CVE-2021-20865: Advanced Custom Fields Info Disclosure

CVE-2021-20865 Overview

CVE-2021-20865 is a missing authorization vulnerability affecting Advanced Custom Fields and Advanced Custom Fields Pro, popular WordPress plugins used for extending content management capabilities. Versions prior to 5.11 contain a flaw in the database browsing functionality that allows unauthorized users to access sensitive data without proper authorization checks.

This vulnerability falls under CWE-862 (Missing Authorization), where the application fails to perform authorization checks when a user attempts to browse database content through unspecified vectors.

Critical Impact

Unauthorized users can browse and access sensitive database information, potentially exposing confidential site data, custom field values, and other protected content stored within the WordPress database.

Affected Products

• Advanced Custom Fields versions prior to 5.11
• Advanced Custom Fields Pro versions prior to 5.11
• WordPress installations using affected ACF plugin versions

Discovery Timeline

• 2021-12-13 - CVE-2021-20865 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-20865

Vulnerability Analysis

The vulnerability exists within the database browsing functionality of Advanced Custom Fields. The plugin fails to implement proper authorization checks when users attempt to access database content, allowing any authenticated user (or potentially unauthenticated users depending on configuration) to browse data they should not have access to.

Advanced Custom Fields is designed to allow WordPress administrators to add custom field groups to posts, pages, and other content types. The vulnerable component handles database queries and data retrieval, but does not verify whether the requesting user has the appropriate permissions to view the requested data.

The missing authorization check means that the application trusts user requests without validating access rights, violating the principle of least privilege. This type of vulnerability is particularly concerning in multi-user WordPress environments where different users have varying levels of access.

Root Cause

The root cause is a missing authorization vulnerability (CWE-862) in the database browsing functionality. The plugin does not properly verify user permissions before allowing access to browse database content. This architectural oversight allows users to bypass intended access controls and retrieve data beyond their authorization level.

Attack Vector

The attack can be conducted remotely over the network without requiring user interaction. An attacker with network access to the vulnerable WordPress installation can exploit this flaw to access unauthorized database content. The attack complexity is low, meaning exploitation does not require sophisticated techniques or specialized conditions.

The exploitation typically involves:

1. Identifying a WordPress installation with a vulnerable version of Advanced Custom Fields
2. Accessing the database browsing functionality through the plugin's interface
3. Retrieving sensitive data without proper authorization validation

Due to the nature of this missing authorization flaw, attackers can potentially access custom field data, post metadata, and other sensitive information stored through the ACF plugin without having legitimate access rights.

Detection Methods for CVE-2021-20865

Indicators of Compromise

• Unusual access patterns to ACF database browsing endpoints
• Access logs showing database queries from low-privilege or anonymous users
• Unexpected data retrieval requests targeting custom field data
• Authentication logs showing access to ACF administrative functions by unauthorized accounts

Detection Strategies

• Monitor WordPress access logs for requests to ACF-related endpoints from unauthorized users
• Implement Web Application Firewall (WAF) rules to detect suspicious database browsing patterns
• Review plugin access logs for unusual query patterns or data access attempts
• Deploy file integrity monitoring to detect unauthorized modifications to ACF plugin files

Monitoring Recommendations

• Enable detailed logging for WordPress plugins, particularly ACF database operations
• Set up alerts for failed or anomalous authorization attempts within the WordPress admin area
• Regularly audit user access patterns to ACF functionality
• Monitor for bulk data retrieval attempts that may indicate exploitation

How to Mitigate CVE-2021-20865

Immediate Actions Required

• Update Advanced Custom Fields to version 5.11 or later immediately
• Update Advanced Custom Fields Pro to version 5.11 or later immediately
• Review access logs for any signs of unauthorized data access
• Audit user permissions and remove unnecessary access to ACF functionality
• Consider temporarily disabling the plugin if immediate update is not possible

Patch Information

The vulnerability has been addressed in Advanced Custom Fields version 5.11 and Advanced Custom Fields Pro version 5.11. Users should update to these versions or later to remediate the vulnerability. The patches can be obtained through the WordPress Plugin Directory or the Advanced Custom Fields Website.

For detailed security information, refer to the JVN Security Advisory.

Workarounds

• Restrict access to the WordPress admin area using IP-based filtering until patching is complete
• Implement additional authentication layers for administrative functions
• Use a Web Application Firewall to block suspicious requests to ACF endpoints
• Limit user roles and capabilities to reduce the potential impact of unauthorized access
• Consider disabling database browsing functionality if not required for operations

# Configuration example - Restrict ACF access via .htaccess
# Add to WordPress .htaccess file to limit admin access

<FilesMatch "admin-ajax.php">
    Order Deny,Allow
    Deny from all
    Allow from 192.168.1.0/24
    Allow from 10.0.0.0/8
</FilesMatch>

# Ensure WordPress and plugins are updated
wp plugin update advanced-custom-fields --allow-root
wp plugin list --status=active --format=table