Yes! Adcash is malware!
Does this happen when using another Wifi network, or when on 3G/4G? If this happens with any browser and with any computer or device, it could mean that you router got infected by drive-by-hijacking.
You visited a site with a malware script that logged into your router and changed the DNS settings. This works only if the router has a default admin password. It just tries and then changes the DNS in the router. Changing the DNS they can reroute all traffic if they want to. All computers that use DHCP and use the router DNS settings will use the changed DNS of the hijackers.
The malware will infect all devices it can infect, so probably the popups will appear if you use another network as well. For devices that cannot be infected, they will work without problem on other (clean) networks.
I can't tell of course if this is really the case. But if so, it's potentially dangerous. Don't use online banking until you're sure this is not the case! Other sites involving money or valuable information (email) - same story!
- First thing you could do is disconnect the router/modem from the internet, login locally and check the DNS.
- Change the password of the router if it still has the default password!
- Check (and change) DNS settings for all your computers and devices as well.
You can change the DNS to 220.127.116.11 and 18.104.22.168 (Google DNS service) or use OpenDNS. This can be set for each computer, which then ignores the router DNS. Even if you don't like Google to see each DNS request, this is a good method to test and to be sure that you're using a service that you can be trust to be reliable.
If DNS hasn't changed, then probably you can forget about this answer. And even if this is not the case - be aware that something like this could happen.