More than one million WordPress websites were potentially impacted by a critical Ninja Forms plugin vulnerability that appears to have been exploited in the wild.
With over one million installations, the popular Ninja Forms plugin helps administrators add customizable forms to their WordPress sites.
The exploited security issue, which was identified in the Merge Tag functionality of the plugin, does not have a CVE identifier yet, but it has a CVSS score of 9.8.
“One feature of Ninja Forms is the ability to add ‘Merge Tags’ to forms that will auto-populate values from other areas of WordPress like Post IDs and logged in user’s names,” the Wordfence team at WordPress security company Defiant explains.
Because of the bug, it was possible to call various Ninja Form classes and abuse them for “a wide range of exploits targeting vulnerable WordPress sites,” Wordfence researchers say.
The researchers also note that the manner in which the NF_MergeTags_Other class handles Merge Tags makes it possible for unauthenticated attackers to supply Merge Tags.
The Ninja Forms plugin contains various classes and functions that could be leveraged as part of multiple exploit chains, Wordfence also notes.
“One potentially critical exploit chain in particular involves the use of the NF_Admin_Processes_ImportForm class to achieve remote code execution via deserialization, though there would need to be another plugin or theme installed on the site with a usable gadget,” the researchers say.
Wordfence claims to have evidence that the vulnerability “is being actively exploited in the wild,” but has yet to share any details on the exploit chains the attackers are using.
The vulnerability was addressed earlier this week with the release of Ninja Forms versions 18.104.22.168, 3.1.10, 3.2.28, 22.214.171.124, 126.96.36.199, 188.8.131.52, and 3.6.11.
WordPress apparently performed a forced update, meaning that the impacted websites should already be on a patched version. However, administrators are advised to check their Ninja Forms iterations to make sure they use a fixed version.