Chrome Extension Review

Chrome Extension Review

Purpose

Review Chrome extensions for Manifest V3 compliance, security (CSP, XSS, privilege escalation), permission minimization, message-passing correctness, content script isolation, and Chrome Web Store policy readiness.

When to use

• Reviewing a new or updated Chrome extension before submitting to the Chrome Web Store.
• Auditing an existing extension's manifest.json, service worker, content scripts, or popup pages.
• Evaluating permission requests for least-privilege compliance.
• Reviewing extension code after a MV2 → MV3 migration.
• Security audit of an extension that handles user credentials, payment data, or PII.

When not to use

• Firefox/Safari WebExtension review where browser-specific APIs differ significantly — flag divergences but note this skill targets Chrome.
• Pure web app review with no chrome.* API usage.
• Native messaging host code review (separate OS-level process).

Procedure

1. Manifest V3 compliance

• Confirm "manifest_version": 3 — MV2 extensions are being phased out and will stop working in Chrome.
• background.service_worker replaces background.scripts / background.page from MV2.
• declarativeNetRequest replaces webRequest blocking mode — verify rules are defined in _rules.json or fetched via updateDynamicRules.
• action replaces browser_action / page_action.
• Remote code execution is prohibited: no eval(), no new Function(string), no chrome.tabs.executeScript with a string body loaded from a remote URL.
• content_scripts must not use document.write().

2. Permissions — least privilege

• List every permission in permissions, optional_permissions, and host_permissions.
• For each permission, verify it is actually used in the code. Remove any unused permission.
• Prefer activeTab over broad host_permissions like <all_urls> or *://*/* when the extension only needs the current tab.
• Sensitive permissions (tabs, history, bookmarks, cookies, webNavigation, downloads, nativeMessaging) require clear justification in the store listing.
• scripting permission: verify chrome.scripting.executeScript is not used to inject remotely-fetched code strings.
• storage vs unlimitedStorage: only request unlimitedStorage if the data volume genuinely requires it.

3. Host permissions

• Scope to the minimum set of origins required. https://api.example.com/* is better than https://*/*.
• Wildcard subdomains (https://*.example.com/*) must be justified — ensure the extension does not accidentally run on all subdomains of a large service.
• Optional host permissions should be requested at runtime via chrome.permissions.request() rather than declared statically for permissions not needed at install time.

4. Content Security Policy

• Extension pages (popup, options, sidepanel) must have a strict CSP in manifest.json under "content_security_policy".
• Minimum acceptable: "script-src 'self'; object-src 'none';" — no unsafe-inline, no unsafe-eval.
• External script sources in CSP are not allowed in MV3 for extension pages.
• Content scripts run in an isolated world by default; verify they do not access unsafeWindow or window.wrappedJSObject without necessity.

5. Message passing security

• chrome.runtime.onMessage.addListener: verify the sender is checked before acting on messages — do not trust messages from arbitrary web pages.
• chrome.runtime.sendMessage from content scripts: the service worker handler must validate the message shape before executing privileged actions.
• chrome.runtime.onMessageExternal (cross-extension messaging): explicitly whitelist allowed extension IDs if used.
• postMessage between content script and page: validate event.origin strictly; never use origin === '*'.

6. Content script isolation and XSS

• Content scripts must not inject innerHTML with page-supplied data: element.innerHTML = response.data is XSS if response.data is attacker-controlled.
• Use textContent for text, createElement + setAttribute for DOM construction.
• DOM-based XSS: watch for document.location, document.URL, document.referrer fed into eval, innerHTML, or document.write.
• Content scripts should not expose functions on the page's window object unless strictly necessary.

7. Service worker (background)

• Service workers are event-driven and terminate when idle — do not rely on in-memory state persisting between events; use chrome.storage.session or chrome.storage.local.
• Avoid long-running setInterval — use chrome.alarms for periodic tasks.
• chrome.alarms.create requires the alarms permission.
• Unhandled promise rejections in service workers silently fail — add .catch() to all async chains.
• chrome.storage.local is not encrypted — do not store sensitive credentials there; use chrome.storage.session (cleared on browser restart) or the OS credential store via native messaging.

8. web_accessible_resources

• List only the resources that must be accessible from web pages.
• Specify matches to restrict which origins can access the resource — avoid "matches": ["<all_urls>"].
• An overly permissive web_accessible_resources allows malicious pages to fingerprint extension presence and load extension assets.

9. Data handling and privacy

• Personal data collected (browsing history, form data, PII) must be disclosed in the store privacy policy and in the manifest "privacy_policy" URL.
• No sending of tab URLs or content to third-party servers without user consent.
• Verify no API keys, client secrets, or tokens are hardcoded in extension source — they are visible to users who inspect the extension.

10. Chrome Web Store policy readiness

• Extension must have a single, clear purpose described in the listing.
• No obfuscated code — the store may reject or remove extensions with intentionally obfuscated JavaScript.
• Update URL must be omitted (managed by the store) for public listings.
• Icons: 16, 48, 128 px PNG required; verify they are included in the package.

Checklist

MV3 compliance:

• manifest_version: 3
• background.service_worker defined (no background.page)
• No eval(), new Function(string), or remote script injection
• declarativeNetRequest used instead of blocking webRequest where applicable

Permissions:

• Every declared permission is used in code
• activeTab preferred over <all_urls> where possible
• Sensitive permissions (tabs, history, cookies) justified
• Host permissions scoped to minimum required origins

Security:

• CSP on extension pages: script-src 'self', no unsafe-inline/unsafe-eval
• runtime.onMessage validates sender before privileged actions
• Content scripts avoid innerHTML with external data
• web_accessible_resources has restricted matches

Service worker:

• No in-memory state relied upon across events
• chrome.alarms used for periodic tasks, not setInterval
• All async promise chains have .catch()

Privacy:

• No hardcoded API keys in source
• No PII sent to third parties without user consent
• Privacy policy URL in manifest

Common issues & anti-patterns

• MV2 background page left in manifest: causes load failure in Chrome 127+ (MV2 phase-out). Replace with service_worker.
• "host_permissions": ["<all_urls>"]: flags for manual review in the store and is almost always broader than needed.
• innerHTML with data from fetch(): if the fetched resource can be influenced by a web page (CORS, redirect), this is a stored XSS vector inside the extension context.
• No sender check in onMessage: a malicious web page can send crafted messages to the extension's background and trigger privileged actions.
• Storing OAuth tokens in chrome.storage.local: readable by any code with access to the extension context. Prefer chrome.storage.session or prompt re-auth.
• eval() in content scripts for template rendering: violates MV3 CSP and the store policy.
• web_accessible_resources: ["*"] with matches: ["<all_urls>"]: allows any web page to detect the extension and load any bundled asset.
• chrome.tabs.query for all tabs: collecting all open tab URLs is a privacy risk and requires justification; prefer activeTab.

Required output

Return a structured report with:

• Summary: pass / needs fixes / blocked (store-rejection risk or security issue).
• Manifest snapshot: key fields reviewed (version, permissions, host_permissions, CSP, service worker).
• Findings table: severity (critical / high / medium / low / info), category, file + line, description, remediation.
• Permission audit: each permission listed with justification status (justified / unused / overly broad).
• Store readiness: checklist of store policy items that pass or require attention.
• Next handoff: specific items to fix before store submission; note any that require store team waiver.

Safety

• Do not install or execute the extension in a browser during review.
• Never extract or print API keys found in the source — flag as critical finding requiring rotation.
• Do not access the extension's storage contents or user data.
• If the extension appears to perform undisclosed data collection (keylogging, form scraping), flag as critical and recommend immediate removal from distribution.