Bypassing Content Sandbox in Grav CMS via Twig Variables

Overview

Grav CMS is a popular open-source content management system. A vulnerability was discovered that allows attackers with page-content edit access to bypass the content sandbox and read sensitive configuration data, including secrets. This is achieved through the way Twig variables are handled in the content sandbox.

Understanding the Vulnerability / Threat

Root Cause Analysis

The root cause of this vulnerability lies in the way Grav CMS handles Twig variables in the content sandbox. Specifically, the `system`, `site`, and `theme` Twig variables are not properly filtered and are passed to the sandboxed render without any restrictions. This allows attackers to access sensitive configuration data, including secrets.

Attack Surface & Vector

The attack surface for this vulnerability is the content sandbox in Grav CMS. An attacker needs to have page-content edit access to exploit this vulnerability. The attack vector is through the Twig variables `system`, `site`, and `theme`, which are not properly filtered and can be accessed by an attacker to read sensitive configuration data.

Exploitation Mechanics — Scenario Walkthrough

Scenario: Compromising a Corporate Grav CMS Instance 1. Initial Position: An attacker has page-content edit access to a Grav CMS instance. 2. Triggering the Flaw: The attacker creates a new page with Twig processing enabled and adds a Twig variable `{{ system.cache.redis.password }}` to the page content. This variable is not properly filtered and allows the attacker to access the sensitive configuration data. 3. What Breaks: The content sandbox fails to restrict access to the `system` Twig variable, allowing the attacker to read sensitive configuration data, including secrets. 4. Attacker's Prize: The attacker can now read sensitive configuration data, including secrets, such as the Redis password.

Real-World Impact

The impact of this vulnerability is significant, as it allows attackers to access sensitive configuration data, including secrets. This can lead to further exploitation of the system, including lateral movement, data exfiltration, and ransomware deployment.

Detection & Defense

Immediate Mitigations

To mitigate this vulnerability, users should upgrade to Grav CMS version 2.0.16 or later. Additionally, users can apply a workaround by modifying the `Twig.php` file to filter the `system`, `site`, and `theme` Twig variables.

Detection Strategies

Defenders can detect exploitation attempts by monitoring for suspicious activity, such as unusual access to configuration files or Twig variables. Additionally, defenders can use security information and event management (SIEM) systems to monitor for anomalies.

Long-Term Hardening

To prevent similar vulnerabilities in the future, Grav CMS developers should implement additional security measures, such as input validation and output encoding. Additionally, developers should regularly review and update the content sandbox to ensure it is secure and effective.

Key Takeaways

* The vulnerability in Grav CMS allows attackers to bypass the content sandbox and read sensitive configuration data, including secrets. * The root cause of the vulnerability lies in the way Twig variables are handled in the content sandbox. * Users should upgrade to Grav CMS version 2.0.16 or later to mitigate the vulnerability. * Defenders should monitor for suspicious activity and implement additional security measures to prevent similar vulnerabilities in the future.

Sources

* GitHub Security Advisories: https://github.com/advisories/GHSA-p597-crqc-m349