In our experience, residential proxies are an often overlooked security threat; one that can be very difficult to remediate for the end user who -in our experience- is entirely unaware of its existence.

A proxy refresh

For those who aren’t familiar with the term “residential proxies,” these exist in end user networks (either landline or cell), as opposed to ones running on servers in a data center.

Proxies in user land are nothing new. In days gone by, proxies used to be either open/misconfigured or installed by malware. These days, most modern residential proxies exist because the end user installed them as part of an application or toolbar, not fully understanding what they are getting. 

What’s hidden?

To make money from their apps, some developers embed software development kits (SDKs) that create proxies. These proxies are then made available to users – at a cost. Developers bury the SDK’s End User License Agreement (EULA) deep inside the one for the app or use such vague text that it’s almost impossible for the end user to understand what they are actually agreeing to. Often end users are tempted by the promise of ad removals or another “carrot” that streamlines the user experience.

These proxies are not only run on desktops, tablets, and mobile phones but also on streaming sticks/boxes, media players, and -yes- doorbells.

Is it malware if you install it yourself and it doesn’t exploit anything?

Once the app is installed and connected to the internet, the proxy is available to paying customers of the proxy network. Some of these proxy networks offer their users access to literally millions of IP addresses in every country and across most networks/ASNs. This illustrates how many people have unwittingly downloaded these proxies.

Since this is the internet, it won’t come as any great surprise to learn that these residential proxies are an excellent platform for a variety of nefarious activities. From Spamhaus’ side, we see these platforms sending spam, which in turn can lead to unsuspecting residential users being unable to send email because their IP address has been placed on a blocklist. And that is one of the lesser of several evils!

Another evil

Consider the implications of a proxy that is under someone else’s control being active on your network. Think about it… if a device containing such a proxy gets onto a corporate network, there is suddenly a route into the said network that shouldn’t be there. Even more worrying, because all traffic goes over HTTPS, it usually is allowed to flow freely.

From Spamhaus’ perspective, what we usually observe is the spam, but cybercriminals can use proxies for all sorts of malicious activities, and they are inside your firewall.

There’s no such thing as a free lunch

For the moment, perhaps the biggest lesson to learn is that when you see software promising something for free, it is usually charging you in some other currency, such as your internet access or your privacy!

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