CVE-2025-3439 - PHP Object Injection Vulnerability in Everest Forms for WordPress (Up to v3.1.1) — Details, Exploit, and What You Need to Know

WordPress website owners: a serious security flaw has been discovered in the popular Everest Forms plugin, officially tracked as CVE-2025-3439. This vulnerability affects all versions up to and including 3.1.1. If you use this plugin to manage your contact, quiz, survey, or newsletter forms, you should pay close attention.

The problem is a classic programming error: PHP Object Injection via the field_value parameter. This issue allows an unauthenticated attacker to inject arbitrary PHP objects into your WordPress site. While this doesn’t mean automatic disaster (yet), if your website also has another plugin or theme installed that contains a so-called “POP chain”, you could be at serious risk — including file deletion, data theft, and even remote code execution.

What Is PHP Object Injection?

In PHP, serialization is used to convert objects to storable strings and later restore them. Object injection happens when unsanitized input is passed to a PHP unserialize() function, letting attackers craft objects or even break into functionality that wasn’t intended (through magic methods like __wakeup, __destruct, etc.).

POP chain (Property Oriented Programming chain) is a series of classes and methods (often across different codebases/plugins) that let an attacker achieve specific goals (like file deletion or code execution) when objects are unserialized.

The tricky part: By itself, Everest Forms doesn’t provide a POP chain. So, on a barebones install just with Everest Forms, an attacker gets nowhere. But if you happen to have another theme or plugin with a vulnerable chain, all bets are off.

Where Does the Problem Live?

The Everest Forms plugin lets users submit data through forms. One hidden parameter, field_value, is passed to the backend and ends up in a PHP unserialize() call with no filtering.

The problematic code lives somewhere like this (simplified for clarity)

// Receives form data through a POST request
$field_value = $_POST['field_value'];

if (!empty($field_value)) {
    // DANGEROUS: Untrusted user input gets unserialized!
    $object = unserialize($field_value);
    // ...further actions...
}

That unserialize() is a doorway wide open for crafted data if attackers know what to send — especially if anything on your system exposes a POP chain.

How Could an Attacker Exploit This?

The vulnerability is “pre-auth,” meaning no login is required. Anyone can POST data to your site. Here’s a rough example of a malicious HTTP request:

POST /wp-admin/admin-ajax.php?action=everest_forms_some_action HTTP/1.1
Host: your-wordpress-site.com
Content-Type: application/x-www-form-urlencoded

field_value=O:8:"EvilClass"::{}

In this example, O:8:"EvilClass"::{} is a serialized PHP object of some “EvilClass”. If there’s a vulnerable class + POP chain on your site (maybe from another poorly coded plugin), the attacker could trigger harmful magic methods and cause real damage.

Without a POP chain, the injected object just… sits there and does nothing. But as plugin ecosystems mix together, the odds of a dangerous chain get higher.

Let’s say you have another plugin installed with a class like this

class FileDeleter {
    public $file;

    function __destruct() {
        unlink($this->file);
    }
}

An attacker could send

field_value=O:11:"FileDeleter":1:{s:4:"file";s:12:"/etc/passwd";}

On unserialization and object destruction, this could try to delete /etc/passwd — the critical Linux password file! (This is just an example; most servers would block this particular file, but you get the idea.)

Attackers need no account; attack can happen remotely

Not affected:
Sites where only Everest Forms is installed and no vulnerable third-party code has a dangerous POP chain. But you’re still at risk if you add more plugins later!

Upgrade! Check for a patch or update to Everest Forms ASAP.

- Official Everest Forms Page
- Audit your plugins/themes. Remove or update any that look suspicious or are known for using unserialize() on user-provided data.

Resources and References

- Original Wordfence Advisory
- Everest Forms Plugin Page
- PHP Object Injection Explainer
- How POP Chains Work

The Bottom Line

CVE-2025-3439 is a wake-up call. Even if Everest Forms itself doesn’t include a POP chain, the sheer modular messiness of WordPress means trouble can sneak in from almost anywhere. Patch your plugins and remember: never trust serialized data from users!

Timeline

Published on: 04/11/2025 13:15:41 UTC
Last modified on: 04/23/2025 16:47:50 UTC