CVE-2023-34252 in Grav
Summary
by MITRE • 06/15/2023
Grav is a file-based Web platform. Prior to version 1.7.42, there is a logic flaw in the `GravExtension.filterFilter()` function whereby validation against a denylist of unsafe functions is only performed when the argument passed to filter is a string. However, passing an array as a callable argument allows the validation check to be skipped. Consequently, a low privileged attacker with login access to Grav Admin panel and page creation/update permissions is able to inject malicious templates to obtain remote code execution. The vulnerability can be found in the `GravExtension.filterFilter()` function declared in `/system/src/Grav/Common/Twig/Extension/GravExtension.php`. Version 1.7.42 contains a patch for this issue. End users should also ensure that `twig.undefined_functions` and `twig.undefined_filters` properties in `/path/to/webroot/system/config/system.yaml` configuration file are set to `false` to disallow Twig from treating undefined filters/functions as PHP functions and executing them.
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Analysis
by VulDB Data Team • 07/13/2023
The vulnerability CVE-2023-34252 represents a critical logic flaw in the Grav web platform that enables remote code execution through improper input validation within the Twig template engine. This issue affects versions prior to 1.7.42 and stems from a fundamental flaw in how the GravExtension.filterFilter() function processes callable arguments. The vulnerability operates under CWE-20, which classifies it as an improper input validation scenario where the system fails to properly validate or sanitize user-provided data before processing it as executable code. The flaw specifically manifests when the filter function receives an array as a callable argument instead of a string, allowing attackers to bypass the intended denylist validation mechanism that only applies to string inputs.
The technical exploitation of this vulnerability occurs within the GravExtension.php file where the filterFilter() function fails to properly validate array-based callable arguments that could contain malicious PHP code execution instructions. When an attacker with administrative privileges creates or modifies pages within the Grav admin panel, they can inject specially crafted template code that leverages this bypass to execute arbitrary PHP commands on the server. This occurs because the system treats array inputs differently than string inputs during validation, effectively creating a bypass path that allows malicious code to be interpreted and executed as PHP functions. The vulnerability directly relates to ATT&CK technique T1059.007 for "Command and Scripting Interpreter: Python" and T1566.001 for "Phishing: Spearphishing Attachment" as it enables attackers to execute malicious code through legitimate administrative interfaces.
The operational impact of this vulnerability is severe as it allows low-privileged attackers with basic administrative access to escalate their privileges and gain full control over the web server. The vulnerability requires minimal prerequisites - only login access to the Grav admin panel and permission to create or modify pages. This makes it particularly dangerous in environments where administrative accounts might be compromised through social engineering or credential theft attacks. The attack chain begins with authentication, followed by template injection, and concludes with remote code execution, providing attackers with complete system compromise capabilities. The vulnerability affects not just the web application but potentially the entire underlying server infrastructure, as the executed code operates with the privileges of the web server process.
Mitigation strategies for CVE-2023-34252 involve multiple layers of defense to protect against exploitation. The primary fix is updating to Grav version 1.7.42 or later, which implements proper validation for both string and array inputs to the filter function. Additionally, administrators should configure the Twig engine by setting both `twig.undefined_functions` and `twig.undefined_filters` properties to `false` in the system configuration file located at `/path/to/webroot/system/config/system.yaml`. This configuration prevents the Twig engine from treating undefined functions or filters as executable PHP code, effectively blocking one potential exploitation vector. Organizations should also implement strict access controls for administrative interfaces, enforce multi-factor authentication for admin accounts, and regularly audit user permissions to minimize the attack surface. Network monitoring should be configured to detect unusual template modifications or code injection attempts, and regular security assessments should validate that the patch has been properly applied and that no other vulnerabilities exist in the Grav installation.