Essential Privacy Extensions

Go: open a terminal, then mkdir extlab and cd extlab. Nothing here installs anything into your browser.

Do: save this as manifest.json. Every browser extension ships one of these — it is the complete, machine-readable list of what the extension may do:

{
  "manifest_version": 3,
  "name": "Dark Mode for Docs",
  "version": "1.2.0",
  "description": "Adds a dark theme to your document editor.",
  "permissions": ["storage"],
  "host_permissions": ["https://docs.example.com/*"]
}

You should see: a small file. It claims two powers: keep its own settings, and act on one named site.

That is a narrow extension. It cannot see other tabs, cannot read cookies, and cannot touch any site except the one it names. The manifest is not marketing — the browser enforces exactly this list and nothing more.

If not: if a later step reports JSONDecodeError, the file has a stray comma or a smart quote from copy-paste. JSON allows only straight double quotes and no trailing comma before }.

Go: the same folder.

Do: save this as explain-perms.py. It turns the manifest's shorthand into plain English and judges the overall footprint:

import json, sys, pathlib

MEANING = {
    "storage":        "keep its own settings — harmless",
    "tabs":           "see the title and URL of every tab you have open",
    "webRequest":     "observe every network request the browser makes",
    "webRequestBlocking": "observe AND alter or block every request",
    "cookies":        "read and write cookies, including login sessions",
    "history":        "read your entire browsing history",
    "bookmarks":      "read and change your bookmarks",
    "downloads":      "see and start downloads",
    "clipboardRead":  "read whatever you have copied",
    "nativeMessaging": "talk to a program installed outside the browser",
    "<all_urls>":     "read and change the contents of EVERY site you visit",
}

path = sys.argv[1] if len(sys.argv) > 1 else "manifest.json"
m = json.loads(pathlib.Path(path).read_text(encoding="utf-8"))

print(f"{m.get('name','?')}  v{m.get('version','?')}")
print(f"  {m.get('description','')}\n")

perms = list(m.get("permissions", [])) + list(m.get("host_permissions", []))
if not perms:
    print("  no permissions requested")
worst = 0
for p in perms:
    if p in ("<all_urls>", "*://*/*", "http://*/*", "https://*/*"):
        print(f"  [FULL ACCESS] {p:<28} {MEANING.get('<all_urls>')}")
        worst = 2
    elif p.endswith("/*") and "*" not in p.split("//")[-1].split("/")[0]:
        print(f"  [one site]    {p:<28} read and change that site only")
        worst = max(worst, 0)
    else:
        note = MEANING.get(p, "no plain-English note for this one — look it up")
        level = "[sensitive]  " if p in ("tabs","webRequest","webRequestBlocking","cookies",
                                         "history","clipboardRead","nativeMessaging") else "[minor]      "
        print(f"  {level} {p:<28} {note}")
        worst = max(worst, 1 if level.strip() == "[sensitive]" else 0)

print()
print(["FOOTPRINT: narrow — it can only touch what it names",
       "FOOTPRINT: broad — it can see things beyond its stated job",
       "FOOTPRINT: TOTAL — this extension can read and modify every page you load"][worst])

Run it on the modest manifest:

python3 explain-perms.py

You should see: the two claims spelled out, and a verdict:

Dark Mode for Docs  v1.2.0
  Adds a dark theme to your document editor.

  [minor]       storage                      keep its own settings — harmless
  [one site]    https://docs.example.com/*   read and change that site only

FOOTPRINT: narrow — it can only touch what it names

This is what a well-scoped extension looks like: the permissions match the description. A dark theme for one site needs exactly this and nothing else.

If not: FileNotFoundError means the two files are in different folders — run ls and check that manifest.json and explain-perms.py both appear.

Go: the same folder.

Do: save this as greedy.json. Read it carefully first: the name is the same, the description is word for word identical, and only the version moved:

{
  "manifest_version": 3,
  "name": "Dark Mode for Docs",
  "version": "1.3.0",
  "description": "Adds a dark theme to your document editor.",
  "permissions": ["storage", "tabs", "cookies", "webRequest", "history"],
  "host_permissions": ["<all_urls>"]
}

Now run the explainer on it:

python3 explain-perms.py greedy.json

You should see: the same extension, one version later, asking for everything:

Dark Mode for Docs  v1.3.0
  Adds a dark theme to your document editor.

  [minor]       storage                      keep its own settings — harmless
  [sensitive]   tabs                         see the title and URL of every tab you have open
  [sensitive]   cookies                      read and write cookies, including login sessions
  [sensitive]   webRequest                   observe every network request the browser makes
  [sensitive]   history                      read your entire browsing history
  [FULL ACCESS] <all_urls>                   read and change the contents of EVERY site you visit

FOOTPRINT: TOTAL — this extension can read and modify every page you load

🔴 This is the actual attack, and it is not hypothetical. Popular extensions get sold, or their developer account gets compromised, and an update widens the permissions. The listing keeps its old name, its old reviews and its old star rating — the description above did not change a single character. <all_urls> plus cookies means it can read your banking session on any page you open.

If not: if the output looks the same as step 2, you ran it without the filename — the command needs greedy.json on the end, or it defaults to manifest.json.

Go: the same folder.

Do: an extension file is an ordinary ZIP archive with a different ending. Build one, then look inside it:

mkdir -p pkg
cp greedy.json pkg/manifest.json
echo 'console.log("extension code");' > pkg/background.js
cd pkg && zip -q -r ../darkmode-1.3.0.xpi . && cd ..
unzip -l darkmode-1.3.0.xpi

You should see: its contents listed, without the browser ever running it:

  Length      Date    Time    Name
---------  ---------- -----   ----
       31  2026-08-28 06:04   background.js
      257  2026-08-28 06:04   manifest.json
---------                     -------
      288                     2 files

Your dates and the exact byte counts will differ — the archive is built at the moment you run it, so an automated re-check of this page will always report this block as differing.

Firefox uses .xpi and Chrome uses .crx; both are ZIP files. Anything you can download, you can read before you trust it.

If not: zip: command not found — install it (sudo apt install zip) or skip ahead: on Windows, rename the file to .zip and open it in Explorer, which does the same job.

Go: the same folder.

Do: pull just the manifest out of the archive and run it through your explainer — the check you would do on a real download:

unzip -p darkmode-1.3.0.xpi manifest.json > extracted.json
python3 explain-perms.py extracted.json

You should see: the same report as step 3, because it is the same manifest — only now it came out of the packaged file rather than the source folder. Its closing lines (the full report is identical to step 3's):

  [sensitive]   history                      read your entire browsing history
  [FULL ACCESS] <all_urls>                   read and change the contents of EVERY site you visit

FOOTPRINT: TOTAL — this extension can read and modify every page you load

You now have a habit worth keeping. Firefox lists an add-on's permissions on its listing page before you install, and again at about:addons afterwards — and it asks again when an update wants more. That prompt is the one people click through fastest, and step 3 is what it is trying to tell you.

Inspecting a real add-on from the store was not re-run for this page — downloading one needs a network connection this page could not make. The technique is identical: the file is a ZIP either way.

If not: caution: filename not matched means the archive has no manifest.json at its top level — run unzip -l again and use the exact path shown, some packages nest their files one folder deep.