CVE-2017-0895 in Server
Summary
by MITRE
Nextcloud Server before 10.0.4 and 11.0.2 are vulnerable to disclosure of calendar and addressbook names to other logged-in users. Note that no actual content of the calendar and addressbook has been disclosed.
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Analysis
by VulDB Data Team • 09/24/2020
The vulnerability described in CVE-2017-0895 represents a significant information disclosure flaw within the Nextcloud server platform that affects versions prior to 10.0.4 and 11.0.2. This issue stems from inadequate access control mechanisms that fail to properly restrict visibility of calendar and addressbook metadata between authenticated users. The vulnerability specifically impacts the privacy of user data by allowing unauthorized disclosure of calendar and addressbook names, creating a scenario where one logged-in user can potentially discover the existence and names of other users' calendar and addressbook resources. This type of information disclosure vulnerability falls under the CWE-200 category of "Information Exposure" and represents a violation of the principle of least privilege in information security. The flaw does not compromise the actual content stored within these resources, but rather exposes the structural metadata that reveals what calendar and addressbook resources exist within the system.
The technical implementation of this vulnerability occurs at the application layer where Nextcloud fails to properly validate and enforce user access controls when serving calendar and addressbook listing requests. When a user accesses calendar or addressbook data, the server should verify that the requesting user has appropriate permissions to view the specific resource. However, in affected versions, the authentication and authorization checks are insufficient, allowing users to enumerate calendar and addressbook names from other authenticated users. This typically manifests through API endpoints or web interface calls that return directory listings or resource names without proper user context validation. The vulnerability can be exploited by any logged-in user who has access to the Nextcloud instance, making it particularly concerning in multi-user environments where users may not be fully trusted or where users may have varying levels of access privileges.
The operational impact of CVE-2017-0895 extends beyond simple information disclosure, as it creates potential for further exploitation and reconnaissance activities within the Nextcloud environment. Attackers can leverage this vulnerability to map the calendar and addressbook structure of an organization, potentially identifying high-value targets or resources that may contain sensitive information. This reconnaissance capability aligns with ATT&CK technique T1087.001 for Account Discovery, as it allows adversaries to identify and enumerate user resources. The vulnerability also undermines user privacy expectations and can lead to social engineering attacks where attackers use the discovered calendar and addressbook names to craft more convincing phishing attempts or targeted attacks. Organizations using Nextcloud in corporate or sensitive environments may face compliance issues with data protection regulations such as GDPR or HIPAA, as this vulnerability exposes user data structures without actual content compromise. The impact is particularly severe in environments where calendar and addressbook data may contain sensitive organizational information, meeting schedules, or personal details that could be used for malicious purposes.
The recommended mitigations for CVE-2017-0895 involve immediate upgrades to Nextcloud server versions 10.0.4 or 11.0.2, which contain the necessary patches to address the access control flaws. Organizations should also implement additional monitoring to detect unusual enumeration patterns or access attempts that may indicate exploitation of this vulnerability. Security teams should review and strengthen their access control policies, ensuring that proper user authentication and authorization mechanisms are in place for all calendar and addressbook operations. Network segmentation and access controls can provide additional defense in depth, limiting access to Nextcloud services to authorized users only. Regular security assessments and vulnerability scanning should be conducted to identify similar issues in other applications within the organization's infrastructure. The patch addresses the root cause by implementing proper access validation checks that ensure users can only access calendar and addressbook resources they are authorized to view, thereby preventing the unauthorized disclosure of resource names while maintaining legitimate access for authorized users. This vulnerability serves as a reminder of the critical importance of proper access control implementation and the potential consequences of inadequate security controls in collaborative platforms.