Security teams that already have their hands full dealing with COVID-19-related threats now have another issue to contend with.

Attacks on WordPress sites have soared in recent days to more than 30 times the normal volume. This week researchers from WordPress firewall provider Defiant reported observing attack attempts on more than 900,000 websites since April 28.

On May 3 alone, Defiant counted in excess of 20 million attacks against some 500,000 WordPress sites. Over the course of the past month, Defiant says it detected more than 24,000 distinct IP addresses being used to launch attacks on WordPress.

According to Defiant, a single threat actor appears to be behind a majority of the attacks, many of which have targeted known cross-site scripting (XSS) vulnerabilities in third-party WordPress plug-ins. The same adversary is also targeting other, older vulnerabilities in WordPress to attack sites, the vendor said in a report this week.

The attacks involving the XSS vulnerabilities are focused primarily on planting a backdoor on the victim site, says Ram Gall, a software quality assurance engineer at Defiant.

The backdoor's payload is designed to add a malicious JavaScript to every page on the site. This would redirect any visitors to sites hosting malicious advertisements.

If a victim is logged into WordPress as an administrator, the script attempts to inject a malicious PHP backdoor into the site, Defiant said. The backdoor then downloads another payload that would ultimately give the attacker control over the domain. The access would allow the threat actor to do things like embed a Web shell, create a malicious administrator account, or delete the content of the site.

"For the non-XSS attacks we saw, the attacker attempted to redirect visitors to the same malvertising campaign by changing the site's home URL," Gall notes.

The sheer volume and variety of the attacks — and the fact they involve older vulnerabilities — suggest the attacks are not targeted. "This campaign was not aimed at any specific category of sites," Gall says. "Ultimately, with any campaign like this the motivation is monetization," he says.

Many of the vulnerabilities the threat actor has targeted have been exploited in previous campaigns as well, Defiant said. According to the vendor, more than half of the attacks involved a plug-in called Easy2Map that was removed from WordPress' repository last August because of security issues. Other vulnerabilities that have been heavily targeted in the campaign include an XSS vulnerability in a plug-in named Blog Designer, another in a WP GDPR Compliance plug-in, and one in a now-pulled plug-in named Total Donations.

"Although it is not readily apparent why these vulnerabilities were targeted, this is a large-scale campaign that could easily pivot to other targets," Defiant warns in its alert.

Third-Party Plug-in Risks
The large-scale attacks are another reminder of the risks WordPress operators run in using third-party plug-ins on their sites. Over the years, tens of thousands of plug-ins have become available for WordPress that allow owners and operators to extend site functionality. But vulnerabilities in these plug-ins — and the fact that many sites often use old and out-of-date plug-ins — have made them hugely popular targets for attackers.

Ameet Naik, security evangelist at PerimeterX, says vulnerable WordPress plug-ins represent a critical third-party risk. "Over 70% of the scripts on a website are third-party, which represents a significant risk for any website owner," he says.

In addition to making security an integral part of the development cycle, website owners can also follow a few simple steps to mitigate third-party plug-in risks. These includes scanning the site for known vulnerable libraries, using content security policies to restrict access, using Web application firewalls to mitigate XSS attacks, and using client-side runtime visibility tools for detecting malicious script activity, Naik says.

For organizations, such attacks demonstrate the need for a layered security approach, Gall says.

"Keep WordPress core, plug-ins, and themes up to date," he notes. "Deactivate and delete any plug-ins or themes that have been removed from the WordPress repository."

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