Urgent: Bit Form Contact Form Plugin (<= 2.20.3) — Unauthenticated Arbitrary File Upload (CVE-2025-6679)

Date: 15 August 2025   |   Severity: Critical / CVSS 10   |   Affected: Bit Form – Contact Form Plugin <= 2.20.3   |   Fixed in: 2.20.4   |   Reporter: Phat RiO – BlueRock

Summary

This disclosure (CVE-2025-6679) describes an unauthenticated arbitrary file upload vulnerability in the Bit Form contact form plugin for WordPress. An unauthenticated attacker can upload arbitrary files — including web shells and executable payloads — enabling full site takeover, data exfiltration, and lateral movement. If you operate WordPress sites with this plugin installed, treat this as an emergency: patch immediately. If immediate patching is not possible, apply the mitigations below without delay.

Note: this advisory is written from the perspective of an experienced Hong Kong security practitioner — practical, prioritized guidance for site owners, administrators, and hosting teams to detect, contain, mitigate and remediate the risk.

Why this is extremely dangerous

• Unauthenticated: no login required — any remote attacker can target the endpoint.
• File upload = shell risk: upload of a PHP file or similar allows remote code execution and persistence.
• High automation potential: attackers quickly weaponize such flaws into mass-exploit scanners and bots.
• Wide reach: contact form plugins are common, increasing the attack surface significantly.

What went wrong (technical overview)

The vendor released a fix in v2.20.4. The root cause is a typical insecure file upload flow combined with missing access controls:

• The plugin exposes an upload handler endpoint that is accessible without authentication.
• Server-side validation is insufficient or absent:
  • No robust extension whitelist or reliable MIME-type verification.
  • File names and paths are not properly sanitized, enabling path traversal or arbitrary filenames.
  • Uploaded files may be stored in web-accessible directories that allow execution.
• Missing CSRF/nonces or weak request validation permit direct POST requests to the upload handler.
• These combined issues allow an attacker to upload a malicious PHP file (web shell) and then execute it via HTTP.

Attackers can craft a multipart/form-data POST to the vulnerable endpoint and drop an arbitrary file which can later be accessed and executed.

Likely exploitation flow (attack chain)

1. Reconnaissance: scan sites for the plugin and probe for the upload endpoint.
2. Upload attempt: send a multipart/form-data POST with a file named like shell.php containing a backdoor.
3. File stored: insufficient validation causes the file to be saved to the server (often wp-content/uploads/ or a plugin folder).
4. Execution: attacker requests the uploaded file (e.g., https://example.com/wp-content/uploads/shell.php) to run server-side.
5. Post-exploitation: persistent backdoors, new admin users, data theft, and lateral movement.

Immediate actions — Checklist (0–60 minutes)

If this plugin is present, follow this triage checklist immediately:

1. Identify installed version:
   • Dashboard: WordPress > Plugins and check Bit Form version.
   • WP-CLI: wp plugin list --status=active --format=csv | grep bit-form
2. If the plugin is at or below 2.20.3, do not wait — proceed with mitigation.
3. Quick protective controls if you cannot update immediately:
   • Disable the plugin temporarily:
     • WP admin: Plugins > deactivate Bit Form.
     • WP-CLI: wp plugin deactivate bit-form
   • If deactivation is not possible for business reasons, restrict access to the upload endpoint via server rules (examples below).
   • Put the site into maintenance mode while you patch where practical.
4. Update to patched version 2.20.4 immediately:
   • WordPress dashboard plugin update or WP-CLI: wp plugin update bit-form --version=2.20.4
5. If you suspect compromise: isolate the site (offline or restrict access), preserve logs, take a filesystem snapshot, and proceed with incident response steps below.

Emergency mitigations when you cannot update immediately

If you cannot update to 2.20.4 right away, apply one or more of the following mitigations. Combining measures reduces risk further.

1. Block the vulnerable upload handler endpoint:

   Add a server or WAF rule to block POST requests to the plugin’s upload URI pattern. If the exact endpoint is unknown, temporarily block unauthenticated POSTs to /wp-content/plugins/bit-form/**.

2. Prevent PHP execution in upload directories:

   Place a webserver configuration or .htaccess to deny script execution in the upload directory.

   # Apache .htaccess example
   <FilesMatch "\.(php|phtml|phar|pl|py|jsp|asp|aspx)$">
     Require all denied
   </FilesMatch>
         

   # Nginx example
   location ~* /wp-content/uploads/.*\.(php|phtml|phar)$ {
       deny all;
   }
         

3. Disable file upload functionality inside the plugin:
   • If possible, turn off attachments via plugin settings.
   • As a temporary measure, you may rename the upload handling file on disk (document changes carefully).
4. File and execution permissions:
   • Ensure uploads are not executable. Typical permissions: files 644, directories 755.
   • Confirm webserver user cannot execute PHP from uploads locations.
5. WAF virtual patching:

   Apply rules to block suspicious multipart uploads, disallow PHP-like extensions, and detect webshell markers (base64_eval, preg_replace with /e, etc.).

6. Rate-limit and anomaly detection:

   Throttle high-volume POSTs and rapidly block suspicious IPs.

Detection — how to know if you were attacked

Check these indicators and logs immediately:

• Web access logs:
  • POST requests to plugin URLs with multipart/form-data bodies.
  • Follow-up GETs to files under wp-content/uploads/ or plugin folders with .php, .phtml, .phar extensions.
  • Suspicious User-Agents or bursts of requests from single IPs.
• File system checks:
  • Search for newly created files with executable extensions under wp-content/uploads, wp-content/plugins, or plugin-specific folders.
  • Example commands:

    find /var/www/html/wp-content/uploads -type f -mtime -7 -print
    find /var/www/html -type f \( -iname "*.php" -o -iname "*.phtml" \) -mtime -7 -print
              

  • Grep for suspicious content:

    grep -R --line-number -E "eval\(|base64_decode\(|gzinflate\(|shell_exec\(|passthru\(" /var/www/html/wp-content/uploads
              

• WordPress logs and database: new admin users, unexpected posts, unknown options entries, and unexpected cron tasks.
• Malware scan: run server-side scanners that detect web shells and known signatures.
• Host/Server telemetry: unexpected CPU spikes, outgoing connections to unfamiliar IPs, new processes, or new SSH keys.

If you find suspect files or activity, treat the site as compromised and follow the incident response steps below.

Incident response and recovery (if compromised)

If compromise is confirmed, act methodically:

1. Isolate: take the site offline or block traffic at network/WAF to prevent further attacker actions.
2. Preserve evidence: copy access/error logs, create database backups, and snapshot the filesystem (read-only if possible).
3. Identify and remove backdoors: search for web shells (small PHP files, base64 payloads, eval/gzinflate calls). Archive evidence before removal.
4. Rebuild from clean backup: restore from a known-good backup taken before the compromise; update core, themes and plugins before restoring public access.
5. Update and harden: update Bit Form to 2.20.4, rotate credentials (admin, FTP/SFTP/SSH, API keys), audit users, and re-scan for malware.
6. Post-incident monitoring: increase logging/monitoring for several weeks to detect persistence.
7. Notification and compliance: if personal or regulated data was exposed, follow applicable breach notification rules and inform stakeholders as required.

WAF rule suggestions and examples

Adapt these examples to your environment. These are compensating controls — they reduce risk but do not replace updating the plugin.

1. Block uploads with PHP-like filenames (generic regex-based)

# ModSecurity example (conceptual)
SecRule REQUEST_HEADERS:Content-Type "multipart/form-data" "phase:2,chain,deny,log,msg:'Block suspicious file upload - php extension'"
SecRule FILES_NAMES|FILES_NAMES:something "@rx \.(php|phtml|phar|pl|py|jsp|asp|aspx)$" "t:none"
  

2. Block direct POSTs to plugin folder (Nginx example)

location ~* /wp-content/plugins/bit-form/.* {
    if ($request_method = POST) {
        return 403;
    }
}
  

Use this temporarily and validate you are not breaking legitimate site features.

3. Block requests containing webshell indicators

Implement rules that detect webshell markers in POST bodies — example indicators: eval(base64_decode(, gzinflate(, system(, shell_exec( — and block and log matches.

4. Enforce CSRF/nonce checks (virtual patch)

If feasible, require a valid WordPress nonce or custom header token for access to plugin endpoints.

5. File-size and extension whitelist

Allow only known-safe file types (images: jpg/png/gif; PDFs if required) and disallow others.

6. Rate-limit upload attempts per IP

Rate-limit POSTs to form-upload endpoints to reduce automated exploitation attempts.

How to update safely (recommended remediation)

1. Backup first: full database and file backup before changes.
2. Apply plugin update:
   • Dashboard: Plugins > update Bit Form to 2.20.4.
   • WP-CLI: wp plugin update bit-form
3. Verify: test contact form functionality in staging and monitor logs for attempts against the old interface.
4. Post-update tasks: re-run malware scans and review recently created files and users for prior compromise.

Detection scripts and commands (practical)

Quick commands for Linux/Unix to look for suspicious files and activity:

# Find recently modified files (last 7 days)
find /var/www/html -type f -mtime -7 -print > /root/recent-files.txt

# Search for PHP webshell patterns
grep -R --line-number -E "eval\(|base64_decode\(|gzinflate\(|preg_replace\(.*/e" /var/www/html || true

# Find risky extensions in content folders
find /var/www/html/wp-content/uploads -type f \( -iname "*.php" -o -iname "*.phtml" -o -iname "*.phar" \) -print

# Check access logs for suspicious POSTs
zgrep -i "POST.*bit-form" /var/log/nginx/access.log* | tail -n 200
zgrep -i "multipart/form-data" /var/log/nginx/access.log* | grep "bit-form" | less

# List WordPress admin users
wp user list --role=administrator
  

Hardening to reduce risk going forward

• Run PHP and file system processes with least privilege.
• Disable unnecessary plugin features (e.g., file uploads) if not required.
• Maintain timely updates for core, themes and plugins.
• Enforce file type whitelists and scan uploads for malicious content.
• Protect upload directories with server-level non-executable policies.
• Implement multi-factor authentication for all admin users.
• Use monitoring and file-integrity checks (hash-based) and keep off-site backups.

Communication guidance for agencies and hosts

If you manage client sites or host multiple tenants, prioritise as follows:

1. Identify all sites with the vulnerable plugin installed.
2. Prioritise updates for high-risk sites (e-commerce, membership, sites with sensitive data).
3. For sites that cannot be updated immediately, apply server-wide WAF rules to block the vulnerable path or block uploads with PHP-like extensions until upgrades are completed.
4. Notify site owners clearly: explain the risk concisely and instruct them to update to 2.20.4 immediately or apply the mitigations listed.
5. For managed hosting, consider temporarily disabling plugin functionality for tenants that cannot update promptly and provide assisted remediation windows.

Examples of suspicious access patterns (what to look for in logs)

• Repeated POSTs to /wp-content/plugins/bit-form/upload.php or similar paths.
• POSTs with Content-Type: multipart/form-data and unusually small or blank User-Agent fields.
• Immediate GETs to /wp-content/uploads/<random>.php returning 200 or 500.
• POST bodies containing “

Prevention is better than cure: layering protection

Adopt a layered approach:

• Preventive: timely patching, least privilege, disable unnecessary features.
• Detective: logging, file integrity monitoring, malware scanning.
• Responsive: WAF virtual patching, incident response playbooks, offline backups.
• Continuous: automated updates where possible, 24/7 monitoring and regular security reviews.

FAQ (quick answers)

Q: If I update to 2.20.4, am I safe?
A: Updating removes the vulnerability from the plugin code. If your site was already exploited, you must investigate, remove backdoors, rotate credentials, and re-scan for malware.

Q: Can I rely on just a WAF?
A: A WAF is a valuable compensating control and can block many exploitation attempts, but it does not replace applying the vendor fix and performing a full cleanup if compromise occurred.

Q: What if I can’t update because the plugin is required and incompatible?
A: Apply strict WAF rules to block the upload endpoint, prevent PHP execution in uploads, and plan to migrate to a supported plugin or implement a code-level fix. Consider contracting a professional incident response team for risk-based remediation.

Quick prioritized remediation plan (one-page for site owners)

1. Check plugin version. If ≤ 2.20.3, act immediately.
2. Backup files and database.
3. Deactivate plugin OR update to 2.20.4.
4. If you cannot update immediately:
   • Block upload endpoint at webserver/WAF level.
   • Deny execution in uploads directory.
   • Enforce strict file-type whitelist.
5. Scan for indicators of compromise and review logs (last 30 days).
6. If compromised: isolate, preserve logs, rebuild from clean backup, rotate credentials.
7. Re-enable features only after thorough verification and monitoring.

Final notes and recommended timelines

• Immediate (within 1 hour): identify affected sites and apply emergency mitigations (deactivate plugin or block endpoint).
• Short term (within 24 hours): update plugin to 2.20.4 on all affected sites.
• Medium term (1–2 weeks): scan and verify no compromises remain; implement server hardening and WAF rules.
• Long term (ongoing): maintain an update policy, continuous monitoring, regular backups, and periodic security reviews.

If you manage multiple sites, automate detection of vulnerable plugin versions and roll out updates centrally where possible. If you require assistance triaging or applying mitigations, engage a qualified security or incident response provider promptly.

Stay vigilant. Patch as soon as possible and treat any evidence of compromise as an urgent incident.