Vulnerability: GHSA-3q49-h8f9-9fr9
Summary: Missing TLS certificate verification
Aliases: CVE-2020-15134
Severity: HIGH
CVSS score: 8
CWE: CWE-295
Published: 2020-07-31T17:39:42Z

Faye uses [em-http-request][6] and [faye-websocket][10] in the Ruby version of
its client. Those libraries both use the [`EM::Connection#start_tls`][1] method
in [EventMachine][2] to implement the TLS handshake whenever a `wss:` URL is
used for the connection. This method does not implement certificate verification
by default, meaning that it does not check that the server presents a valid and
trusted TLS certificate for the expected hostname. That means that any `https:`
or `wss:` connection made using these libraries is vulnerable to a
man-in-the-middle attack, since it does not confirm the identity of the server
it is connected to.

The first request a Faye client makes is always sent via normal HTTP, but later
messages may be sent via WebSocket. Therefore it is vulnerable to the same
problem that these underlying libraries are, and we needed both libraries to
support TLS verification before Faye could claim to do the same. Your client
would still be insecure if its initial HTTPS request was verified, but later
WebSocket connections were not.

This has been a requested feature in EventMachine for many years now; see for
example [#275][3], [#378][4], and [#814][5]. In June 2020, em-http-request
published an [advisory][7] related to this problem and fixed it by [implementing
TLS verification][8] in their own codebase; although EventMachine does not
implement certificate verification itself, it provides an extension point for
the caller to implement it, called [`ssl_verify_peer`][9]. Based on this
implementation, we have incorporated similar functionality into faye-websocket.

After implementing verification in v1.1.6, em-http-request has elected to leave
the `:verify_peer` option switched off by default. We have decided to _enable_
this option by default in Faye, but are publishing a minor release with added
functionality for configuring it. We are mindful of the fact that this may break
existing programs, but we consider it much more important that all clients have
TLS verification turned on by default. A client that is not carrying out
verification is either:

- talking to the expected server, and will not break under this change
- being attacked, and would benefit from being alerted to this fact
- deliberately talking to a server that would be rejected by verification

The latter case includes situations like talking to a non-public server using a
self-signed certificate. We consider this use case to be "working by accident",
rather than functionality that was actively supported, and it should be properly
and explicitly supported instead.

We are releasing Faye v1.4.0, which enables verification by default and provides
a way to opt out of it:

```rb
client = Faye::Client.new('https://example.com/', tls: { verify_peer: false })
```

Unfortunately we can't offer an equivalent of the `:root_cert_file` option that
has been added to faye-websocket, because em-http-request does not support it.
If you need to talk to servers whose certificates are not recognised by your
default root certificates, then you need to add its certificate (or another one
that can verify it) to your system's root set.

The same functionality is now supported in the Node.js version, with a `tls`
option whose values will be passed to the `https` and `tls` modules as
appropriate when making connections. For example, you can provide your own CA
certificate:

```js
var client = new faye.Client('https://example.com/', {
  tls: {
    ca: fs.readFileSync('path/to/certificate.pem')
  }
});
```

For further background information on this issue, please see [faye#524][12] and
[faye-websocket#129][13]. We would like to thank [Tero Marttila][14] and [Daniel
Morsing][15] for providing invaluable assistance and feedback on this issue.

[1]: https://www.rubydoc.info/github/eventmachine/eventmachine/EventMachine/Connection:start_tls
[2]: https://rubygems.org/gems/eventmachine
[3]: https://github.com/eventmachine/eventmachine/issues/275
[4]: https://github.com/eventmachine/eventmachine/pull/378
[5]: https://github.com/eventmachine/eventmachine/issues/814
[6]: https://rubygems.org/gems/em-http-request
[7]: https://securitylab.github.com/advisories/GHSL-2020-094-igrigorik-em-http-request
[8]: https://github.com/igrigorik/em-http-request/pull/340
[9]: https://www.rubydoc.info/github/eventmachine/eventmachine/EventMachine/Connection:ssl_verify_peer
[10]: https://rubygems.org/gems/faye-websocket
[11]: https://faye.jcoglan.com/
[12]: https://github.com/faye/faye/issues/524
[13]: https://github.com/faye/faye-websocket-ruby/pull/129
[14]: https://github.com/SpComb
[15]: https://github.com/DanielMorsing