The action followed a study that the company conducted together with researchers from University of California Berkeley and which found that more than five percent of Web users who accessed Google websites had an "ad injector" installed.
The deceptive Chrome extensions were detected as part of that study, but the researchers also found ad injectors affecting browsers such as Internet Explorer and Mozilla Firefox, on both Windows and Mac OS X.
Google doesn't specifically ban extensions published on the Chrome Web Store from injecting ads into Web traffic, as long as they clearly inform users about what they do, but the study found that around a third of extensions with such functionality were actually malware.
The larger problem is that rogue ad injection, which is used for advertising fraud, is not done only through browser extensions. There are software applications that can do this at the network layer, outside of the browser, or by hooking the browser process without installing an extension.
Some of these applications are outright malware, while others fall into a category that antivirus firms call "potentially unwanted programs," or PUPs. Such was the case of Superfish, a program that Lenovo pre-installed on some of its consumer laptops. Unfortunately, aside from injecting ads into Web pages, the application also opened a serious security hole on users' computers.
Google has recently taken action against PUPs as well, by starting to display warnings in Chrome whenever users try to download potentially harmful software. While commendable, the company's effort will likely only stop a small percentage of PUP installations, because many programs of this type are not directly downloaded by users from the Internet. Instead, they are bundled with and installed by other freeware applications.
Moreover, ad hijacking programs that fall in the malware category are distributed through exploits, malicious email attachments and other nefarious methods that don't involve Web downloads through the browser.
And even when computers are clean of malicious browser extensions, PUPs and ad fraud malware, they might continue to be plagued by rogue ads. Just last week, security researchers from a company called AraLabs warned about an attack that changed the DNS settings of consumer routers to inject rogue ads into websites when viewed on computers behind those devices.
The Domain Name System (DNS) is like the Internet's phone book. It's used to convert domain names that people remember easily into numerical Internet Protocol (IP) addresses that computers use to communicate with each other.
Computers on a local network are typically configured to pass DNS lookup requests to the network's router, which then passes those queries to DNS servers run by the ISP. If attackers manage to replace the DNS servers configured on a router with rogue servers they control, they can then spoof websites.
In the attack reported by AraLabs, hackers used this router-based DNS hijacking technique to intercept browser requests to google-analytics.com, a popular Web analytics service run by Google, and to serve malicious JavaScript code that injected rogue ads into websites when viewed by users behind the compromised routers.
Such network layer attacks that happen outside the computer are almost impossible to detect by antivirus software, the browser, Google or the victims themselves.
The problems go even higher up the chain, to the legitimate advertising networks that place ads on websites. Cybercriminals frequently manage to trick these networks or their partners into distributing malicious ads, which then end up on popular websites and infect users with malware.
These attacks, collectively known as malvertising, have been going on for years with no end in sight. Ad networks repeatedly claim that they have defenses to prevent such incidents, but time and time again attackers find a way to bypass them. The scale of the problem even prompted harsh criticism by the U.S. Senate and calls for increased regulation of online advertising practices.
The fact that Google has begun scanning Chrome extensions for deceptive ad injecting behavior is a good thing, even though it comes years after security researchers warned about such threats. But, it's also worth keeping in mind that rogue browser extensions are just a small part of the problem.