The Wordfence team at WordPress security company Defiant warns of an increase in attacks targeting an unpatched vulnerability in the Kaswara addon for the WPBakery Page Builder WordPress plugin.
Tracked as CVE-2021-24284 (CVSS score of 10) and disclosed in April 2021, the critical-severity security bug allows an unauthenticated attacker to upload malicious PHP files to a vulnerable site, potentially achieving remote code execution.
When discovered, the flaw was being actively exploited, and Wordfence warned WordPress website administrators that the plugin had been closed without a patch, urging them to remove it immediately.
Although more than a year has passed since the zero-day was disclosed, between 4,000 and 8,000 sites continue to use the plugin, which exposes them to malicious attacks.
Over the past two weeks, Wordfence has seen a massive surge in the number of attack attempts targeting the vulnerability, at an average of 440,000 per day. The attacks come from 10,215 attacking IP addresses, with five IP addresses being responsible for the majority of assaults.
The attackers, Wordfence explains, are probing more than 1,5 million WordPress sites for the vulnerable plugin, but the vast majority of them are not impacted, given that they do not use the plugin.
“The majority of the attacks we have seen are sending a POST request to /wp-admin/admin-ajax.php using the uploadFontIcon AJAX action found in the plugin to upload a file to the impacted website. Your logs may show the following query string on these events: /wp-admin/admin-ajax.php?action=uploadFontIcon HTTP/1.1,” Wordfence says.
Most of the attacks attempt to upload a .ZIP archive containing a malicious PHP file that is extracted to the /wp-content/uploads/kaswara/icons/ directory, and which allows the attackers to deploy additional payloads.
“At this time the plugin has been closed, and the developer has not been responsive regarding a patch. The best option is to fully remove the Kaswara Modern WPBakery Page Builder Addons plugin from your WordPress website,” Wordfence notes.