CVE-2026-110899: Ninja Forms Stored XSS Vulnerability

CVE-2026-110899 Overview

CVE-2026-110899 is a stored cross-site scripting (XSS) vulnerability in the Ninja Forms WordPress plugin developed by Kevin Stover. The flaw affects all versions of Ninja Forms up to and including 3.15.5. The plugin fails to properly neutralize user-supplied input during web page generation, allowing attackers to inject persistent malicious scripts [CWE-79]. Successful exploitation requires user interaction and executes attacker-controlled JavaScript in the context of the vulnerable WordPress site. The scope change in the Common Vulnerability Scoring System (CVSS) vector reflects impact beyond the vulnerable component, affecting site visitors and administrators who view the injected content.

Critical Impact
Attackers can inject persistent JavaScript into Ninja Forms output, enabling session theft, administrative account takeover, and arbitrary actions in victim browsers.

Affected Products

Ninja Forms plugin for WordPress — versions through 3.15.5
WordPress sites running the vulnerable ninja-forms plugin
All sites where unauthenticated or low-privileged users can submit form data processed by the plugin

Discovery Timeline

2026-10-21 - CVE CVE-2026-110899 published to the National Vulnerability Database (NVD)
2026-10-21 - Last updated in NVD database

Technical Details for CVE-2026-110899

Vulnerability Analysis

The vulnerability is a stored XSS flaw in Ninja Forms, a widely deployed WordPress form builder plugin. Ninja Forms accepts user input through form fields and persists this data server-side. When the stored data is later rendered in a web page, the plugin fails to sanitize or encode HTML metacharacters. This allows an attacker to submit input containing <script> tags or event-handler attributes that execute when another user — including an administrator reviewing submissions — loads the affected page. Because the payload persists, every subsequent viewer of the rendered content triggers the injected script.

Root Cause

The root cause is improper neutralization of input during web page generation [CWE-79]. The plugin writes attacker-controlled strings into HTML output without applying context-appropriate escaping functions such as esc_html(), esc_attr(), or wp_kses(). Any input flowing from form submissions to administrative dashboards, confirmation pages, or email previews is rendered as live markup rather than inert text.

Attack Vector

Exploitation occurs over the network and requires user interaction. An attacker submits a crafted payload through an exposed Ninja Forms form on a target site. The payload is stored in the WordPress database. When a victim — typically an administrator viewing submissions in wp-admin — loads the page containing the stored input, the browser parses and executes the injected JavaScript. The attacker can hijack the session, perform actions as the administrator, exfiltrate data, or pivot to install malicious plugins. See the Patchstack Ninja Forms XSS Vulnerability advisory for additional technical context.

Detection Methods for CVE-2026-110899

Indicators of Compromise

Form submissions in the WordPress database containing HTML tags such as <script>, <img onerror=>, or <svg onload=> in Ninja Forms submission tables
Unexpected outbound requests from administrator browsers to attacker-controlled domains after viewing form submissions
New or modified WordPress administrator accounts created shortly after a form submission was viewed
Unauthorized plugin installations or theme file modifications in wp-content/

Detection Strategies

Query the wp_nf3_submissions and related Ninja Forms tables for stored input containing HTML metacharacters or JavaScript event handlers
Monitor the WordPress access.log for POST requests to Ninja Forms submission endpoints followed by administrative page loads
Review Content Security Policy (CSP) violation reports for inline script executions on pages that render form data

Monitoring Recommendations

Enable WordPress audit logging to capture submission content, administrator session activity, and user role changes
Forward web server and WordPress logs to a centralized analytics platform for correlation of submission events with administrative access
Alert on administrator sessions that originate from unexpected IP addresses immediately after high-risk pages load

How to Mitigate CVE-2026-110899

Immediate Actions Required

Update the Ninja Forms plugin to a version later than 3.15.5 as soon as the vendor publishes a fixed release
Audit existing Ninja Forms submissions for stored HTML or JavaScript payloads and purge malicious entries
Rotate credentials and session cookies for any WordPress administrator who viewed submissions before patching
Review installed plugins, themes, and administrator accounts for unauthorized changes

Patch Information

Consult the Patchstack Ninja Forms XSS Vulnerability advisory for the current patched version. The vulnerability affects Ninja Forms through 3.15.5; apply the vendor's security release once available through the WordPress plugin repository.

Workarounds

Temporarily disable the Ninja Forms plugin on sites that cannot be patched immediately
Deploy a Web Application Firewall (WAF) rule that blocks form submissions containing HTML tags or JavaScript event handlers
Enforce a strict Content Security Policy that disallows inline scripts on pages rendering form data
Restrict access to wp-admin by IP address to reduce exposure of administrator sessions

bash

# Example WAF rule concept (ModSecurity) to block script tags in POST bodies
SecRule REQUEST_METHOD "@streq POST" \
  "chain,phase:2,deny,status:403,id:1109900,msg:'Potential XSS in Ninja Forms submission'"
SecRule REQUEST_BODY "@rx (?i)(<script|onerror=|onload=|javascript:)" "t:none"