Browser Extension Permissions: Security Implications for JSON Formatters

A JSON formatter extension is usually safe only when its permissions match its feature set. If it formats the current tab only after you click the toolbar button, it can often work with limited permissions such as "activeTab", "scripting", and "storage". Risk rises when a formatter asks for persistent access to every site, clipboard reading, cookies, or network interception.

That distinction matters because browser permission screens are intentionally broad. In current Chromium browsers, a request like "<all_urls>" can surface the warning "Read and change all your data on all websites". In Firefox, host permissions for Manifest V3 extensions are shown at install time, but Firefox documentation notes that updates adding new host permissions are not surfaced in the update prompt the same way. For a search visitor deciding whether a JSON formatter is safe, the real question is not whether it works, but whether it needs that level of access to work.

Match the Permission to the Feature

Most safe choices become obvious once you map the requested permission to the actual job the extension is doing:

Lower-risk permissions you should expect

These permissions are common for a formatter that works locally and only touches the current page when you ask it to:

"activeTab"

This is one of the least invasive permissions. In Chrome, it grants temporary access to the current tab only after a user gesture such as clicking the extension action, a context-menu item, or a keyboard shortcut. The access is temporary and is revoked when the tab is closed or moves away from that page context.

• Power: Lets the extension inspect the active tab and, with "scripting", inject code into that tab after the user invokes it.
• Security implications: Relatively low risk because it is user-initiated and temporary. If the extension is compromised, an attacker still has to wait for the user to trigger it.

"storage"

This is the normal place to keep preferences such as indentation width, collapsed-node defaults, and theme choices.

• Power: Stores extension settings separately from website storage.
• Security implications: Usually low risk. The main concern is what the extension chooses to store, not the permission itself.

"clipboardWrite"

This is common for a copy button in a popup or side panel.

• Power: Lets the extension place formatted JSON onto the clipboard.
• Security implications: Lower risk than clipboard reading, but it can still alter copied content. It should be attached to an obvious user action such as clicking Copy.

Permissions That Deserve Extra Scrutiny

The following requests are not automatically malicious, but they need a stronger explanation from the developer because they meaningfully widen the blast radius if the extension is compromised.

"clipboardRead"

A formatter may ask for this if it offers a one-click Paste button. That can be convenient, but it is more sensitive than "clipboardWrite" because the browser warns that the extension can read what you copy and paste.

• When it may be justified: An explicit user-triggered paste workflow inside the extension.
• When it is suspicious: If the extension mostly formats pages in-place and cannot explain why it needs clipboard reads.

Host permissions such as "<all_urls>"

This is where the biggest trust decision sits. Host permissions tell the browser which sites the extension is allowed to interact with. In Chrome, broad host access is what leads to warnings like "Read and change all your data on all websites".

• Power: Lets the extension read page content on matching sites, inject scripts, access tab metadata for those origins, and make privileged requests from extension pages to those hosts.
• Security implications: High risk when the pattern is broad. A formatter that only needs to work when clicked usually should not need persistent access to every site you visit.

There is an important nuance here: host permissions alone do not grant every browser capability. But they do create the foundation for more invasive behavior, especially when paired with other permissions such as "cookies", "webRequest", or broad content-script matching rules.

Cookies and network permissions

A basic formatter rarely needs "cookies", "webRequest", or "declarativeNetRequestWithHostAccess". Once those appear alongside broad host access, you are no longer evaluating a simple viewer. You are evaluating software that can meaningfully observe or influence web traffic.

What Over-Permissive JSON Formatters Can Really Do

When a JSON formatter asks for broad access, the security problem is not just the current code. It is the amount of damage the extension could do later if the developer account is compromised, the project is sold, or a bad update is published.

Data Exposure & Theft ()

Broad site access lets the extension inspect the content of pages that match its allowed origins. That can include raw JSON responses, internal API docs, admin panels, and any sensitive information rendered in the page. If the extension also has cookie or network permissions, the exposure can become much deeper.

Action Execution on Your Behalf ()

If an extension can inject scripts into pages automatically, it can manipulate what you see and interact with forms or controls on your behalf. For a JSON formatter, that is usually unnecessary power.

Data Exfiltration ()

The biggest practical red flag is not just reading data but sending it elsewhere. A trustworthy formatter should process JSON locally in the browser, without uploading page contents or pasted payloads to a remote service.

Supply Chain Attacks ()

This is the central extension risk. A popular formatter can start out legitimate and later become dangerous through an account takeover or ownership change. The broader the granted permissions, the more valuable that installed base becomes to an attacker.

Current Browser Behavior That Changes the Risk

A good security decision is not just about the manifest. Modern browsers also give users some runtime control over site access:

• Chromium browsers: Users can usually restrict an extension to run on click, on specific sites, or on all requested sites. For a JSON formatter with broader host access, changing site access to on-click or a short allowlist materially reduces risk.
• Chrome install prompts: Broad host permissions and clipboard access trigger explicit warnings. That warning text is a useful signal, not boilerplate to ignore.
• Firefox 127 and later: Manifest V3 host permissions are shown in the install prompt. Firefox documentation also notes that update prompts do not surface newly requested host permissions the same way, which makes periodic permission reviews worthwhile.
• File URLs and incognito: In Chrome, these are separate user-controlled toggles on the extension details page. A formatter usually does not need either unless you explicitly use it in those contexts.

How to Evaluate a JSON Formatter Before Installing It

• Start with the workflow: If you only want to prettify copied JSON, you should not need an extension with all-sites access.
• Prefer on-demand access: For in-page formatting, favor "activeTab"and on-click behavior over persistent "<all_urls>".
• Be skeptical of permission stacking: A formatter asking for "<all_urls>", "clipboardRead", "cookies", and network permissions at the same time deserves a very strong explanation.
• Use browser controls after install: Set site access to on-click or a small list of trusted origins if the browser offers that choice.
• Turn off extra surfaces: Leave file-URL and incognito access disabled unless you specifically need them.
• Check the trust signals: Read the store listing, privacy policy, update history, and whether the developer clearly says processing happens locally.
• Reduce extension count: Every installed extension increases browser attack surface, even if you use it only occasionally.

Practical Guidance for Developers

A secure JSON formatter is mostly a least-privilege design exercise. The smaller the permission set, the more trustworthy the product feels and the less damage a compromise can do.

• Default to on-demand formatting: If the user clicks the extension on a JSON page, "activeTab" plus "scripting" is often enough.
• Keep processing local: Do not send raw page content, pasted JSON, or API responses to your own backend just to pretty-print them.
• Request host access only when the feature truly needs it: Auto-formatting on a known set of sites is a better fit for narrow host patterns or optional host permissions than for "<all_urls>".
• Ask later when possible: Optional host permissions let you explain the feature first, then request access when the user enables it.
• Explain every warning in plain language: The store description should tell users exactly why a permission is needed and what data never leaves the device.
• Avoid unrelated power permissions: A formatter rarely needs "cookies", "history", "webRequest", or "declarativeNetRequestWithHostAccess".
• Protect the release pipeline: Use strong account security and review updates carefully, as extension supply-chain abuse often arrives through legitimate update channels.

{
  "manifest_version": 3,
  "name": "My Secure JSON Formatter",
  "version": "1.0.0",
  "description": "Formats JSON in the active tab when the user clicks the extension.",
  "permissions": [
    "activeTab",
    "scripting",
    "storage",
    "clipboardWrite"
  ],
  "action": {
    "default_title": "Format JSON on this page"
  },
  "background": {
    "service_worker": "service-worker.js"
  }
}

Bottom Line

The safest JSON formatter extension is the one that processes data locally and asks for access only when you use it. If a formatter mostly needs to help you read JSON, then a permission bundle centered on "activeTab", "scripting", and "storage" is usually easier to justify than permanent all-sites access. The farther an extension moves toward clipboard reading, cookie access, network interception, and "<all_urls>", the more carefully you should scrutinize it.

For many users, the best security choice is to avoid installing an extension at all unless in-page formatting is truly necessary. If you only need to paste, prettify, and inspect JSON, a local or browser-based formatter outside the extension permission model removes this entire category of risk.