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Unrestricted File UploadWeakness · CWE-434

CVE-2026-4883

CRITICAL · 9.8 CVSS v3.1 Published 2026-05-19
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
The Piotnet Forms plugin for WordPress is vulnerable to arbitrary file upload due to missing file type validation in the 'piotnetforms_ajax_form_builder' function in all versions up to, and including, 2.1.40. The plugin uses an incomplete extension blacklist that only blocks php, phpt, php5, php7, and exe extensions, while allowing dangerous extensions such as .phar or .phtml to be uploaded. This makes it possible for unauthenticated attackers to upload arbitrary files on the affected site's server which may make remote code execution possible. Note: The exploit can only be exploited if a file field is added to the form.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The Piotnet Forms WordPress plugin versions up to 2.1.40 contains an arbitrary file upload vulnerability in the 'piotnetforms_ajax_form_builder' function. The plugin uses an incomplete blacklist that only blocks php/phpt/php5/php7/exe extensions while allowing dangerous extensions like .phar and .phtml, enabling unauthenticated attackers to upload arbitrary files and achieve remote code execution. Exploitation requires a file field to be present on the form.

MitigationUpdate Piotnet Forms to a version beyond 2.1.40 that implements proper file type validation (whitelist approach), or disable the plugin and remove any forms containing file upload fields until a patch is available.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify Piotnet Forms plugin version
    Navigate to WordPress admin > Plugins > Installed Plugins and locate 'Piotnet Forms' in the list. The version number is displayed below the plugin name. Alternatively, check the plugin's main PHP file header for the Version field.
    Affected if The installed version is 2.1.40 or any earlier version
  2. Confirm file upload fields exist in any form
    In WordPress admin, go to Piotnet Forms > All Forms and review each form. Check if any form contains a 'File Upload' or similar field element. This can also be verified by examining the form's shortcode or database entries in wp_posts where post_type relates to piotnet forms.
    Affected if At least one form contains an enabled file upload field
  3. Verify the plugin file upload endpoint is accessible
    Test access to the AJAX endpoint used by piotnetforms_ajax_form_builder by submitting a request to /wp-admin/admin-ajax.php with action=piotnetforms_ajax_form_builder. This requires checking network logs or attempting a benign request.
    Affected if The endpoint responds without authentication errors, indicating it is accessible to unauthenticated users
  4. Review file extension blacklist configuration
    Inspect the plugin source code, specifically the function handling file uploads (piotnetforms_ajax_form_builder). Look for the list of blocked extensions in the file validation logic.
    Affected if The blacklist only includes php/phpt/php5/php7/exe extensions and does NOT include .phar or .phtml extensions

A user is affected if Piotnet Forms version 2.1.40 or earlier is installed AND at least one form contains a file upload field that is publicly accessible.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Update Piotnet Forms to a version beyond 2.1.40 that implements proper file type validation (whitelist approach), or disable the plugin and remove any forms containing file upload fields until a patch is available.

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-4883 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
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