Executive Summary

CVE-2026-42843 is a privilege escalation vulnerability in the Grav CMS API plugin, specifically in the UsersController::update method.

It allows any authenticated user with basic API access (api.access permission) to modify their own permission configuration due to an insecure direct object reference and a logic flaw.

An attacker can exploit this flaw by sending a crafted PATCH request to change their access permissions, escalating their privileges to Super Administrator (admin.super and api.super).

This leads to full system compromise, including the ability to modify content, alter configurations, upload malicious plugins, or edit Twig templates to achieve remote code execution (RCE).

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Impact Analysis

This vulnerability can have severe impacts as it allows an attacker to escalate their privileges to Super Administrator within the Grav CMS.

With superuser access, the attacker can fully compromise the system by modifying site content, changing configurations, uploading malicious plugins, or executing arbitrary code remotely.

Such a compromise can lead to loss of data confidentiality, integrity, and availability, potentially causing significant damage to the website and its users.

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Detection Guidance

This vulnerability can be detected by monitoring for unusual PATCH requests to the Grav API plugin's UsersController::update endpoint, especially those attempting to modify the 'access' permissions field.

A practical approach is to inspect web server logs or API access logs for PATCH requests that include changes to permission configurations by authenticated users.

• Use tools like curl or HTTP request logging to identify suspicious PATCH requests modifying user permissions.
• Example command to check logs for suspicious PATCH requests (assuming Apache logs):
• grep 'PATCH' /var/log/apache2/access.log | grep 'UsersController::update'
• Alternatively, monitor API traffic with a network analyzer (e.g., Wireshark) for PATCH requests containing 'access' permission changes.

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Mitigation Strategies

The immediate mitigation step is to upgrade the Grav API plugin to version 1.0.0-beta.15 or later, where this vulnerability is fixed.

Until the upgrade can be applied, restrict API access to trusted users only and monitor for suspicious permission modification attempts.

Additionally, consider disabling or limiting the API access permissions (api.access) for users who do not require it.

Review and tighten user permission configurations to prevent unauthorized privilege escalation.

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Compliance Impact

The vulnerability allows an authenticated user to escalate their privileges to Super Administrator, leading to full system compromise including unauthorized access and modification of content, configurations, and potential remote code execution.

Such unauthorized privilege escalation and full system compromise can lead to breaches of confidentiality, integrity, and availability of data, which are critical requirements under common standards and regulations like GDPR and HIPAA.

Therefore, exploitation of this vulnerability could result in non-compliance with these regulations due to potential unauthorized access to sensitive personal or protected health information, failure to maintain data integrity, and inadequate system security controls.

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