9528a7139b
Change-Id: Iaeb9fab5a80597469e5efdf24655ae07dca48fe1 Reviewed-on: https://cl.tvl.fyi/c/depot/+/3058 Reviewed-by: tazjin <mail@tazj.in> Tested-by: BuildkiteCI
158 lines
6.2 KiB
Markdown
158 lines
6.2 KiB
Markdown
TIP: WatchGuard has
|
|
[responded](https://www.reddit.com/r/netsec/comments/5tg0f9/reverseengineering_watchguard_mobile_vpn/dds6knx/)
|
|
to this post on Reddit. If you haven\'t read the post yet I\'d recommend
|
|
doing that first before reading the response to have the proper context.
|
|
|
|
------------------------------------------------------------------------
|
|
|
|
One of my current clients makes use of
|
|
[WatchGuard](http://www.watchguard.com/help/docs/fireware/11/en-US/Content/en-US/mvpn/ssl/mvpn_ssl_client-install_c.html)
|
|
Mobile VPN software to provide access to the internal network.
|
|
|
|
Currently WatchGuard only provides clients for OS X and Windows, neither
|
|
of which I am very fond of. In addition an OpenVPN configuration file is
|
|
provided, but it quickly turned out that this was only a piece of the
|
|
puzzle.
|
|
|
|
The problem is that this VPN setup is secured using 2-factor
|
|
authentication (good!), but it does not use OpenVPN's default
|
|
[challenge/response](https://openvpn.net/index.php/open-source/documentation/miscellaneous/79-management-interface.html)
|
|
functionality to negotiate the credentials.
|
|
|
|
Connecting with the OpenVPN config that the website supplied caused the
|
|
VPN server to send me a token to my phone, but I simply couldn't figure
|
|
out how to supply it back to the server. In a normal challenge/response
|
|
setting the token would be supplied as the password on the second
|
|
authentication round, but the VPN server kept rejecting that.
|
|
|
|
Other possibilities were various combinations of username&password
|
|
(I've seen a lot of those around) so I tried a whole bunch, for example
|
|
`$password:$token` or even a `sha1(password, token)` - to no avail.
|
|
|
|
At this point it was time to crank out
|
|
[Hopper](https://www.hopperapp.com/) and see what's actually going on
|
|
in the official OS X client - which uses OpenVPN under the hood!
|
|
|
|
Diving into the client
|
|
----------------------
|
|
|
|
The first surprise came up right after opening the executable: It had
|
|
debug symbols in it - and was written in Objective-C!
|
|
|
|
|
|
|
|
A good first step when looking at an application binary is going through
|
|
the strings that are included in it, and the WatchGuard client had a lot
|
|
to offer. Among the most interesting were a bunch of URIs that looked
|
|
important:
|
|
|
|
|
|
|
|
I started with the first one
|
|
|
|
%@?action=sslvpn_download&filename=%@&fw_password=%@&fw_username=%@
|
|
|
|
and just curled it on the VPN host, replacing the username and
|
|
password fields with bogus data and the filename field with
|
|
`client.wgssl` - another string in the executable that looked like a
|
|
filename.
|
|
|
|
To my surprise this endpoint immediately responded with a GZIPed file
|
|
containing the OpenVPN config, CA certificate, and the client
|
|
*certificate and key*, which I previously thought was only accessible
|
|
after logging in to the web UI - oh well.
|
|
|
|
The next endpoint I tried ended up being a bit more interesting still:
|
|
|
|
/?action=sslvpn_logon&fw_username=%@&fw_password=%@&style=fw_logon_progress.xsl&fw_logon_type=logon&fw_domain=Firebox-DB
|
|
|
|
Inserting the correct username and password into the query parameters
|
|
actually triggered the process that sent a token to my phone. The
|
|
response was a simple XML blob:
|
|
|
|
```xml
|
|
<?xml version="1.0" encoding="UTF-8"?>
|
|
<resp>
|
|
<action>sslvpn_logon</action>
|
|
<logon_status>4</logon_status>
|
|
<auth-domain-list>
|
|
<auth-domain>
|
|
<name>RADIUS</name>
|
|
</auth-domain>
|
|
</auth-domain-list>
|
|
<logon_id>441</logon_id>
|
|
<chaStr>Enter Your 6 Digit Passcode </chaStr>
|
|
</resp>
|
|
```
|
|
|
|
Somewhat unsurprisingly that `chaStr` field is actually the challenge
|
|
string displayed in the client when logging in.
|
|
|
|
This was obviously going in the right direction so I proceeded to the
|
|
procedures making use of this string. The first step was a relatively
|
|
uninteresting function called `-[VPNController sslvpnLogon]` which
|
|
formatted the URL, opened it and checked whether the `logon_status` was
|
|
`4` before proceeding with the `logon_id` and `chaStr` contained in the
|
|
response.
|
|
|
|
*(Code snippets from here on are Hopper's pseudo-Objective-C)*
|
|
|
|
|
|
|
|
It proceeded to the function `-[VPNController processTokenPrompt]` which
|
|
showed the dialog window into which the user enters the token, sent it
|
|
off to the next URL and checked the `logon_status` again:
|
|
|
|
(`r12` is the reference to the `VPNController` instance, i.e. `self`).
|
|
|
|
|
|
|
|
If the `logon_status` was `1` (apparently \"success\" here) it proceeded
|
|
to do something quite interesting:
|
|
|
|
|
|
|
|
The user's password was overwritten with the (verified) OTP token -
|
|
before OpenVPN had even been started!
|
|
|
|
Reading a bit more of the code in the subsequent
|
|
`-[VPNController doLogin]` method revealed that it shelled out to
|
|
`openvpn` and enabled the management socket, which makes it possible to
|
|
remotely control an `openvpn` process by sending it commands over TCP.
|
|
|
|
It then simply sent the username and the OTP token as the credentials
|
|
after configuring OpenVPN with the correct config file:
|
|
|
|
|
|
|
|
... and the OpenVPN connection then succeeds.
|
|
|
|
TL;DR
|
|
-----
|
|
|
|
Rather than using OpenVPN's built-in challenge/response mechanism, the
|
|
WatchGuard client validates user credentials *outside* of the VPN
|
|
connection protocol and then passes on the OTP token, which seems to be
|
|
temporarily in a 'blessed' state after verification, as the user's
|
|
password.
|
|
|
|
I didn't check to see how much verification of this token is performed
|
|
(does it check the source IP against the IP that performed the challenge
|
|
validation?), but this certainly seems like a bit of a security issue -
|
|
considering that an attacker on the same network would, if they time the
|
|
attack right, only need your username and 6-digit OTP token to
|
|
authenticate.
|
|
|
|
Don't roll your own security, folks!
|
|
|
|
Bonus
|
|
-----
|
|
|
|
The whole reason why I set out to do this is so I could connect to this
|
|
VPN from Linux, so this blog post wouldn't be complete without a
|
|
solution for that.
|
|
|
|
To make this process really easy I've written a [little
|
|
tool](https://github.com/tazjin/watchblob) that performs the steps
|
|
mentioned above from the CLI and lets users know when they can
|
|
authenticate using their OTP token.
|