Wormhole, funnel-shaped tunnel that can connect one universe with another

Intentional security holes

Sinkholes and watering holes are two expressions not automatically associated with computer security, yet they are in use to describe two tactics that are used in this field.

Both are set up in order to disrupt the “normal” flow of things. This post aims to introduce both these expressions and explain the differences, so you won’t get them confused.


A DNS sinkhole in cyberspace is a means of taking away traffic from the intended target. It is often used as a defense mechanism against botnets. The DNS of the Command and Control (C&C) server(s) is interrupted and the traffic can either be dropped or rerouted for analysis. One objective of analysis is to get an overview of the drones in the botnet that are under control of the C&C.

The WIndows hosts file that blocks traffic to known malicious domains can be considered a miniature sinkhole as it can be used to ‘drop’ the traffic to all the domains listed in the hosts file, by rerouting it to (localhost).  In computer networking, localhost is a hostname that resolves to ‘this computer’ so the traffic never leaves the computer.

On a larger scale, network administrators can use DNS sinkholing to prevent access of malicious URLs at an enterprise level by deploying an internal DNS sinkhole server. The request can trigger a custom page telling the user that the requested domain is blacklisted. However, this will not work against threats that use their own DNS resolver.

A very special way of sinkholing against botnets is done by Kaspersky in the first Hlux/Kelihos takedown. After reverse engineering the workings of the botnet, they managed to introduce a sinkhole and make all the drones talk to that machine instead of the other controllers.


Watering holes

Watering holes are used as an aimed attack strategy. The attacker infects a website where he knows his intended victim(s) visits regularly. Depending on the nature of the infection, he can single out his intended target(s) or just infect anyone that visits the site unprotected. The watering hole strategy is a mix of social engineering, hacking, and drive-by infections which require a high level of knowledge and a well-thought out strategy.

This is normally used against high-profile targets and organizations of great importance as a way to get a foothold inside such an organization by infecting one or more of their systems.

The attacker needs the following knowledge to perform the watering hole technique successfully:

  • A website that is visited on a regular basis by the target
  • A vulnerability on the targets system that can be exploited
  • A way to infect the site with their exploit of choice

Telling them apart

An easy way to remember what’s what is to keep in mind their real life equivalents. A sinkhole absorbs anything that comes near and a watering hole is a pub, a place that attracts people and where they are more likely to show their weaknesses.


Understanding DNS Sinkholes – A weapon against malware

Building a sinkhole that never clogs

Pieter Arntz


Pieter Arntz

Malware Intelligence Researcher

Was a Microsoft MVP in consumer security for 12 years running. Can speak four languages. Smells of rich mahogany and leather-bound books.