Some of the Netherlands' most popular websites have fallen victim to a malvertising campaign that managed to compromise a widely used ad platform, security researchers reported on Monday.

The malicious ads were served over at least 11 sites including marktplaats.nl, the Netherlands equivalent to eBay and the country's seventh most visited website, according to a blog post published by security firm Fox IT. Other affected sites included news site nu.nl (which is ranked No. 14), weather site buienradar.nl (54), and startpagina.nl (67). Other widely visited sites were operated by commercial TV stations and magazines.

According to the blog post:

The malvertising is occurring through an advertisement platform which is actively used on the above mentioned websites. From the websites, external scripts are loaded which in turn redirect further towards the exploit kit. We’ve observed the Angler Exploit Kit being active on these redirects during this campaign. We have not seen any successful infections at our customer yet. One of the redirects towards the Angler exploit kit as observed by our monitoring platform:

The report comes a month after visitors to The New York Times, the BBC, MSN, AOL, and other big-name websites were hit by a malvertising campaign that spread the Teslacrypt ransomware package. That attack also relied on the Angler exploit kit. To maintain stealth, the exploits contained highly obfuscated code that avoided infecting visitors running certain types of security software. Malicious ads generally exploit security vulnerabilities in Adobe Flash, the Java Software Framework, or older versions of Internet Explorer and prey upon people who have not yet installed updates. Occasionally, the underlying exploit kits exploit zero-day vulnerabilities , which by definition have not yet been patched.

So far, none of the affected websites have said how the campaign managed to slip past defenses designed to detect malicious ads or explained what they're doing to prevent similar malvertising campaigns in the future.