# Important Security Notes

## Separate domains

We do not recommend running Element from the same domain name as your Matrix
homeserver. The reason is the risk of XSS (cross-site-scripting)
vulnerabilities that could occur if someone caused Element to load and render
malicious user generated content from a Matrix API which then had trusted
access to Element (or other apps) due to sharing the same domain.

We have put some coarse mitigations into place to try to protect against this
situation, but it's still not good practice to do it in the first place. See
<https://github.com/element-hq/element-web/issues/1977> for more details.

## Configuration best practices

Unless you have special requirements, you will want to add the following to
your web server configuration when hosting Element Web:

- The `X-Frame-Options: SAMEORIGIN` header, to prevent Element Web from being
  framed and protect from [clickjacking][owasp-clickjacking].
- The `frame-ancestors 'self'` directive to your `Content-Security-Policy`
  header, as the modern replacement for `X-Frame-Options` (though both should be
  included since not all browsers support it yet, see
  [this][owasp-clickjacking-csp]).
- The `X-Content-Type-Options: nosniff` header, to [disable MIME
  sniffing][mime-sniffing].
- The `X-XSS-Protection: 1; mode=block;` header, for basic XSS protection in
  legacy browsers.

[mime-sniffing]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types#mime_sniffing
[owasp-clickjacking-csp]: https://cheatsheetseries.owasp.org/cheatsheets/Clickjacking_Defense_Cheat_Sheet.html#content-security-policy-frame-ancestors-examples
[owasp-clickjacking]: https://cheatsheetseries.owasp.org/cheatsheets/Clickjacking_Defense_Cheat_Sheet.html

If you are using nginx, this would look something like the following:

```
add_header X-Frame-Options SAMEORIGIN;
add_header X-Content-Type-Options nosniff;
add_header X-XSS-Protection "1; mode=block";
add_header Content-Security-Policy "frame-ancestors 'self'";
```

For Apache, the configuration looks like:

```
Header set X-Frame-Options SAMEORIGIN
Header set X-Content-Type-Options nosniff
Header set X-XSS-Protection "1; mode=block"
Header set Content-Security-Policy "frame-ancestors 'self'"
```

Note: In case you are already setting a `Content-Security-Policy` header
elsewhere, you should modify it to include the `frame-ancestors` directive
instead of adding that last line.

# Building From Source

Element is a modular webapp built with modern ES6 and uses a Node.js build system.
Ensure you have the latest LTS version of Node.js installed.

Using `pnpm` instead of `npm` is recommended. Please see the pnpm [install
guide](https://pnpm.io/installation#using-corepack) if you do not have it already.

1. Install or update `node.js` so that your `node` is at least the current recommended LTS.
1. Install `pnpm` if not present already.
1. Clone the repo: `git clone https://github.com/element-hq/element-web.git`.
1. Switch to the element-web directory: `cd element-web/apps/web`.
1. Install the prerequisites: `pnpm install`.
    - If you're using the `develop` branch, then it is recommended to set up a
      proper development environment (see [Setting up a dev
      environment](../../developer_guide.md#setting-up-a-dev-environment) below). Alternatively, you
      can use <https://develop.element.io> - the continuous integration release of
      the develop branch.
1. Configure the app by copying `config.sample.json` to `config.json` and
   modifying it. See the [configuration docs](../../docs/config.md) for details.
1. `pnpm dist` to build a tarball to deploy. Untaring this file will give
   a version-specific directory containing all the files that need to go on your
   web server.

Note that `pnpm dist` is not supported on Windows, so Windows users can run `pnpm build`,
which will build all the necessary files into the `webapp` directory. The version of Element
will not appear in Settings without using the dist script. You can then mount the
`webapp` directory on your web server to actually serve up the app, which is
entirely static content.

# config.json

Element supports a variety of settings to configure default servers, behaviour, themes, etc.
See the [configuration docs](../../docs/config.md) for more details.

# Labs Features

Some features of Element may be enabled by flags in the `Labs` section of the settings.
Some of these features are described in [labs.md](https://github.com/element-hq/element-web/blob/develop/docs/labs.md).

# Caching requirements

Element requires the following URLs not to be cached, when/if you are serving Element from your own webserver:

```
/config.*.json
/i18n
/version
/index.html
```

We also recommend that you force browsers to re-validate any cached copy of Element on page load by configuring your
webserver to return `Cache-Control: no-cache` for `/`. This ensures the browser will fetch a new version of Element on
the next page load after it's been deployed. Note that this is already configured for you in the nginx config of our
Dockerfile.

# Development

Please read through the following:

1. [Developer guide](../../developer_guide.md)
2. [Code style](../../code_style.md)
3. [Contribution guide](../../CONTRIBUTING.md)

# Extending Element Web with Modules

Element Web supports a module system that allows you to extend or modify functionality at runtime. Modules are loaded dynamically and provide a safe, predictable API for customization.

## What are modules?

Modules are extensions that can add or modify Element Web's functionality. They are:

- Built using the [`@element-hq/element-web-module-api`](https://github.com/element-hq/element-web/tree/develop/packages/module-api)
- Loaded in EW via [config.json](../../docs/config.md#modules)
