CVE-2020-35489
📋 TL;DR
This vulnerability in Contact Form 7 WordPress plugin allows unrestricted file upload due to improper filename validation. Attackers can upload malicious files containing special characters, leading to remote code execution. All WordPress sites using Contact Form 7 versions before 5.3.2 are affected.
💻 Affected Systems
- Contact Form 7 WordPress Plugin
📦 What is this software?
Contact Form 7 by Rocklobster
⚠️ Risk & Real-World Impact
Worst Case
Complete server compromise allowing attackers to execute arbitrary code, deface websites, steal data, install backdoors, or pivot to internal networks.
Likely Case
Website defacement, malware distribution, credential theft, or installation of cryptocurrency miners through uploaded malicious files.
If Mitigated
File uploads blocked or sanitized, preventing execution of malicious payloads while maintaining form functionality.
🎯 Exploit Status
Exploits are publicly available and require no authentication. Attackers can directly upload malicious files through contact forms.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.3.2 and later
Vendor Advisory: https://contactform7.com/2020/12/17/contact-form-7-532/
Restart Required: No
Instructions:
1. Log into WordPress admin panel. 2. Go to Plugins → Installed Plugins. 3. Find Contact Form 7. 4. Click 'Update Now' if available. 5. Alternatively, download version 5.3.2+ from WordPress repository and replace plugin files.
🔧 Temporary Workarounds
Disable File Uploads
allTemporarily disable file upload functionality in all Contact Form 7 forms
Edit each contact form and remove file upload fields
Web Application Firewall Rules
linuxBlock requests containing special characters in filenames
ModSecurity rule: SecRule ARGS "@rx [\\\/\|\*\?\<\>\:\"]" "id:1001,phase:2,deny,status:403,msg:'Blocked special chars in filename'"
WAF configuration to block uploads with special characters in Content-Disposition headers
🧯 If You Can't Patch
- Implement strict file upload restrictions at web server level (Apache/Nginx)
- Use security plugins like Wordfence or Sucuri to monitor and block malicious uploads
🔍 How to Verify
Check if Vulnerable:
Check WordPress admin → Plugins → Contact Form 7 → Version. If version is below 5.3.2, you are vulnerable.Check Version:
wp plugin list --name=contact-form-7 --field=version (if WP-CLI installed) or check wp-content/plugins/contact-form-7/readme.txtVerify Fix Applied:
Verify Contact Form 7 version is 5.3.2 or higher. Test file upload functionality with filenames containing special characters - they should be rejected.📡 Detection & Monitoring
Log Indicators:
- HTTP POST requests to /wp-admin/admin-ajax.php with file uploads
- Web server logs showing uploads of files with special characters in names
- PHP error logs showing file execution attempts
Network Indicators:
- Unusual file uploads to contact form endpoints
- POST requests with multipart/form-data containing special characters in filenames
SIEM Query:
source="web_server_logs" AND (uri_path="/wp-admin/admin-ajax.php" OR uri_path="/wp-json/contact-form-7/") AND http_method="POST" AND (filename CONTAINS ".." OR filename CONTAINS "/" OR filename CONTAINS "\" OR filename CONTAINS ":")🔗 References
- https://contactform7.com/2020/12/17/contact-form-7-532/
- https://wordpress.org/plugins/contact-form-7/#developers
- https://wpscan.com/vulnerability/10508
- https://www.getastra.com/blog/911/plugin-exploit/contact-form-7-unrestricted-file-upload/
- https://www.jinsonvarghese.com/unrestricted-file-upload-in-contact-form-7/
- https://contactform7.com/2020/12/17/contact-form-7-532/
- https://wordpress.org/plugins/contact-form-7/#developers
- https://wpscan.com/vulnerability/10508
- https://www.getastra.com/blog/911/plugin-exploit/contact-form-7-unrestricted-file-upload/
- https://www.jinsonvarghese.com/unrestricted-file-upload-in-contact-form-7/
