CVE-2025-8871

Executive Summary

CVE-2025-8871 is a PHP Object Injection vulnerability in the Everest Forms (Pro) WordPress plugin, affecting all versions up to and including 1.9.7. The flaw resides in the mime_content_type() function, which deserializes untrusted input without proper sanitization. An unauthenticated attacker can exploit this by crafting a malicious file upload that triggers PHP object deserialization. The attack requires a form with a non-required signature field and an image upload field. While no known POP (Property Oriented Programming) chain exists in the Everest Forms plugin itself, the vulnerability becomes critical if another plugin or theme on the same site provides a suitable POP chain. Successful exploitation could lead to arbitrary file deletion, sensitive data retrieval, or remote code execution. The vulnerability is only exploitable on PHP versions prior to 8.0.

Supply Chain & Package Security

This vulnerability affects the WordPress ecosystem, specifically the Everest Forms plugin. The plugin is distributed via the WordPress plugin repository and commercial channels. Since the patch status is unknown, users must rely on manual mitigation or removal of the plugin. The lack of a POP chain in the core plugin reduces immediate risk, but the supply chain risk increases if the site uses additional plugins or themes that contain exploitable POP chains.

CVSS Breakdown & Threat Level

• CVSS Score: 5.6 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: Low (C:L)
• Integrity Impact: Low (I:L)
• Availability Impact: Low (A:L)

Threat Level: Medium. The vulnerability is easy to exploit (no authentication required) but requires a specific form configuration and a POP chain from another component. The impact is limited without a POP chain.

Deep Dive & Mechanics

The vulnerability is a classic PHP Object Injection (CWE-502) triggered by insecure deserialization. The mime_content_type() function in PHP is used to determine the MIME type of a file. In Everest Forms, this function is called on uploaded files, but the input is not properly validated before being passed to unserialize() or a similar deserialization function.

Attack Flow:

1. An attacker identifies a WordPress site using Everest Forms with a form that includes a non-required signature field and an image upload field.
2. The attacker uploads a specially crafted file (e.g., a PNG image with embedded serialized PHP object data).
3. The plugin calls mime_content_type() on the uploaded file, which internally deserializes the embedded object.
4. If a POP chain exists (e.g., from another plugin like WooCommerce or a custom theme), the deserialized object triggers arbitrary actions such as file deletion, data exfiltration, or code execution.

Why PHP < 8.0?
PHP 8.0 introduced stricter type handling and deprecated certain deserialization behaviors, making this attack vector less viable. The vulnerability is only exploitable on PHP 7.x and earlier.

Vulnerable vs. Patched Code

Since no official patch is available, the vulnerable code is presumed to be in the file upload handling logic. A typical vulnerable pattern might look like:

// Vulnerable code (hypothetical)
$mime = mime_content_type($_FILES['file']['tmp_name']);
// The function internally uses unserialize() on file metadata

A patched version would replace the insecure deserialization with safe alternatives:

// Patched code (hypothetical)
$mime = mime_content_type($_FILES['file']['tmp_name']);
// Use finfo or other safe MIME detection
$finfo = finfo_open(FILEINFO_MIME_TYPE);
$mime = finfo_file($finfo, $_FILES['file']['tmp_name']);
finfo_close($finfo);

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• WordPress site with Everest Forms <= 1.9.7
• PHP version < 8.0
• A form with a non-required signature field and an image upload field
• A POP chain from another plugin/theme (e.g., WooCommerce, Gravity Forms, or custom code)

Exploitation Steps:

1. Identify the target form and its fields.
2. Craft a malicious file containing a serialized PHP object that triggers the POP chain. For example, if a POP chain exists in WooCommerce that allows file deletion, the payload might look like:

   O:10:"WooCommerce":0:{}
   

3. Upload the file via the form's image upload field.
4. The mime_content_type() function deserializes the object, executing the POP chain.

Example Payload (using a hypothetical POP chain):

// payload.php
class EvilClass {
    public $cmd = 'rm -rf /tmp/evil';
    public function __destruct() {
        system($this->cmd);
    }
}
$payload = serialize(new EvilClass());
file_put_contents('evil.png', $payload);

Blue Team (Detection & Defense)

Detection:

• Monitor file uploads for unusual MIME types or file sizes.
• Log PHP errors related to deserialization (unserialize() errors).
• Use WAF rules to block serialized PHP objects in file uploads.

Defense:

• Upgrade PHP to version 8.0 or later.
• Disable the Everest Forms plugin if not essential.
• Remove any unused plugins or themes that may contain POP chains.
• Implement file upload validation using safe MIME detection (e.g., finfo).

Indicators of Compromise (IOCs)

• File Hashes: Suspicious uploaded files with embedded serialized PHP objects.
• Network Traffic: POST requests to form endpoints with unusual file uploads.
• Logs: PHP warnings/errors containing unserialize() or mime_content_type().
• System Changes: Unexpected file deletions, new files in web root, or outbound connections.

Example IOC (YARA rule):

rule php_object_injection_everest_forms {
    meta:
        description = "Detects serialized PHP objects in file uploads targeting Everest Forms"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $serialized = /O:\d+:"[^"]+":\d+:\{/
    condition:
        $serialized
}

Detection Engineering

YARA Rule

rule cve_2025_8871_everest_forms_php_object_injection {
    meta:
        description = "Detects PHP object injection attempts in Everest Forms file uploads"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-8871"
    strings:
        $serialized = /O:\d+:"[^"]+":\d+:\{/
        $mime_func = "mime_content_type"
    condition:
        $serialized and $mime_func
}

Sigma Rule

title: CVE-2025-8871 Everest Forms PHP Object Injection
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects exploitation attempts of PHP object injection in Everest Forms plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        c-uri|contains: '/wp-admin/admin-ajax.php'
        cs-uri-query|contains: 'action=everest_forms_upload'
        sc-status: 200
    condition: selection
falsepositives:
    - Legitimate file uploads
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-8871 Everest Forms PHP Object Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/wp-admin/admin-ajax.php"; http_uri; content:"action=everest_forms_upload"; http_client_body; pcre:"/O:\d+:\"[^\"]+\":\d+:\{/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade PHP to version 8.0 or later to disable the vulnerable deserialization path.
2. Disable the Everest Forms plugin until a patch is released.
3. Remove unused plugins/themes that may contain POP chains.

Long-term Fix:

• Monitor the Everest Forms vendor for a security patch.
• Implement file upload validation using safe MIME detection (e.g., finfo).
• Use a Web Application Firewall (WAF) to block serialized PHP objects in uploads.

Workaround:

• Modify the form configuration to require the signature field, which may reduce attack surface (though not fully mitigate).

Report generated by Valters IT DevSecOps & CTI Division
Valters IT – Securing Digital Frontiers