CVE-2021-24884
📋 TL;DR
This vulnerability in the Formidable Form Builder WordPress plugin allows unauthenticated attackers to inject malicious HTML links containing JavaScript. When authenticated users click these links, attackers can execute arbitrary code with the user's privileges, potentially leading to account takeover or remote code execution. All WordPress sites using vulnerable versions of this plugin are affected.
💻 Affected Systems
- Formidable Form Builder WordPress Plugin
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
An attacker could achieve remote code execution, compromise the entire WordPress site, steal sensitive data, and potentially pivot to other systems.
Likely Case
Attackers would typically use this to hijack administrator accounts, modify site content, or install backdoors for persistent access.
If Mitigated
With proper user awareness training and least privilege principles, impact would be limited to the privileges of the tricked user.
🎯 Exploit Status
Exploitation requires tricking an authenticated user to click a malicious link, but the technical barrier is low with public proof-of-concept available.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.09.05 and later
Vendor Advisory: https://github.com/Strategy11/formidable-forms/pull/335/files
Restart Required: No
Instructions:
1. Log into WordPress admin panel. 2. Navigate to Plugins → Installed Plugins. 3. Find 'Formidable Forms' and check version. 4. If below 4.09.05, click 'Update Now'. 5. Verify update to 4.09.05 or higher.
🔧 Temporary Workarounds
Disable vulnerable feature
allDisable the web-based entry inspection page if not required
Content Security Policy
allImplement strict CSP headers to prevent script execution from untrusted sources
🧯 If You Can't Patch
- Implement web application firewall rules to block HTML injection attempts
- Restrict plugin access to trusted users only and implement clickjacking protection
🔍 How to Verify
Check if Vulnerable:
Check WordPress admin panel → Plugins → Formidable Forms → Version number. If version is below 4.09.05, you are vulnerable.Check Version:
wp plugin list --name=formidable --field=versionVerify Fix Applied:
After updating, verify version is 4.09.05 or higher in WordPress admin panel.📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to form entry inspection pages
- Multiple failed attempts to access admin functions from new IPs
Network Indicators:
- HTTP requests containing suspicious HTML tags in form data parameters
- Unexpected outbound connections after form submissions
SIEM Query:
source="wordpress.log" AND ("data-frmverify" OR "frmverify") AND status=200🔗 References
- https://github.com/S1lkys/XSS-in-Formidable-4.09.04/blob/main/XSS-in-Formidable-4.09.04.pdf
- https://github.com/Strategy11/formidable-forms/pull/335/files
- https://wpscan.com/vulnerability/b57dacdd-43c2-48f8-ac1e-eb8306b22533
- https://github.com/S1lkys/XSS-in-Formidable-4.09.04/blob/main/XSS-in-Formidable-4.09.04.pdf
- https://github.com/Strategy11/formidable-forms/pull/335/files
- https://wpscan.com/vulnerability/b57dacdd-43c2-48f8-ac1e-eb8306b22533
