While IPv6 adoption is increasing on the internet, company networks that use IPv6 internally are quite rare. However, most companies are unaware that while IPv6 might not be actively in use, all Windows versions since Windows Vista (including server variants) have IPv6 enabled and prefer it over IPv4. In this blog, an attack is presented that abuses the default IPv6 configuration in Windows networks to spoof DNS replies by acting as a malicious DNS servers and redirect traffic to an attacker specified endpoint. In the second phase of this attack, a new method is outlined to exploit the (infamous) Windows Proxy Auto Discovery (WPAD) feature in order to relay credentials and authenticate to various services within the network. The tool Fox-IT created for this is called mitm6, and is available from the Fox-IT GitHub.
Similar to the slow IPv6 adoption, resources about abusing IPv6 are much less prevalent than those describing IPv4 pentesting techniques. While every book and course mentions things such as ARP spoofing, IPv6 is rarely touched on and the tools available to test or abuse IPv6 configurations are limited. The THC IPV6 Attack toolkit is one of the available tools, and was an inspiration for mitm6. The attack described in this blog is a partial version of the SLAAC attack, which was first described by in 2011 by Alex Waters from the Infosec institute. The SLAAC attack sets up various services to man-in-the-middle all traffic in the network by setting up a rogue IPv6 router. The setup of this attack was later automated with a tool by Neohapsis called suddensix.
The downside of the SLAAC attack is that it attempts to create an IPv6 overlay network over the existing IPv4 network for all devices present. This is hardly a desired situation in a penetration test since this rapidly destabilizes the network. Additionally the attack requires quite a few external packages and services to work. mitm6 is a tool which focusses on an easy to setup solution that selectively attacks hosts and spoofs DNS replies, while minimizing the impact on the network’s regular operation. The result is a python script which requires almost no configuration to set up, and gets the attack running in seconds. When the attack is stopped, the network reverts itself to it’s previous state within minutes due to the tweaked timeouts set in the tool.
The mitm6 attack
Attack phase 1 – Primary DNS takeover
mitm6 starts with listening on the primary interface of the attacker machine for Windows clients requesting an IPv6 configuration via DHCPv6. By default every Windows machine since Windows Vista will request this configuration regularly. This can be seen in a packet capture from Wireshark:
mitm6 will reply to those DHCPv6 requests, assigning the victim an IPv6 address within the link-local range. While in an actual IPv6 network these addresses are auto-assigned by the hosts themselves and do not need to be configured by a DHCP server, this gives us the opportunity to set the attackers IP as the default IPv6 DNS server for the victims. It should be noted that mitm6 currently only targets Windows based operating systems, since other operating systems like macOS and Linux do not use DHCPv6 for DNS server assignment.
mitm6 does not advertise itself as a gateway, and thus hosts will not actually attempt to communicate with IPv6 hosts outside their local network segment or VLAN. This limits the impact on the network as mitm6 does not attempt to man-in-the-middle all traffic in the network, but instead selectively spoofs hosts (the domain which is filtered on can be specified when running mitm6).
The screenshot below shows mitm6 in action. The tool automatically detects the IP configuration of the attacker machine and replies to DHCPv6 requests sent by clients in the network with a DHCPv6 reply containing the attacker’s IP as DNS server. Optionally it will periodically send Router Advertisment (RA) messages to alert client that there is an IPv6 network in place and that clients should request an IPv6 adddress via DHCPv6. This will in some cases speed up the attack but is not required for the attack to work, making it possible to execute this attack on networks that have protection against the SLAAC attack with features such as RA Guard.
Attack phase 2 – DNS spoofing
On the victim machine we see that our server is configured as DNS server. Due to the preference of Windows regarding IP protocols, the IPv6 DNS server will be preferred to the IPv4 DNS server. The IPv6 DNS server will be used to query both for A (IPv4) and AAAA (IPv6) records.
Now our next step is to get clients to connect to the attacker machine instead of the legitimate servers. Our end goal is getting the user or browser to automatically authenticate to the attacker machine, which is why we are spoofing URLs in the internal domain
testsegment.local. On the screenshot in step 1 you see the client started requesting information about
wpad.testsegment.local immediately after it was assigned an IPv6 address. This is the part we will be exploiting during this attack.
A (short) history of WPAD abuse
The Windows Proxy Auto Detection feature has been a much debated one, and one that has been abused by penetration testers for years. Its legitimate use is to automatically detect a network proxy used for connecting to the internet in corporate environments. Historically, the address of the server providing the wpad.dat file (which provides this information) would be resolved using DNS, and if no entry was returned, the address would be resolved via insecure broadcast protocols such as Link-Local Multicast Name Resolution (LLMNR). An attacker could reply to these broadcast name resolution protocols, pretend the WPAD file was located on the attackers server, and then prompt for authentication to access the WPAD file. This authentication was provided by default by Windows without requiring user interaction. This could provide the attacker with NTLM credentials from the user logged in on that computer, which could be used to authenticate to services in a process called NTLM relaying.
In 2016 however, Microsoft published a security bulletin MS16-077, which mitigated this attack by adding two important protections:
– The location of the WPAD file is no longer requested via broadcast protocols, but only via DNS.
– Authentication does not occur automatically anymore even if this is requested by the server.
While it is common to encounter machines in networks that are not fully patched and are still displaying the old behaviour of requesting WPAD via LLMNR and automatically authenticating, we come across more and more companies where exploiting WPAD the old-fashioned way does not work anymore.
Exploiting WPAD post MS16-077
The first protection, where WPAD is only requested via DNS, can be easily bypassed with mitm6. As soon as the victim machine has set the attacker as IPv6 DNS server, it will start querying for the WPAD configuration of the network. Since these DNS queries are sent to the attacker, it can just reply with its own IP address (either IPv4 or IPv6 depending on what the victim’s machine asks for). This also works if the organization is already using a WPAD file (though in this case it will break any connections from reaching the internet).
To bypass the second protection, where credentials are no longer provided by default, we need to do a little more work. When the victim requests a WPAD file we won’t request authentication, but instead provide it with a valid WPAD file where the attacker’s machine is set as a proxy. When the victim now runs any application that uses the Windows API to connect to the internet or simply starts browsing the web, it will use the attackers machine as a proxy. This works in Edge, Internet Explorer, Firefox and Chrome, since they all respect the WPAD system settings by default.
Now when the victim connects to our “proxy” server, which we can identify by the use of the
CONNECT HTTP verb, or the presence of a full URI after the
GET verb, we reply with a HTTP 407 Proxy Authentication required. This is different from the HTTP code normally used to request authentication, HTTP 401.
IE/Edge and Chrome (which uses IEs settings) will automatically authenticate to the proxy, even on the latest Windows versions. In Firefox this setting can be configured, but it is enabled by default.
Windows will now happily send the NTLM challenge/response to the attacker, who can relay it to different services. With this relaying attack, the attacker can authenticate as the victim on services, access information on websites and shares, and if the victim has enough privileges, the attacker can even execute code on computers or even take over the entire Windows Domain. Some of the possibilities of NTLM relaying were explained in one of our previous blogs, which can be found here.
The full attack
The previous sections described the global idea behind the attack. Running the attack itself is quite straightforward. First we start mitm6, which will start replying to DHCPv6 requests and afterwards to DNS queries requesting names in the internal network. For the second part of our attack, we use our favorite relaying tool, ntlmrelayx. This tool is part of the impacket Python library by Core Security and is an improvement on the well-known smbrelayx tool, supporting several protocols to relay to. Core Security and Fox-IT recently worked together on improving ntlmrelayx, adding several new features which (among others) enable it to relay via IPv6, serve the WPAD file, automatically detect proxy requests and prompt the victim for the correct authentication. If you want to check out some of the new features, have a look at the relay-experimental branch.
To serve the WPAD file, all we need to add to the command prompt is the host is the
-wh parameter and with it specify the host that the WPAD file resides on. Since mitm6 gives us control over the DNS, any non-existing hostname in the victim network will do. To make sure ntlmrelayx listens on both IPv4 and IPv6, use the
-6 parameter. The screenshots below show both tools in action, mitm6 selectively spoofing DNS replies and ntlmrelayx serving the WPAD file and then relaying authentication to other servers in the network.
Defenses and mitigations
The only defense against this attack that we are currently aware of is disabling IPv6 if it is not used on your internal network. This will stop Windows clients querying for a DHCPv6 server and make it impossible to take over the DNS server with the above described method.
For the WPAD exploit, the best solution is to disable the Proxy Auto detection via Group Policy. If your company uses a proxy configuration file internally (PAC file) it is recommended to explicitly configure the PAC url instead of relying on WPAD to detect it automatically.
While writing this blog, Google Project Zero also discovered vulnerabilities in WPAD, and their blog post mentions that disabling the
WinHttpAutoProxySvc is the only reliable way that in their experience disabled WPAD.
Lastly, the only complete solution to prevent NTLM relaying is to disable it entirely and switch to Kerberos. If this is not possible, our last blog post on NTLM relaying mentions several mitigations to minimize the risk of relaying attacks.