Following a link from an unsolicited email or browsing shady sites comes with common consequences, and topping the list is exposure to malware through fake downloads and malvertising campaigns. But many users assume mainstream websites, including news portals, entertainment sites and highbrow political commentary offerings, are immune to this kind of trickery.

According to Trend Micro, that’s no longer the case: Last weekend, several security vendors discovered a massive malvertising campaign linked to the Angler exploit kit targeting top-tier websites. While most of the affected sites have fixed the problem and are back on track, tens of thousands of browsers may have been exposed. Here’s what researchers know so far.

Bait and Switch

According to Joseph C. Chen of Trend Micro, there was a large uptick in Angler activity on March 13 when this malvertising campaign got underway. When users visited any page hosting the compromised ad code, the ad itself automatically redirected to two malware servers, one of which downloaded the Angler exploit kit.

The kit then downloaded a BEDEP variant and dropped the TROJ_AVRECON malware. Of course, this begs the question: How did mainstream sites partnered with large, legitimate digital ad agencies get infected in the first place?

As noted by PCWorld, it likely started with an old domain named brentsmedia.com, which may have been connected with above-board online advertising. Once in the hands of Angler developers, however, it was used to trick big ad companies into publishing infected ads on the websites of mainstream clients.

A closer look at some JSON code from the BrentsMedia site found that it had 12,000 lines of concealed JavaScript code. That code attempted to discover whether victim computers were running specific security tools. If not, they were quickly sent to a new, Angler-infected landing page.

Tricky Tools for Malvertising

This isn’t the first foray into mainstream media for Angler this year. In late February, Threatpost reported that Angler added browser-based anti-malware defenses and then targeted a popular software site, exposing almost 1 million users to potential TeslaCrypt infection.

Beta News, meanwhile, noted that malvertising tools are now using fingerprinting techniques to detect whether a target computer is actually a honeypot set up to trap their code or whether it is under the control of a security researcher. This allows cybercriminals to screen targets and only infect computers that aren’t protected by security software and have no virtual sandboxes.

This step-up for Angler shouldn’t come as a surprise. What malware-makers can’t take by force they’ll grab by deception, and mainstream sites offer such a massive user pool it was only a matter of time before malvertisers found a way inside.

While there’s no way to completely safeguard any online experience, users can limit their risk by running up-to-date security tools, never allowing code to execute without permission and always patching browser plugins. Bottom line? Any ad is a suspect ad that may be looking to multiply device risk, divide and conquer PC defenses and jeopardize user privacy.