PHP Object Injection and Code Execution in Grav CMS

Executive Summary

Grav CMS versions before 2.0.0-beta.2 have multiple critical security flaws. These include three unsafe unserialize() calls that deserialize untrusted data without restricting allowed classes, which enables PHP object injection and arbitrary code execution through a gadget chain. Additionally, the InstallCommand's git clone operation improperly handles branch, url, and path parameters by passing them into a shell command without escaping, allowing OS command injection during plugin or theme installation, which requires admin access. There is also a Twig security blocklist bypass that leads to server-side template injection. All these issues are fixed in version 2.0.0-beta.2.

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Detection Guidance

Detection of CVE-2026-56700 involves identifying the presence of vulnerable Grav CMS versions prior to 2.0.0-beta.2 and monitoring for exploitation attempts targeting unsafe unserialize() calls, OS command injection, or server-side template injection.

You can start by checking the installed Grav CMS version with a command like:

• grep 'version' /path/to/grav/system/defines.php

To detect exploitation attempts, monitor web server logs for suspicious serialized PHP objects or unusual parameters in plugin/theme installation requests that might exploit the git clone command injection.

Example commands to search logs for suspicious unserialize payloads or command injection attempts include:

• grep -r --include='*.log' 'O:' /var/log/apache2/
• grep -r --include='*.log' 'git clone' /var/log/apache2/

Additionally, scanning for files or requests containing serialized PHP objects or unusual shell commands in plugin/theme installation paths can help detect exploitation.

Ultimately, upgrading Grav CMS to version 2.0.0-beta.2 or later is the recommended mitigation.

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Impact Analysis

This vulnerability can have severe impacts including allowing attackers to execute arbitrary code on the server running Grav CMS. Through PHP object injection and OS command injection, attackers can gain control over the system, potentially leading to data breaches, unauthorized access, and full system compromise. The server-side template injection can also be exploited to execute malicious code. These impacts can disrupt service, compromise sensitive data, and damage the integrity and availability of the affected system.

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Compliance Impact

The vulnerability in Grav CMS allows remote code execution through unsafe unserialize() calls, OS command injection, and server-side template injection. Such critical security flaws can lead to unauthorized access, data breaches, and manipulation of sensitive information.

These risks can impact compliance with common standards and regulations like GDPR and HIPAA, which require protection of personal and sensitive data against unauthorized access and breaches.

Exploitation of these vulnerabilities could result in exposure or alteration of protected data, potentially leading to non-compliance with data protection and privacy regulations.

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Mitigation Strategies

The vulnerability is fixed in Grav CMS version 2.0.0-beta.2. Immediate mitigation steps include upgrading Grav CMS to version 2.0.0-beta.2 or later.

Additionally, restrict access to the admin interface to trusted users only, as some attacks require admin access.

Avoid installing untrusted plugins or themes, since the vulnerability includes OS command injection via plugin/theme installation.

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