CVE-2017-0885 in Server
Summary
by MITRE
Nextcloud Server before 9.0.55 and 10.0.2 suffers from a error message disclosing existence of file in write-only share. Due to an error in the application logic an adversary with access to a write-only share may enumerate the names of existing files and subfolders by comparing the exception messages.
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Analysis
by VulDB Data Team • 08/25/2020
The vulnerability described in CVE-2017-0885 affects Nextcloud Server versions prior to 9.0.55 and 10.0.2, representing a significant information disclosure flaw that undermines the security of write-only share functionality. This issue stems from inadequate error handling within the application logic, where the system fails to properly sanitize exception messages that are generated during file operations. The flaw specifically manifests when users with write-only permissions attempt to access or enumerate files within shared directories, creating a scenario where adversaries can exploit the system's response to gain unauthorized knowledge about the existence and structure of files and subfolders.
The technical implementation of this vulnerability resides in the application's error message handling mechanism within the sharing and file access components. When a user with write-only access attempts to perform operations that would normally result in an access denial, the system generates exception messages that inadvertently reveal whether specific files or directories exist within the shared context. This occurs because the error handling logic does not properly distinguish between different types of access failures, instead providing verbose error responses that include file path information. The vulnerability aligns with CWE-200, which addresses improper information exposure, and represents a classic case of information leakage through error messages that should remain generic and non-descriptive to maintain security boundaries.
The operational impact of this vulnerability extends beyond simple information disclosure, as it enables adversaries to perform systematic enumeration of shared resources. An attacker with write-only share access can systematically test various file paths and observe the differences in error messages to build a comprehensive directory structure of the shared content. This enumeration capability allows for targeted attacks on specific files or folders that may contain sensitive information, effectively bypassing the intended security restrictions of write-only shares. The vulnerability demonstrates a failure in the principle of least privilege, where the system's response to unauthorized access provides more information than necessary, enabling attackers to map the underlying file structure without proper read permissions.
From a threat modeling perspective, this vulnerability aligns with ATT&CK technique T1083, which involves discovering file and directory permissions on compromised systems. The flaw enables adversaries to gather intelligence about shared resources without requiring additional privileges, making it particularly dangerous in environments where sensitive data is shared through Nextcloud. The vulnerability also represents a failure in secure coding practices, specifically in the area of proper error handling and response generation. Organizations using affected Nextcloud versions face increased risk of data exposure, as attackers can use this information to identify valuable targets within shared directories and plan more sophisticated attacks against specific files or folders.
The recommended mitigation strategy involves upgrading to Nextcloud Server versions 9.0.55 or 10.0.2, which contain patches addressing the improper error handling logic. Additionally, administrators should implement proper input validation and ensure that all error messages generated during file access operations remain generic and do not reveal file system structure information. Security teams should conduct regular vulnerability assessments to identify similar information disclosure issues in other applications and ensure that error handling follows security best practices. The fix implemented by Nextcloud developers likely involved modifying the exception handling code to sanitize error messages and ensure that write-only share users receive consistent generic responses regardless of whether files exist, thereby preventing the enumeration capability that made this vulnerability exploitable.