CVE-2026-27206: Zumba Json Serializer RCE Vulnerability

CVE-2026-27206 Overview

CVE-2026-27206 is an Insecure Deserialization vulnerability in the Zumba Json Serializer library, a PHP library used to serialize PHP variables into JSON format. In versions 3.2.2 and below, the library allows deserialization of PHP objects from JSON using a special @type field. The deserializer instantiates any class specified in the @type field without restriction, creating a significant security risk when processing untrusted JSON input.

When an attacker-controlled JSON payload is passed into JsonSerializer::unserialize() and the application contains classes with dangerous magic methods (such as __wakeup() or __destruct()), this vulnerability may lead to PHP Object Injection and potentially Remote Code Execution (RCE), depending on available gadget chains in the application or its dependencies.

Critical Impact

This vulnerability enables attackers to instantiate arbitrary PHP classes through malicious JSON input, potentially leading to Remote Code Execution if exploitable gadget chains exist in the application or its dependencies.

Affected Products

• Zumba Json Serializer versions 3.2.2 and below
• PHP applications using vulnerable JsonSerializer::unserialize() with untrusted input
• Applications with exploitable gadget chains in their class dependencies

Discovery Timeline

• 2026-02-21 - CVE CVE-2026-27206 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-27206

Vulnerability Analysis

The vulnerability stems from the Zumba Json Serializer's handling of the @type field during JSON deserialization. When processing JSON data, the library parses this special field and instantiates the corresponding PHP class without any validation or restriction. This behavior mirrors the security risks associated with PHP's native unserialize() function when used without the allowed_classes restriction.

The weakness is classified as CWE-502 (Deserialization of Untrusted Data). Applications are vulnerable only when untrusted or attacker-controlled JSON is passed into JsonSerializer::unserialize() and the application or its dependencies contain classes that can be leveraged as a gadget chain. The attack requires network access and can be performed without authentication, though exploitation complexity is higher as it depends on the presence of suitable gadget chains in the target application.

Root Cause

The root cause of this vulnerability is the unrestricted class instantiation during JSON deserialization. The library's design allowed any class specified in the @type field to be instantiated without validation, enabling attackers to trigger arbitrary class constructors and magic methods. The absence of an allowlist mechanism for permitted classes created this security gap.

Attack Vector

An attacker can exploit this vulnerability by crafting a malicious JSON payload containing an @type field that specifies a dangerous class available in the target application's codebase or dependencies. When the application deserializes this JSON using JsonSerializer::unserialize(), the specified class is instantiated, triggering its constructor or magic methods (__wakeup(), __destruct(), etc.). If the application contains classes with dangerous side effects in these methods (gadget chains), the attacker may achieve Remote Code Execution.

The attack requires:

1. The ability to supply untrusted JSON input to the application
2. The JSON being processed by JsonSerializer::unserialize()
3. The presence of exploitable gadget chains in the application or its dependencies

      */
     protected $undefinedAttributeMode = self::UNDECLARED_PROPERTY_MODE_SET;
 
+    /**
+     * Allowed classes for deserialization.
+     * Null means all classes are allowed (default, backward-compatible).
+     * An empty array means no classes are allowed.
+     * A non-empty array restricts deserialization to only the listed classes.
+     *
+     * @var array|null
+     */
+    protected $allowedClasses = null;
+
     /**
      * Constructor.
      *

Source: GitHub Commit Update

The patch introduces an $allowedClasses property that enables applications to restrict which classes can be instantiated during deserialization. When set to null (default), all classes are allowed for backward compatibility. An empty array blocks all class instantiation, while a populated array restricts deserialization to only the explicitly listed classes.

Detection Methods for CVE-2026-27206

Indicators of Compromise

• Unusual JSON payloads containing @type fields with unexpected class names in application logs
• Error messages or exceptions related to unknown class instantiation during JSON processing
• Anomalous process execution or file system activity following JSON API requests
• Web application firewall logs showing JSON requests with serialized object patterns

Detection Strategies

• Monitor application logs for JsonSerializer::unserialize() errors involving unexpected class names
• Implement input validation logging to detect JSON payloads containing @type fields with suspicious class references
• Deploy web application firewalls with rules to detect PHP object injection patterns in JSON data
• Review PHP error logs for class instantiation failures or magic method execution anomalies

Monitoring Recommendations

• Enable verbose logging for JSON deserialization operations in development and staging environments
• Set up alerts for unusual patterns in API endpoints that accept JSON input
• Monitor for new process creation or file system modifications following JSON processing operations
• Implement anomaly detection for class loading patterns in PHP runtime environments

How to Mitigate CVE-2026-27206

Immediate Actions Required

• Upgrade Zumba Json Serializer to version 3.2.3 or later immediately
• Audit all code paths where JsonSerializer::unserialize() is called with external input
• Identify and restrict any deserialization of untrusted JSON data
• Review application dependencies for known gadget chain classes

Patch Information

The vulnerability has been addressed in Zumba Json Serializer version 3.2.3. The fix introduces an $allowedClasses property that enables applications to restrict which classes can be instantiated during JSON deserialization. Organizations should update their Composer dependencies to include version 3.2.3 or later. For detailed information, refer to the GitHub Security Advisory GHSA-v7m3-fpcr-h7m2 and the release notes for version 3.2.3.

Workarounds

• Never deserialize untrusted JSON with JsonSerializer::unserialize() until the library is upgraded
• Validate and sanitize all JSON input before deserialization, specifically rejecting payloads containing @type fields
• Disable @type-based object instantiation wherever possible in your application configuration
• Implement input validation to reject JSON containing serialized object references from untrusted sources

# Update Zumba Json Serializer via Composer
composer require zumba/json-serializer:^3.2.3

# Verify installed version
composer show zumba/json-serializer | grep versions