CVE-2021-40102 in Concrete
Summary
by MITRE • 09/24/2021
An issue was discovered in Concrete CMS through 8.5.5. Arbitrary File deletion can occur via PHAR deserialization in is_dir (PHP Object Injection associated with the __wakeup magic method).
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Analysis
by VulDB Data Team • 10/02/2021
The vulnerability identified as CVE-2021-40102 represents a critical security flaw within Concrete CMS versions 8.5.5 and earlier, where an attacker can exploit PHAR deserialization mechanisms to achieve arbitrary file deletion on the affected system. This vulnerability specifically leverages the is_dir function's behavior during PHP object injection attacks, utilizing the __wakeup magic method to execute malicious code. The flaw exists within the content management system's handling of serialized data structures, creating an attack surface that allows remote exploitation without authentication requirements. The vulnerability stems from insufficient input validation and improper sanitization of user-supplied data during the deserialization process, which is a common pattern in PHP applications that fail to properly secure object instantiation.
The technical exploitation of this vulnerability involves crafting a malicious PHAR archive that, when processed by the Concrete CMS application, triggers the __wakeup method of a serialized object. This method execution occurs within the context of the is_dir function, which typically validates directory paths but becomes compromised when handling untrusted serialized data. The attack vector demonstrates a classic PHP object injection scenario where the attacker can manipulate the serialized object's properties to control the execution flow and ultimately delete arbitrary files on the server filesystem. This type of vulnerability is classified under CWE-502 as Deserialization of Untrusted Data, which is a well-documented weakness in web applications that handle serialized objects from external sources.
The operational impact of this vulnerability is severe as it provides attackers with the capability to delete critical files, potentially leading to complete system compromise and service disruption. An attacker could target configuration files, database connection details, user authentication data, or even core application files that would result in system instability or complete application failure. The vulnerability's remote exploitability means that attackers do not require local system access or prior authentication, making it particularly dangerous for publicly accessible web applications. This flaw could enable attackers to escalate privileges, persist in the system through file deletion, or create conditions that facilitate further attacks within the network infrastructure. The potential for data loss and system compromise aligns with ATT&CK technique T1485 which covers data destruction and T1059 which encompasses command and scripting interpreter usage.
Organizations using Concrete CMS versions prior to 8.5.6 should implement immediate mitigations to protect against this vulnerability. The primary remediation involves upgrading to the patched version of Concrete CMS, which addresses the deserialization flaw by implementing proper input validation and sanitization of serialized data. Additionally, administrators should consider implementing web application firewalls that can detect and block malicious PHAR file uploads or requests containing serialized objects. Input validation measures should include strict sanitization of all user-supplied data, particularly when handling file uploads or any data that might be serialized and later deserialized. The implementation of proper access controls and file permission settings can help limit the impact of successful exploitation attempts, while regular security audits and monitoring of system logs should be conducted to detect any unauthorized file deletion activities. Network segmentation and regular backup procedures are essential defensive measures that can minimize the potential damage from such an attack.