Mitigate A New CERT Vulnerability (#598349) With An Entry In /etc/hosts

Originally published at: Mitigate A New CERT Vulnerability (#598349) With An Entry In /etc/hosts – Pi-hole

There is a new CERT vulnerability that can leave you vulnerable to a Man-in-the-Middle attack. You can mitigate this vulnerability today by adding these two lines to your /etc/hosts file: wpad 
:: wpad is a stand in for your local domain. So replace with whatever your local domain is.

The essence of this vulnerability is that an attacker can add a device to the network named wpad and get a DHCP lease, thus inserting the name in the local DNS pointing to the attacker's machine. The presence of that A record allows control of the proxy settings of any browser in the network.

You can learn more about the technology behind this attack at Google's Project Zero page--it's an older article, but gives some insight into the inner workings of the attack.

The next release of dnsmasq includes an option (dhcp-ignore-names) that can be used to mitigate the attack at the source, but we haven't heard how Simon will act on this new vulnerability.

Since FTLDNS is just our fork of dnsmasq, we can easily merge in any upstream changes from him, but we wanted to let you know of the /etc/hosts fix that you can immediately implement.


What would be the correct settings for a domain with no name?



My pihdole is not inside my lan.

must I set this on my local pc on the pihole server?

I don't have pihole serve my DHCP.... my router does it but I am unsure what my domain is...

  1. How can one tell what the local domain is?
  2. Should I add those lines to my router's /etc/hosts?

Run this command on the terminal on your Pi: domainname

Put on the /etc/hosts file for the device which is providing DNS resolution - in this case your Pi-Hole.

Thank you for the reply @jfb. I don't have domainname it seems. The distro is Arch ARM. I am not sure any package supplies that program :confused:

Probably wpad would be sufficient.

Someone on Reddit mentioned that when domainname displays (none) and you check the webinterface the domain name is "lan". Does this mean we should use wpad wpad.lan
:: wpad wpad.lan

Or should we just use wpad wpad
:: wpad wpad

Or wpad
:: wpad

Sorry if this is a basic question, but I'm new to this. :slight_smile:

You're defining the redirect/no need to double it. Likely, your router is appending .lan to your hostname. Try to ping it, ie

ping foo.lan

If it that returns bytes, you can use both wpad wpad.lan

A long time ago, I responded to a topic, regarding excessive wpad queries and provided a solution to have lighttpd respond.
Does this imply that the solution is no longer a good solution?

I'm aware the solution doesn't have an entry for IPv6, but this can be easily fixed, NOT sure if lighttpd will also provide the response...

The solution was further discussed in this topic, section Q/A (Q: Why so many local requests?)

checked to see if IPv6 requests are responded to by lighttpd -> YES

IPv4 request (, entry in the lighttpd log

1536659104||GET /wpad.dat HTTP/1.1|200|56

IPv6 request (http://[2a02:1810:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx]/wpad.dat), entry in the lighttpd log

1536659116|[2a02:1810:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx]|GET /wpad.dat HTTP/1.1|200|56


What about using the Pi-Hole blacklist instead editing the hosts file?

Seems to work fine for me, by just adding


as a Regex to the blacklist.

You're right, thanks msatter. Thought it needs to have a * to match wpad.custom_tld (ie but it will Interpret ^wpad as the beginning of a domain string anyway matching wpad and .

Using ^wpad* would also match wpadddd which is overkill.

Anyway is there any drawback in using the blacklist instead of the hosts?

This is a good question....

Not stop learning :slight_smile:

When using


it will block wpad and but not wpadabcd or
Think this is even better.

This is weird, I demonstrated this exploit back in the early naughties .
You also want to aim for registering wpad.[tld], if a Windows machine can't find wpad in it's local domain name, it will go up till it does, example:
when wpad does not exist, windows tries:
if that exists, bingo machine hyjacked.

I am using


in the blacklist of pihole now for some days with a good experience. It will block every request, that is of the character wpad.* (where * is any domain like or I prefer this variant over the hosts file because it is part of the pihole mechanism and I get statistics as usual.

I guess this is the reason why I always disable any Proxy related settings in all my browsers and also check if any unneeded Services are running via services.msc :smiley:

Thnx @ Pi-Hole team for this notification tho! :+1:t2::+1:t2::+1:t2::+1:t2::+1:t2:

Bit late to the party here, but is this still valid?
Is it advised to modify /etc/hosts or use regex?
domainname returns (none) for me.

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Both is possible and it doesn't really matter regarding security.

But I still prefer the blacklist method. Using this one can see which device is quering wpad in the statistics of Pi-hole. Having this information you can reach out to the vulnerable device and deactivate the "auto proxy" setting directly.

Hi team,

If you are tempted to create a "fix" for this wpad thingy...
Please make sure it is configuarable as I use wpad.
If pihole starts blocking this as security feature, there will be issues with users like me.

Many use the wpad dns entry to let clients know where to find wpad.dat or proxy.pac.
DNS wpad entry is picked up by clients, as clients send wpad question to dchp when getting ip stuff.
wpad entry in DNS is based on IP or hostname. And the wpad must be served from a webserver on port 80.

A solution could be adding a wpad thingly in the gui. For users without wpad, just enter
For me: I would use or fqdn name.

DNSmasq can also be used to NOT serve a DHCP IP to a hostname:
Perhaps this works? Have not tested it:

Thanks in advance,

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