CVE-2021-41233 in Server
Summary
by MITRE • 03/11/2022
Nextcloud text is a collaborative document editing using Markdown built for the nextcloud server. Due to an issue with the Nextcloud Text application, which is by default shipped with Nextcloud Server, an attacker is able to access the folder names of "File Drop". For successful exploitation an attacker requires knowledge of the sharing link. It is recommended that users upgrade their Nextcloud Server to 20.0.14, 21.0.6 or 22.2.1. Users unable to upgrade should disable the Nextcloud Text application in the application settings.
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Analysis
by VulDB Data Team • 03/14/2022
The vulnerability identified as CVE-2021-41233 affects the Nextcloud text application, which provides collaborative document editing capabilities using Markdown within the Nextcloud server environment. This application is shipped by default with Nextcloud Server installations, making it a widespread concern for organizations relying on Nextcloud for their file sharing and collaboration needs. The flaw specifically resides in how the Nextcloud Text application handles access controls for folder names within the File Drop functionality, creating an information disclosure vulnerability that could potentially expose sensitive directory structure information to unauthorized parties.
The technical implementation of this vulnerability stems from inadequate access control mechanisms within the Nextcloud Text application's handling of File Drop folder names. When users share files through the File Drop feature, the application should properly restrict access to folder metadata based on user permissions and sharing links. However, the flaw allows an attacker to retrieve folder names associated with File Drop shares even when they do not have direct access to the underlying files or directories. This represents a violation of the principle of least privilege and demonstrates a weakness in the application's authorization logic. The vulnerability requires an attacker to already possess knowledge of a valid sharing link, which means it operates as a privilege escalation issue within the context of an existing share rather than a complete bypass of authentication mechanisms.
The operational impact of this vulnerability extends beyond simple information disclosure, as it could enable attackers to map the directory structure of Nextcloud installations and potentially identify sensitive organizational patterns or file naming conventions. This information could be leveraged in subsequent attacks to target specific files or directories more effectively, or to conduct reconnaissance for further exploitation attempts. The vulnerability affects multiple Nextcloud Server versions including 20.0.0 through 20.0.13, 21.0.0 through 21.0.5, and 22.0.0 through 22.2.0, making it a significant concern for organizations that have not yet patched their systems. The attack vector is particularly concerning because it operates within the legitimate sharing functionality of Nextcloud, making it more difficult to detect through standard security monitoring mechanisms. According to CWE standards, this vulnerability maps to CWE-200 Information Exposure, and from an ATT&CK perspective, it aligns with T1213 Data from Information Repositories and T1083 File and Directory Discovery techniques.
The recommended remediation approach involves upgrading Nextcloud Server installations to version 20.0.14, 21.0.6, or 22.2.1, which contain the necessary patches to address the access control flaw in the Nextcloud Text application. Organizations unable to perform immediate upgrades should disable the Nextcloud Text application entirely through the application settings, effectively removing the attack surface until a proper upgrade can be completed. This mitigation strategy aligns with standard security practices for managing vulnerabilities where immediate patching is not feasible, providing a temporary but effective defense mechanism. Security administrators should also implement monitoring for unusual access patterns related to File Drop functionality and consider conducting regular vulnerability assessments to identify similar issues in other applications within their Nextcloud deployments. The vulnerability demonstrates the importance of comprehensive security testing for collaborative applications and highlights the need for robust access control validation in file sharing systems, particularly those that handle sensitive organizational data through shared access mechanisms.