Critical flaw in Ninja Forms WordPress Plugin actively exploited in the wild

A critical vulnerability in Ninja Forms plugin potentially impacted more than one million WordPress websites

In middle June, the Wordfence Threat Intelligence team noticed a back-ported security update in the popular WordPress plugin Ninja Forms, which has over one million active installations. The analysis of the updates revealed that they patched a code injection vulnerability that an unauthenticated attacker can exploit to execute arbitrary code or delete arbitrary files on the websites where a separate POP chain was present.

The researchers believe that this vulnerability is being actively exploited in the wild, it has been rated with a CVSS score of 9.8.

Ninja Forms is a drag-and-drop form builder plugin for WordPress builder that allows users to easily create complex forms within just a few minutes.

The vulnerability resides in the Merge Tag feature of the plugin.

“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. Unfortunately, this functionality had a flaw that made it possible to call various Ninja Form classes that could be used for a wide range of exploits targeting vulnerable WordPress sites.” reads the advisory published by Wordfence. “Without providing too many details on the vulnerability, the Merge Tag functionality does an is_callable() check on a supplied Merge Tags.”When a callable class and method is supplied as a Merge Tag, the function is called and the code executed. These Merge Tags can be supplied by unauthenticated users due to the way NF_MergeTags_Other class handles Merge Tags. We determined that this could lead to a variety of exploit chains due to the various classes and functions that the Ninja Forms plugin contains.”

The experts explained that the issue can be abused for a wide range of exploits.

One of the potentially critical exploit chain described by the researchers involves the use of the NF_Admin_Processes_ImportForm class to achieve remote code execution via deserialization.

“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.” added the researchers.

The flaw was addressed with the release of Ninja Forms versions 3.0.34.2, 3.1.10, 3.2.28, 3.3.21.4, 3.4.34.2, 3.5.8.4, and 3.6.11.

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Pierluigi Paganini

(SecurityAffairs – hacking, WordPress)

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