CVE-2023-1196: Advanced Custom Fields RCE Vulnerability

CVE-2023-1196 Overview

CVE-2023-1196 is a PHP Object Injection vulnerability affecting the Advanced Custom Fields (ACF) Free and Pro WordPress plugins. The vulnerability exists in versions 6.x before 6.1.0 and 5.x before 5.12.5, where the plugin unsafely unserializes user-controllable data. This flaw allows authenticated users with a role of Contributor or above to perform PHP Object Injection attacks when a suitable gadget chain is present in the WordPress installation.

Advanced Custom Fields is one of the most widely deployed WordPress plugins, used by millions of websites to add custom fields to posts, pages, and other content types. The widespread adoption of this plugin significantly increases the potential attack surface for this vulnerability.

Critical Impact

Authenticated attackers with Contributor-level access can exploit insecure deserialization to achieve remote code execution, data manipulation, or denial of service depending on available gadget chains in the WordPress environment.

Affected Products

• Advanced Custom Fields (ACF) Free plugin versions 5.x before 5.12.5
• Advanced Custom Fields (ACF) Free plugin versions 6.x before 6.1.0
• Advanced Custom Fields (ACF) Pro plugin versions 5.x before 5.12.5
• Advanced Custom Fields (ACF) Pro plugin versions 6.x before 6.1.0

Discovery Timeline

• 2023-05-02 - CVE-2023-1196 published to NVD
• 2025-01-30 - Last updated in NVD database

Technical Details for CVE-2023-1196

Vulnerability Analysis

This vulnerability is classified as CWE-502 (Deserialization of Untrusted Data). The Advanced Custom Fields plugin fails to properly validate and sanitize serialized data before passing it to PHP's unserialize() function. When user-controllable input is deserialized without adequate security controls, attackers can craft malicious serialized objects that, upon deserialization, trigger unintended code execution paths.

The attack requires authentication with at least Contributor-level privileges, which is a common role assignment in WordPress sites that accept user-generated content. While this limits the attack to authenticated users, the Contributor role is often granted to guest bloggers, content creators, or other semi-trusted users, making this a realistic attack scenario.

The practical exploitation of this vulnerability depends on the presence of exploitable "gadget chains" within the WordPress installation. Gadget chains are sequences of existing classes and methods that can be chained together during the deserialization process to achieve malicious outcomes such as arbitrary file operations, command execution, or SQL injection.

Root Cause

The root cause of CVE-2023-1196 lies in the plugin's handling of serialized PHP data without implementing proper input validation or using safer alternatives to native PHP deserialization. The unserialize() function in PHP is inherently dangerous when processing untrusted input because it instantiates objects and can trigger magic methods like __wakeup(), __destruct(), or __toString() that may execute arbitrary code.

The ACF plugin accepts user input that gets passed directly or indirectly to the unserialize function without adequate sanitization, signature verification, or type constraints that would prevent malicious payload injection.

Attack Vector

The attack vector is network-based and requires low-privilege authentication. An attacker with Contributor-level access can exploit this vulnerability by:

1. Crafting a malicious serialized PHP object payload containing gadget chain references
2. Submitting the payload through a vulnerable ACF plugin endpoint or field
3. Triggering the deserialization of the malicious payload
4. Achieving code execution, file manipulation, or other impacts depending on available gadget chains

The vulnerability is exploited by constructing a serialized PHP object that leverages existing classes in the WordPress core, installed plugins, or themes. When the malicious serialized data is processed by the vulnerable unserialize() call, the attacker-controlled object properties are used to hijack application logic through magic method invocations.

For detailed technical analysis and proof-of-concept information, refer to the WPScan Vulnerability Report.

Detection Methods for CVE-2023-1196

Indicators of Compromise

• Unusual or malformed serialized data in ACF plugin database entries or POST requests
• Unexpected PHP object instantiation logged in error logs
• Suspicious file creation or modification in WordPress directories following ACF interactions
• Anomalous process execution originating from PHP/WordPress worker processes

Detection Strategies

• Monitor WordPress access logs for suspicious POST requests to ACF-related endpoints containing serialized PHP data patterns (e.g., O: or a: prefixes followed by numeric values)
• Implement Web Application Firewall (WAF) rules to detect and block serialized PHP object injection attempts
• Review plugin and theme installations for known gadget chain classes that could be exploited in conjunction with this vulnerability
• Deploy file integrity monitoring to detect unauthorized file modifications

Monitoring Recommendations

• Enable detailed WordPress debug logging to capture deserialization errors and unusual plugin behavior
• Configure SIEM alerts for patterns indicative of PHP Object Injection attempts in web server logs
• Monitor user activity for Contributor-level accounts performing unusual administrative actions
• Regularly audit WordPress user accounts to ensure Contributor access is only granted to trusted individuals

How to Mitigate CVE-2023-1196

Immediate Actions Required

• Update Advanced Custom Fields Free plugin to version 5.12.5 or later (for 5.x branch) or 6.1.0 or later (for 6.x branch)
• Update Advanced Custom Fields Pro plugin to version 5.12.5 or later (for 5.x branch) or 6.1.0 or later (for 6.x branch)
• Audit all user accounts with Contributor or higher roles and remove unnecessary access
• Review WordPress installations for potentially exploitable gadget chain classes in other plugins and themes

Patch Information

The vulnerability has been addressed by the Advanced Custom Fields vendor in version 5.12.5 for the 5.x branch and version 6.1.0 for the 6.x branch. Organizations should update to these versions or later to remediate the vulnerability. The patched versions implement proper input validation and sanitization to prevent PHP Object Injection attacks.

For additional vulnerability details, see the WPScan Vulnerability Report #cf376ca2.

Workarounds

• Temporarily restrict the Contributor role by removing capabilities that interact with ACF fields until the patch can be applied
• Implement a Web Application Firewall (WAF) with rules to block serialized PHP object patterns in request parameters
• Consider disabling the ACF plugin temporarily if it is not critical to site operations until updates are applied
• Apply the principle of least privilege by auditing and reducing user role assignments across the WordPress site

# WordPress CLI commands to check and update ACF plugin
# Check current ACF version
wp plugin list --name=advanced-custom-fields --fields=name,version,update_version

# Update ACF Free to latest patched version
wp plugin update advanced-custom-fields

# Verify the update was successful
wp plugin list --name=advanced-custom-fields --fields=name,version,status