CVE-2026-55803

Executive Summary

CVE-2026-55803 is a critical Improperly Controlled Modification of Dynamically-Determined Object Attributes vulnerability in Drupal core, leading to Object Injection. This flaw allows an unauthenticated attacker to manipulate object properties during deserialization or dynamic attribute assignment, potentially resulting in remote code execution (RCE), data exfiltration, or complete site compromise. The vulnerability affects all Drupal core versions from 0.0.0 up to 10.5.12, 10.6.0 to 10.6.11, 11.2.0 to 11.2.14, 11.3.0 to 11.3.12, and all 11.0.* and 11.1.* releases. No public proof-of-concept (PoC) or exploit code is currently available, but the high CVSS score and widespread usage of Drupal make this a high-priority threat.

Supply Chain & Package Security

Drupal is a PHP-based CMS distributed via its own package manager and tarballs. This vulnerability affects the core framework, meaning any site running a vulnerable version is at risk. Organizations should verify their Drupal version immediately and monitor the Drupal security advisory page for patch releases. Since Drupal core is often extended by contributed modules, the attack surface may increase if modules use dynamic object properties unsafely.

CVSS Breakdown & Threat Level

CVSS Score: 9.8 (Critical) Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

• Attack Vector (AV): Network – Exploitable remotely.
• Attack Complexity (AC): Low – No special conditions required.
• Privileges Required (PR): None – No authentication needed.
• User Interaction (UI): None – No user action required.
• Scope (S): Unchanged – Exploit does not affect other components.
• Confidentiality (C): High – Full data access possible.
• Integrity (I): High – Full data modification possible.
• Availability (A): High – Full denial of service possible.

Threat Level: Critical. Immediate patching or mitigation is required.

Deep Dive & Mechanics

The vulnerability resides in Drupal’s handling of dynamically-determined object attributes (CWE-915). When user-supplied input is used to set object properties without proper sanitization or validation, an attacker can inject arbitrary object properties. In PHP, this can lead to object injection if the attacker controls the class name and property values, potentially triggering magic methods like __destruct, __wakeup, or __toString that execute malicious code.

The flaw likely exists in Drupal’s form API, configuration handling, or serialization routines where user input is passed to functions like drupal_array_set_nested_value() or similar property-setting mechanisms. By crafting a malicious request (e.g., via HTTP POST parameters, JSON payloads, or file uploads), an attacker can set object properties that lead to instantiation of arbitrary classes with controlled parameters.

Attack Flow:

1. Attacker sends a crafted request to a vulnerable Drupal endpoint.
2. Drupal processes the input and uses it to set object attributes dynamically.
3. The attacker-controlled attributes cause the instantiation of a gadget chain (e.g., using PHP’s built-in classes like Monolog\Handler\FingersCrossedHandler or custom Drupal classes).
4. The gadget chain triggers code execution, allowing the attacker to execute system commands, read/write files, or exfiltrate data.

Vulnerable vs. Patched Code

Vulnerable Code (Example – Drupal core <= 10.5.12):

// In some form or configuration handler
$object = new stdClass();
$property = $_POST['property_name'];
$value = $_POST['property_value'];
$object->$property = $value; // No validation on $property or $value

Patched Code (Expected – Drupal core >= 10.5.13):

// Whitelist allowed properties or use a safe setter
$allowedProperties = ['title', 'body', 'status'];
$property = $_POST['property_name'];
if (in_array($property, $allowedProperties, true)) {
    $object->$property = sanitizeInput($value);
} else {
    throw new \InvalidArgumentException('Invalid property');
}

The patch should restrict dynamic property assignment to a whitelist of safe properties and sanitize values to prevent object injection.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Environment:

• Drupal 10.5.12 (vulnerable) installed on Ubuntu 22.04 with PHP 8.1.
• Attacker machine: Kali Linux.

Exploitation Steps:

1. Identify a vulnerable endpoint (e.g., /user/register, /node/add, or custom form).
2. Craft a payload that sets a dangerous property, such as __php_code or a class name for gadget chain.
3. Use a PHP object injection gadget chain (e.g., using Monolog or SwiftMailer if available).
4. Execute the payload via HTTP POST or GET parameter.

Example Payload (Conceptual):

POST /user/register HTTP/1.1
Host: target.com
Content-Type: application/x-www-form-urlencoded

form_id=user_register_form&property_name=__php_code&property_value=system('id')

Note: No public exploit code is available. This is a conceptual example.

Blue Team (Defensive)

Detection:

• Monitor for unusual HTTP requests with unexpected parameter names (e.g., property_name, __php_code).
• Look for PHP error logs indicating object instantiation failures or unexpected class loading.
• Use WAF rules to block requests containing __php_code, O:, or other serialization markers.

Mitigation:

• Apply virtual patching via WAF (e.g., ModSecurity rule to block O:\d+:.* patterns).
• Disable unnecessary PHP classes that could be used in gadget chains.
• Restrict file uploads and input validation.

Indicators of Compromise (IOCs)

• Network: HTTP requests with parameters containing O:, __php_code, or serialized PHP objects.
• File System: Unexpected files in /tmp or Drupal’s sites/default/files directory (e.g., .php files).
• Logs: PHP error logs showing __destruct, __wakeup, or unserialize() errors.
• Process: Unusual PHP processes executing system commands (e.g., id, whoami, curl).

Example IOC (Sigma Rule):

title: Drupal Object Injection Attempt
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects attempts to exploit CVE-2026-55803 via HTTP parameters
logsource:
    category: webserver
    product: apache
detection:
    selection:
        http.method: 'POST'
        http.url|contains: '/user/register'
        http.body|contains: 'property_name'
    condition: selection
falsepositives:
    - Legitimate Drupal form submissions with custom fields
level: high

Detection Engineering

YARA Rule

rule CVE_2026_55803_Drupal_Object_Injection {
    meta:
        description = "Detects PHP object injection patterns in Drupal requests"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-55803"
    strings:
        $php_serialized = /O:\d+:/
        $dangerous_prop = /__php_code|__destruct|__wakeup/
        $drupal_form = /form_id=/
    condition:
        $php_serialized and $dangerous_prop and $drupal_form
}

Sigma Rule

title: Drupal CVE-2026-55803 Object Injection Attempt
id: 22345678-2234-2234-2234-223456789abc
status: experimental
description: Detects HTTP requests with object injection indicators targeting Drupal
logsource:
    category: webserver
    product: apache
detection:
    selection:
        http.body|contains: 
            - 'O:'
            - '__php_code'
            - 'property_name'
    condition: selection
falsepositives:
    - Legitimate serialized data in Drupal forms
level: critical

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-55803 Drupal Object Injection Attempt"; flow:to_server; content:"POST"; http_method; content:"/user/register"; http_uri; content:"property_name"; http_client_body; pcre:"/O:\d+:/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Drupal core to the latest patched version (10.5.13, 10.6.12, 11.2.15, 11.3.13, or 11.0./11.1. latest) as soon as available.
2. Apply virtual patching using WAF rules to block serialized PHP objects in HTTP requests.
3. Disable unnecessary PHP modules that may contain gadget chains (e.g., Monolog, SwiftMailer).
4. Restrict input validation for all user-supplied data, especially in form handlers.

Long-Term Measures:

• Implement strict property whitelisting in all custom Drupal modules.
• Use static analysis tools to detect unsafe dynamic property assignments.
• Regularly audit Drupal core and contributed modules for similar vulnerabilities.
• Enable Drupal’s built-in security features (e.g., form tokens, CSRF protection).

Patch Details: No official patch released yet. Monitor Drupal Security Advisories for updates.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert