNavigation + <all_urls>

Full visibility into everything you do in the browser.

How to Evaluate an Extension's Permissions

Before installing any extension, ask these questions:

1. Does the functionality justify the permissions?

A password manager legitimately needs broad access. A theme or new tab page does not.

Test: Can you explain why this extension needs each permission?

2. Are the permissions narrowly scoped?

Good extensions request specific sites, not all sites. Good extensions use activeTab instead of tabs when possible.

Test: Could this extension function with fewer permissions?

3. What's the publisher's track record?

• How long have they been publishing extensions?
• Do they have a website, company, support channels?
• Have they been involved in security incidents?

Test: Would you trust this publisher with the access they're requesting?

4. Is the extension actively maintained?

Abandoned extensions don't get security updates. They can also be sold to malicious actors.

Test: When was the last update? Does the publisher respond to reviews?

5. What do security researchers say?

Search for "[extension name] security" or "[extension name] malware" before installing.

Test: Are there credible reports of security issues?

Manifest V3: What's Changing

Chrome is transitioning extensions from Manifest V2 to Manifest V3, which changes how permissions work:

Key Security Improvements in MV3

1. Service Workers Replace Background Pages: Extensions can no longer run persistent background scripts
2. webRequestBlocking Restricted: Only enterprise-deployed extensions can block requests
3. Remote Code Execution Blocked: Extensions can't execute code fetched from remote servers
4. Host Permission Changes: Users get more granular control

What This Means for Security

MV3 reduces the attack surface significantly. Extensions lose the ability to run persistent background code and can't easily modify network requests. However, sophisticated attackers will adapt.

The transition is ongoing—expect most extensions to migrate by late 2025.

Enterprise Recommendations

For security teams managing browser extensions across an organization:

1. Audit Current Permissions

Use Extension Auditor to get visibility into what permissions are active across your fleet.

2. Create a Permission Policy

Define which permissions are acceptable:

3. Implement Allowlisting

Block all extensions by default, then approve specific extensions that pass your review:

{
  "ExtensionInstallBlocklist": ["*"],
  "ExtensionInstallAllowlist": [
    "approved_extension_id_1",
    "approved_extension_id_2"
  ]
}

4. Monitor for Permission Changes

Extensions can request additional permissions through updates. Set up monitoring to detect when approved extensions request new capabilities.

5. Regular Reviews

Permissions that were acceptable when an extension was approved may become problematic after updates. Review your allowlist quarterly.

Quick Reference: Permission Lookup Table

Conclusion

Chrome extension permissions are the keys to your browser. Understanding what each permission allows—and what combinations create risk—is essential for both individual users and enterprise security teams.

The next time you're about to click "Add extension," take 30 seconds to review the permissions. Ask yourself: "Does this extension really need this access?"

For organizations, the question is bigger: "What permissions are active across our entire browser fleet, and who approved them?"

If you can't answer that question, Extension Auditor can help. We analyze extension permissions, score risk, and give you the visibility you need to secure this critical attack surface.

Related Resources

• How to Check Your Chrome Extensions for Hidden Threats
• Chrome Extension Permission Warnings
• Manifest V3 Migration Guide