Manifest V3 Extension Audits: A Developer's Guide to Secure Sandboxes

With the global rollout of Google Chrome's Manifest V3 framework complete, the architecture of browser extensions has undergone a profound security revolution. By deprecating the old blocking webRequest API and replacing it with the highly constrained declarativeNetRequest structure, Google has taken massive strides toward protecting user privacy. However, a browser extension is still a powerful piece of client-side software that possesses programmatic access to the Document Object Model (DOM). For enterprise IT departments, digital compliance officers, and security-conscious professionals, executing a systematic static security audit is the only way to guarantee that an extension does not represent a severe data-leak liability.

At Lumière Labs, our tools—including our flagship text-expansion utility Just My Type—are built on a zero-knowledge, local-first paradigm. In this technical guide, we outline the exact step-by-step methodologies to unpack, analyze, and static audit any Manifest V3 extension, providing you with the guidelines needed to verify sandboxed integrity.

Step 1: Unpacking the Source Code (.CRX to ZIP)

Chrome Web Store extensions are compiled and distributed as signed .crx archive files. To perform a static analysis of their Javascript execution contexts, we must first unpack the source package. Follow these command-line steps to access the local source-tree:

• Locate the target extension ID. For example, Just My Type’s Chrome Store ID is cneliigbfmgjjjejckpkobnoigegpkjg.
• Navigate to your local Chrome User Data profile directory on your disk:
  %LocalAppData%\Google\Chrome\User Data\Default\Extensions\cneliigbfmgjjjejckpkobnoigegpkjg
• Copy the inner version folder to a separate sandbox workspace directory, or download the .crx package directly and rename the extension suffix to a standard .zip format. Uncompress the zip file to reveal the root source files.

Step 2: Decoding the Permission Schema (manifest.json)

The core configuration file that establishes the extension's boundaries is manifest.json. The primary focal point of your security audit must be the permissions, optional_permissions, and host_permissions arrays. These dictate what system APIs the extension can execute and what external network domains it can read.

Here is a secure, sandboxed manifest.json configuration template modeled on Just My Type. It displays a highly restricted permission schema with strict Content Security Policies (CSP) to block all remote network calls:

{
 "manifest_version": 3,
 "name": "Just My Type (Sandboxed Local-First)",
 "version": "1.4.2",
 "description": "Secure, privacy-first local text expansion utility.",
 "permissions": [
 "storage",
 "activeTab"
 ],
 "host_permissions": [],
 "content_security_policy": {
 "extension_pages": "default-src 'self'; script-src 'self'; object-src 'none'; connect-src 'none';"
 },
 "background": {
 "service_worker": "background.js",
 "type": "module"
 },
 "content_scripts": [
 {
 "matches": ["<all_urls>"],
 "js": ["content.js"],
 "run_at": "document_idle",
 "all_frames": true
 }
 ]
}

Permission Red Flags to Audit

When reviewing the manifest schema of any browser automation or utility tool, look out for these highly dangerous permission settings:

1. Broad Host Permissions ("<all_urls>" inside host_permissions): If an extension lists universal host matchers, it has the ability to read all cookies, execute HTTP requests, and capture sensitive form values across every single website you visit, representing a massive attack surface. In contrast, safe local-first tools do not request host permissions and rely on on-demand triggers or strictly constrained sandboxes.
2. Permissive Content Security Policy (connect-src): If the CSP connect-src block is omitted or allows generic wildcards (*), the extension's scripts are free to transmit captured browser text, clipboard snippets, or session variables to remote cloud tracking databases. A safe extension locks connect-src directly to 'none' to prevent all telemetry leakage by design.
3. Optional/Dynamic Permissions: Verify if the extension requests dynamic API grants at runtime. While useful for specialized features, these bypass initial installation checks and must be audited line-by-line in the execution code.

Step 3: Static Analysis of Injection Scripts (content.js)

Content scripts (like content.js) are injected directly into active browser tabs and run within the context of the web page. These scripts have direct access to the page's DOM, enabling them to read input fields and handle keystrokes. To ensure compliance:

• Trace DOM Mutation Observers: Search the code for MutationObserver instances. Ensure they are used solely to locate inputs and fields (e.g., dynamically rendered chat input boxes) rather than scraping paragraph content.
• Scan Keypress Event Handling: Ensure that addEventListener('keydown') or keyup listeners strictly process key combinations or shortcut keywords locally. They must never collect or persist general typing behaviors.
• Audit Remote Data Transmission: Audit all instances of chrome.runtime.sendMessage. Ensure data passed to background service workers consists strictly of localized config requests, rather than raw text variables or customer credentials.

The E-E-A-T Technical Security Pledge

This technical analysis is researched and verified by David L., Technical Security Lead at Lumière Labs. David has over 10 years of experience in browser-native security research, browser sandbox configurations, and cryptography. Lumière Labs is committed to complete technical transparency, guaranteeing that 100% of our code is open to customer inspections and operates completely on-device without cloud sync tracking dependencies.