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-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 client 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!
-
-![Debug symbols](/static/img/watchblob_1.webp)
-
-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:
-
-![Some URIs](/static/img/watchblob_2.webp)
-
-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)*
-
-![sslvpnLogon](/static/img/watchblob_3.webp)
-
-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`).
-
-![processTokenPrompt](/static/img/watchblob_4.webp)
-
-If the `logon_status` was `1` (apparently \"success\" here) it proceeded
-to do something quite interesting:
-
-![processTokenPrompt2](/static/img/watchblob_5.webp)
-
-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:
-
-![doLogin](/static/img/watchblob_6.webp)
-
-... 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.