High-severity security announcement: Authentication and extension bypass in Faye

[James Coglan]

On 20 April 2020 it was reported to me that the potential for
authentication bypass exists in Faye’s extension system. This
vulnerability has existed in the Node.js and Ruby versions of the
server since version 0.5.0, when extensions were first introduced, in
July 2010. It is patched in versions 1.0.4, 1.1.3 and 1.2.5, which we
are releasing today.

The vulnerability allows any client to bypass checks put in place by
server-side extensions, by appending extra segments to the message
channel. For example, the Faye extension docs suggest that users
implement access control for subscriptions by checking incoming
messages for the /meta/subscribe channel, for example:

server.addExtension({
incoming: function(message, callback) {
if (message.channel === '/meta/subscribe') {
if (message.ext.authToken !== 'my super secret password') {
message.error = 'Invalid auth token';
}
}
callback(message);
}
});

A bug in the server’s code for recognising the special /meta/*
channels, which trigger connection and subscription events, means that
a client can bypass this check by sending a message to
/meta/subscribe/x rather than /meta/subscribe:

{
"channel": "/meta/subscribe/x",
"clientId": "3jrc6602npj4gyp6bn5ap2wqzjtb2q3",
"subscription": "/foo"
}

This message will not be checked by the above extension, as it checks
the message’s channel is exactly equal to /meta/subscribe. But it will
still be processed as a subscription request by the server, so the
client becomes subscribed to the channel /foo without supplying the
necessary credentials.

The vulnerability is caused by the way the Faye server recognises meta
channels. It will treat a message to any channel that’s a prefix-match
for one of the special channels /meta/handshake, /meta/connect,
/meta/subscribe, /meta/unsubscribe or /meta/disconnect, as though it
were an exact match for that channel. So, a message to
/meta/subscribe/x is still processed as a subscription request, for
example.

An authentication bypass for subscription requests is the most serious
effect of this but all other meta channels are susceptible to similar
manipulation.

This parsing bug in the server is fixed in versions 1.0.4, 1.1.3 and
1.2.5. These should be drop-in replacements for prior versions and you
should upgrade immediately if you are running any prior version.

If you are unable to install one of these versions, you can make your
extensions catch all messages the server would process by checking the
channel begins with the expected channel name, for example:

server.addExtension({
incoming: function(message, callback) {
if (message.channel.startsWith('/meta/subscribe')) {
// authentication logic
}
callback(message);
}
});

--
James Coglan
http://jcoglan.com