CVE-2026-4056 in User Registration & Membership Plugin
Summary
by MITRE • 03/24/2026
The User Registration & Membership plugin for WordPress is vulnerable to unauthorized modification of data due to a missing capability check on the Content Access Rules REST API endpoints in versions 5.0.1 through 5.1.4. This is due to the `check_permissions()` method only checking for `edit_posts` capability instead of an administrator-level capability. This makes it possible for authenticated attackers, with Contributor-level access and above, to list, create, modify, toggle, duplicate, and delete site-wide content restriction rules, potentially exposing restricted content or denying legitimate user access.
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Analysis
by VulDB Data Team • 03/28/2026
The vulnerability identified in CVE-2026-4056 affects the User Registration & Membership plugin for WordPress, specifically targeting versions 5.0.1 through 5.1.4. This represents a critical authorization flaw that undermines the security model of the plugin's content access control system. The issue stems from improper capability validation within the Content Access Rules REST API endpoints, creating a significant privilege escalation vector that allows attackers to manipulate site-wide content restriction policies. The vulnerability is classified under CWE-284 which specifically addresses insufficient access control mechanisms, making it a direct violation of fundamental security principles that should protect administrative functions from unauthorized access.
The technical flaw manifests in the `check_permissions()` method implementation which incorrectly validates user capabilities by only requiring the `edit_posts` capability instead of implementing proper administrator-level authorization checks. This design decision creates a dangerous access boundary where users with Contributor level permissions or higher can execute privileged operations within the plugin's administrative interface. The REST API endpoints become susceptible to unauthorized modifications, enabling attackers to perform a comprehensive range of operations including listing existing rules, creating new restrictions, modifying existing configurations, toggling rule states, duplicating access policies, and deleting critical content access controls. This comprehensive attack surface allows for both passive content exposure and active denial of service scenarios that can severely impact site functionality and user experience.
The operational impact of this vulnerability extends beyond simple data modification, creating potential for significant security breaches and service disruption. Attackers with Contributor-level access can effectively bypass content protection mechanisms that are meant to restrict access to specific user groups or roles, potentially exposing sensitive information to unauthorized users while simultaneously creating access control conflicts that may prevent legitimate users from accessing content they should be entitled to. This vulnerability directly impacts the principle of least privilege and can lead to cascading security issues where compromised accounts at the Contributor level can be leveraged to establish persistent access to critical site configuration parameters. The implications are particularly severe in multi-user environments where content access controls are essential for maintaining data integrity and user privacy standards.
Organizations affected by this vulnerability should implement immediate mitigations including upgrading to the patched version of the plugin, which should include proper capability validation that requires administrator-level permissions for content access rule modifications. Security administrators should also consider implementing additional monitoring of REST API endpoints for unauthorized access attempts and review existing user roles to ensure that Contributor accounts do not possess unnecessary permissions that could be exploited. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting the T1078 credential access and T1548 privilege escalation tactics. Network segmentation and API rate limiting can provide additional defensive layers, while regular security audits of WordPress plugins should include capability validation checks to prevent similar issues in other third-party components that may be subject to similar authorization flaws.