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Permissions are defined in the extension's **`manifest.json`** file using the **`permissions`** property and allow access to almost anything a browser can access (Cookies or Physical Storage):
The previous manifest declares that the extension requires the `storage` permission. This means that it can use [the storage API](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage) to store its data persistently. Unlike cookies or `localStorage` APIs which give users some level of control, **extension storage can normally only be cleared by uninstalling the extension**.
An extension will request the permissions indicated in its **`manifest.json`** file and After installing the extension, you can **always check its permissions in your browser**, as shown in this image:
You can find the [**complete list of permissions a Chromium Browser Extension can request here**](https://developer.chrome.com/docs/extensions/develop/concepts/declare-permissions#permissions) and a [**complete list for Firefox extensions here**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions#api\_permissions)**.**
### `host_permissions`
The optional but powerful setting **`host_permissions`** indicates with which hosts the extension is going to be able to interact via apis such as [`cookies`](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/cookies), [`webRequest`](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest), and [`tabs`](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs).
The following `host_permissions` basically allow every web:
```json
"host_permissions": [
"*://*/*"
]
// Or:
"host_permissions": [
"http://*/*",
"https://*/*"
]
// Or:
"host_permissions": [
"<all_urls>"
]
```
These are the hosts that the browser extension can access freely. This is because when a browser extension calls **`fetch("https://gmail.com/")`** it's not restricted by CORS.
## Abusing `permissions` and `host_permissions`
### Tabs
Moreover, **`host_permissions`** also unlock “advanced” [**tabs API**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs) **functionality.** They allow the extension to call [tabs.query()](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/query) and not only get a **list of user’s browser tabs** back but also learn which **web page (meaning address and title) is loaded**.
{% hint style="danger" %}
Not only that, listeners like [**tabs.onUpdated**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/onUpdated) **become way more useful as well**. These will be notified whenever a new page loads into a tab.
Content scripts aren’t necessarily written statically into the extension manifest. Given sufficient **`host_permissions`**, **extensions can also load them dynamically by calling** [**tabs.executeScript()**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/executeScript) **or** [**scripting.executeScript()**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/scripting/executeScript).
Both APIs allow executing not merely files contained in the extensions as content scripts but also **arbitrary cod**e. The former allows passing in JavaScript code as a string while the latter expects a JavaScript function which is less prone to injection vulnerabilities. Still, both APIs will wreak havoc if misused.
{% hint style="danger" %}
In addition to the capabilities above, content scripts could for example **intercept credentials** as these are entered into web pages. Another classic way to abuse them is **injecting advertising** on each an every website. Adding **scam messages** to abuse credibility of news websites is also possible. Finally, they could **manipulate banking** websites to reroute money transfers.
Some extension privileges **don’t have to be explicitly declared**. One example is the [tabs API](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs): its basic functionality is accessible without any privileges whatsoever. Any extension can be notified when you open and close tabs, it merely won’t know which website these tabs correspond with.
Sounds too harmless? The [tabs.create() API](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/create) is somewhat less so. It can be used to **create a new tab**, essentially the same as [window.open()](https://developer.mozilla.org/en-US/docs/Web/API/Window/open) which can be called by any website. Yet while `window.open()` is subject to the **pop-up blocker, `tabs.create()` isn’t**.
An extension can create any number of tabs whenever it wants.
{% endhint %}
If you look through possible `tabs.create()` parameters, you’ll also notice that its capabilities go way beyond what `window.open()` is allowed to control. And while Firefox doesn’t allow `data:` URIs to be used with this API, Chrome has no such protection. **Use of such URIs on the top level has been** [**banned due to being abused for phishing**](https://bugzilla.mozilla.org/show\_bug.cgi?id=1331351)**.**
[**tabs.update()**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/update) is very similar to `tabs.create()` but will **modify an existing tab**. So a malicious extension can for example arbitrarily load an advertising page into one of your tabs, and it can activate the corresponding tab as well.
### Webcam, geolocation and friends <a href="#webcam-geolocation-and-friends" id="webcam-geolocation-and-friends"></a>
You probably know that websites can request special permissions, e.g. in order to access your webcam (video conferencing tools) or geographical location (maps). It’s features with considerable potential for abuse, so users each time have to confirm that they still want this.
{% hint style="danger" %}
Not so with browser extensions. **If a browser extension** [**wants access to your webcam or microphone**](https://developer.mozilla.org/en-US/docs/Web/API/MediaDevices/getUserMedia)**, it only needs to ask for permission once**
{% endhint %}
Typically, an extension will do so immediately after being installed. Once this prompt is accepted, **webcam access is possible at any time**, even if the user isn’t interacting with the extension at this point. Yes, a user will only accept this prompt if the extension really needs webcam access. But after that they have to trust the extension not to record anything secretly.
With access to [your exact geographical location](https://developer.mozilla.org/en-US/docs/Web/API/Geolocation) or [contents of your clipboard](https://developer.mozilla.org/en-US/docs/Web/API/Clipboard\_API), granting permission explicitly is unnecessary altogether. **An extension simply adds `geolocation` or `clipboard` to the** [**permissions entry**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions) **of its manifest**. These access privileges are then granted implicitly when the extension is installed. So a malicious or compromised extension with these privileges can create your movement profile or monitor your clipboard for copied passwords without you noticing anything.
Adding the **`history`** keyword to the [permissions entry](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions) of the extension manifest grants **access to the** [**history API**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/history). It allows retrieving the user’s entire browsing history all at once, without waiting for the user to visit these websites again.
The **`bookmarks`** **permission** has similar abuse potential, this one allows **reading out all bookmarks via the** [**bookmarks API**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/bookmarks).
The extension storage is merely a key-value collection, very similar to [localStorage](https://developer.mozilla.org/en-US/docs/Web/API/Window/localStorage) that any website could use. So no sensitive information should be stored here.
However, advertising companies could also abuse this storage.
### More permissions
You can find the [**complete list of permissions a Chromium Browser Extension can request here**](https://developer.chrome.com/docs/extensions/develop/concepts/declare-permissions#permissions) and a [**complete list for Firefox extensions here**](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions#api\_permissions)**.**
Google’s developer policies [explicitly prohibit](https://developer.chrome.com/docs/webstore/program\_policies/#permissions) requesting more privileges that necessary for the extension to function. In my experience this rule in fact works. I can only think of one case where a browser extension [requested too many privileges](https://palant.info/2020/01/13/pwning-avast-secure-browser-for-fun-and-profit/#selecting-a-target), and this particular extension was being distributed with the browser rather than via some add-on store.
In some cases browsers could do better to **limit the abuse potential** of extension privileges. For example, Chrome allows screen recording via [tabCapture](https://developer.chrome.com/docs/extensions/reference/tabCapture/) or [desktopCapture](https://developer.chrome.com/docs/extensions/reference/desktopCapture/) APIs. The abuse potential is low because the former can only be started as a **response to a user action** (typically clicking the extension icon) whereas the latter brings up a prompt to select the application window to be recorded. Both are sufficient to prevent extensions from silently starting to record in the background.
Such security improvements have the tendency to make extensions **less flexible and less user-friendly** however. A good example here is the [activeTab permission](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/permissions#activetab\_permission). Its purpose is to make requesting host privileges for the entire internet unnecessary. Instead, the **extension can access the current tab when the extension is explicitly activated**, typically by clicking its icon.
That approach works well for some extensions, particularly those where the user needs to explicitly trigger an action. It **doesn’t work in scenarios where extensions have to perform their work automatically** however (meaning being more convenient for the user) or where the extension action cannot be executed immediately and requires preparation.
<summary><strong>Learn AWS hacking from zero to hero with</strong><ahref="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/carlospolopm)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
