Executive Summary

This vulnerability is an Insecure Direct Object Reference (IDOR) in the ACF to REST API WordPress plugin up to version 3.3.4. It occurs because the update_item_permissions_check() method only verifies that a user has the general 'edit_posts' capability, without checking more specific permissions related to the object being modified (such as edit_post($id), edit_user($id), or manage_options). As a result, authenticated users with Contributor-level access or higher can modify Advanced Custom Fields (ACF) on posts they do not own, any user account, comments, taxonomy terms, and even the global options page via the REST API endpoints under /wp-json/acf/v3/{type}/{id}, as long as they can authenticate to the site. [1]

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Impact Analysis

This vulnerability allows authenticated users with relatively low privileges (Contributor-level and above) to modify ACF fields on posts they do not own, as well as on other sensitive objects like user accounts, comments, taxonomy terms, and global options. This can lead to unauthorized data modification, potential privilege escalation, data integrity issues, and unauthorized changes to site configuration or content, which can compromise the security and reliability of the WordPress site. [1]

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Detection Guidance

This vulnerability can be detected by monitoring REST API requests to endpoints matching the pattern /wp-json/acf/v3/{type}/{id}, especially POST, PUT, or PATCH requests attempting to update ACF fields. You can use network monitoring tools like tcpdump or Wireshark to capture HTTP requests to these endpoints. For example, using tcpdump to filter HTTP traffic to your WordPress server's REST API: tcpdump -i any -A 'tcp port 80 or tcp port 443' | grep '/wp-json/acf/v3/'. Additionally, inspecting web server logs (e.g., access.log) for suspicious authenticated requests to these endpoints can help detect exploitation attempts. Since the vulnerability requires authenticated access with Contributor-level or higher, reviewing user activity logs for unusual REST API usage is recommended. [1]

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Mitigation Strategies

Immediate mitigation steps include updating the ACF to REST API plugin to a version later than 3.3.4 where the vulnerability is fixed. If an update is not immediately available, restrict access to the /wp-json/acf/v3/ REST API endpoints by limiting permissions or blocking these endpoints via firewall or security plugins. Additionally, review and tighten user roles and capabilities to ensure that only trusted users have Contributor-level or higher access. Monitoring and logging REST API usage can also help detect and respond to exploitation attempts. [1]

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Compliance Impact

This vulnerability allows authenticated attackers with Contributor-level access to modify ACF fields on posts they do not own, user accounts, comments, taxonomy terms, and global options via the REST API. Such unauthorized modifications could lead to improper handling or alteration of personal or sensitive data, potentially violating data protection requirements under standards like GDPR or HIPAA. Specifically, the lack of proper object-specific permission checks increases the risk of unauthorized data changes, which may compromise data integrity and confidentiality required by these regulations. [1]

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