It's not clear how many users were impacted, but the affected websites have over 250 million monthly visits combined, according to researchers from Malwarebytes who spotted and analyzed the attack.
The malicious ads were posted through an advertising network called AdXpansion that was abused in similar incidents in the past.
The attackers managed to distribute through the network a Flash-based ad that attempted to exploit a vulnerability in Flash Player.
The flaw affects Flash Player through version 17.0.0.134, which was released within the last two months, the Malwarebytes researchers said in a blog post Thursday. Affected sites listed in the blog post include Drtuber.com, Nuvid.com, Hardsextube.com and Justporno.tv.
Unlike more subtle malvertising attacks, where rogue ads redirect the user's browser to a third-party server that hosts an exploit kit, in this attack the ad launched the exploit code directly.
This approach might be an attempt to evade detection by security tools that track exploit kit servers and traffic patterns.
"It is interesting to see the trend of exploit kits taking the appearance of advertisers by leveraging Flash for serving the 'creative' and exploit in one single package," the researchers said. "It is a minimalist type of approach which seems to work quite efficiently."
Malvertising attacks have been frequent occurrences in recent months and have affected a large number of ad networks, despite efforts from their owners to prevent them.
On Thursday, security researchers from Trend Micro reported a separate attack, where cybercriminals posted malicious ads through the compromised servers of an advertising company called Mad Ads Media.