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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Pierluigi Paganini

Pierluigi Paganini is member of the ENISA (European Union Agency for Network and Information Security) Threat Landscape Stakeholder Group and Cyber G7 Group, he is also a Security Evangelist, Security Analyst and Freelance Writer. Editor-in-Chief at "Cyber Defense Magazine", Pierluigi is a cyber security expert with over 20 years experience in the field, he is Certified Ethical Hacker at EC Council in London. The passion for writing and a strong belief that security is founded on sharing and awareness led Pierluigi to find the security blog "Security Affairs" recently named a Top National Security Resource for US. Pierluigi is a member of the "The Hacker News" team and he is a writer for some major publications in the field such as Cyber War Zone, ICTTF, Infosec Island, Infosec Institute, The Hacker News Magazine and for many other Security magazines. Author of the Books "The Deep Dark Web" and “Digital Virtual Currency and Bitcoin”.