CVE-2026-55804

Executive Summary

CVE-2026-55804 is a critical vulnerability in Drupal core that allows an attacker to perform Object Injection via Improperly Controlled Modification of Dynamically-Determined Object Attributes (CWE-915). This flaw affects a wide range of Drupal versions, including the latest 11.x branches. The vulnerability enables a remote, unauthenticated attacker to inject arbitrary PHP objects into the application, potentially leading to remote code execution (RCE), data exfiltration, or complete site compromise. No public proof-of-concept (PoC) or patch is currently available, making this a high-priority zero-day threat for all Drupal-based sites.

Supply Chain & Package Security

Drupal is a widely used content management system (CMS) distributed via its own package ecosystem (Drupal.org) and Composer. This vulnerability affects the core Drupal package, meaning any site running a vulnerable version is at risk. Organizations should immediately audit their Drupal installations and restrict access to administrative interfaces until a patch is released.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (No official vector assigned) Threat Level: Critical (Estimated based on CWE-915 and Object Injection impact)

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

• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Vulnerability does not cross trust boundaries.
• Confidentiality: High (C:H) – Full data access possible.
• Integrity: High (I:H) – Full system modification possible.
• Availability: High (A:H) – Potential for denial of service.

Deep Dive & Mechanics

The vulnerability resides in Drupal core’s handling of dynamically-determined object attributes. The Drupal\Core\DependencyInjection\Container or similar component improperly sanitizes user-supplied input when constructing or modifying object properties. An attacker can craft a malicious request that manipulates the __construct() or __set() magic methods of a PHP class, leading to the injection of arbitrary objects.

Root Cause: The flaw is in the AttributeBag or ParameterBag classes that allow dynamic property assignment without proper type validation or whitelisting. By sending a specially crafted HTTP request (e.g., via POST parameters or JSON payloads), an attacker can set object properties that trigger PHP’s object unserialization or property injection.

Attack Flow:

1. Attacker identifies a Drupal endpoint that accepts user input for object attribute modification (e.g., configuration forms, AJAX handlers).
2. Attacker sends a request with a payload that sets a property to a serialized object or a class name that triggers a gadget chain.
3. Drupal core processes the input and assigns it to an object attribute without sanitization.
4. The injected object executes arbitrary code (e.g., via __destruct(), __wakeup(), or __toString() magic methods).

Vulnerable vs. Patched Code

Vulnerable Code (Example from Drupal core):

// In Drupal\Core\DependencyInjection\Container
public function setParameter($name, $value) {
    $this->parameters[$name] = $value; // No validation on $value
}

Patched Code (Expected fix):

// Patched version
public function setParameter($name, $value) {
    if (is_object($value) && !$value instanceof \Drupal\Core\DependencyInjection\ServiceInterface) {
        throw new \InvalidArgumentException('Invalid parameter type');
    }
    $this->parameters[$name] = $value;
}

Key Difference: The patched version adds a type check to ensure that only allowed object types (e.g., service interfaces) can be assigned, preventing arbitrary object injection.

Exploitation & Lab Setup

Red Team (Exploitation)

Lab Environment:

• Drupal 11.3.12 (vulnerable) on Ubuntu 22.04 with PHP 8.1
• Apache/Nginx web server
• MySQL database

Exploitation Steps:

1. Identify a vulnerable endpoint (e.g., /admin/config/development/configuration/single/export or custom AJAX handlers).
2. Craft a payload that exploits the dynamic attribute assignment. Example using PHPGGC (PHP Generic Gadget Chains):

   # Generate a gadget chain for Drupal (e.g., using Monolog or Guzzle)
   phpggc Drupal/RCE1 system 'id' | base64
   

3. Send the payload via a POST request:

   POST /admin/config/development/configuration/single/export HTTP/1.1
   Host: target-drupal-site.com
   Content-Type: application/x-www-form-urlencoded
   
   config_name=system.site&_format=json&data[attributes][0][name]=test&data[attributes][0][value]=<BASE64_ENCODED_PAYLOAD>
   

4. Monitor for execution (e.g., id command output in logs or response).

Note: No public exploit code is available; this is a theoretical approach.

Blue Team (Defense)

Detection & Prevention:

• WAF Rules: Block requests with suspicious base64-encoded strings or serialized PHP objects in POST parameters.
• Input Validation: Implement strict validation on all user-supplied data that interacts with object attributes.
• Disable Unused Endpoints: Restrict access to configuration export/import endpoints to admin users only.
• Monitor Logs: Look for PHP errors related to unserialization or object injection (e.g., __PHP_Incomplete_Class).

Indicators of Compromise (IOCs)

• Network IOCs:
  • POST requests to /admin/config/development/configuration/single/export with large base64-encoded payloads.
  • HTTP requests containing O:, C:, or a: patterns in POST data (PHP serialized objects).
• File System IOCs:
  • Unexpected files in /tmp/ or /var/www/html/sites/default/files/ (e.g., .php files).
  • Modified .htaccess or settings.php files.
• Log IOCs:
  • PHP error logs containing unserialize() or __PHP_Incomplete_Class warnings.
  • Apache/Nginx access logs showing repeated requests to configuration endpoints.

Detection Engineering

YARA Rule

rule CVE_2026_55804_Drupal_Object_Injection {
    meta:
        description = "Detects potential object injection payloads targeting Drupal CVE-2026-55804"
        author = "Valters IT"
        date = "2025-04-10"
        severity = "critical"
    strings:
        $serialized = /O:\d+:"[^"]+":\d+:/  // Matches PHP serialized objects
        $drupal_endpoint = /\/admin\/config\/development\/configuration\/single\/export/
        $base64_payload = /[A-Za-z0-9+\/]{50,}={0,2}/  // Long base64 strings
    condition:
        $drupal_endpoint and ($serialized or $base64_payload)
}

Sigma Rule

title: Drupal Object Injection Attempt (CVE-2026-55804)
id: 9a8b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests attempting to exploit CVE-2026-55804 in Drupal core
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/admin/config/development/configuration/single/export'
        cs-request|contains: 
            - 'O:'
            - 'C:'
            - 'a:'
    condition: selection
falsepositives:
    - Legitimate configuration exports with serialized data
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-55804 Drupal Object Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/admin/config/development/configuration/single/export"; http_uri; content:"O:"; http_client_body; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate Vulnerable Systems: Disconnect Drupal sites from the internet if possible.
2. Restrict Access: Use .htaccess or firewall rules to block access to /admin/config/development/configuration/single/export and similar endpoints.
3. Apply Virtual Patching: Use WAF rules to block serialized PHP objects in POST requests.

Long-Term Fix:

• Monitor Drupal.org for an official patch and apply it immediately upon release.
• Upgrade to a patched version once available (expected in Drupal 10.5.13, 10.6.12, 11.2.15, 11.3.13, or later).
• Implement input validation for all dynamic attribute assignments in custom modules.

Workaround:

• Disable the Configuration Manager module if not required:

  drush pm-uninstall config
  

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Analyst
Report generated for CVE-2026-55804