CVE-2017-0894 in Server
Summary
by MITRE
Nextcloud Server before 11.0.3 is vulnerable to disclosure of valid share tokens for public calendars due to a logical error. Thus granting an attacker potentially access to publicly shared calendars without knowing the share token.
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Analysis
by VulDB Data Team • 09/24/2020
The vulnerability identified as CVE-2017-0894 affects Nextcloud Server versions prior to 11.0.3 and represents a critical logical flaw in the calendar sharing mechanism that undermines the security model of public calendar access. This vulnerability specifically impacts the way Nextcloud handles share tokens for public calendars, creating an unintended access path that bypasses normal authentication requirements. The flaw exists within the server-side logic that manages calendar sharing permissions, where valid share tokens are being exposed or made discoverable through improper access control validation.
The technical implementation of this vulnerability stems from a design error in how Nextcloud validates and manages share tokens for public calendar resources. When users create public calendar shares, the system should generate and maintain secure, randomized tokens that are required for access to those calendar resources. However, due to the logical error present in versions before 11.0.3, the system fails to properly enforce token-based access control, allowing unauthorized parties to potentially access publicly shared calendars without possessing the legitimate share token. This represents a direct violation of the principle of least privilege and undermines the fundamental security assumptions of the sharing mechanism.
From an operational impact perspective, this vulnerability creates significant risks for organizations using Nextcloud for calendar management and collaboration. Attackers who discover or exploit this vulnerability can gain unauthorized access to public calendars that should only be accessible to legitimate recipients who possess the correct share token. This exposure can lead to data leakage of calendar information including meeting schedules, personal appointments, and potentially sensitive business-related calendar entries. The vulnerability particularly affects organizations that rely on Nextcloud for internal calendar sharing, as it could allow external parties to access confidential scheduling information that might reveal business operations, executive availability, or other sensitive temporal data.
The security implications extend beyond simple information disclosure, as this vulnerability aligns with several attack patterns documented in the attack mitigation framework. This issue can be categorized under CWE-284 (Improper Access Control) and demonstrates weaknesses in access control validation mechanisms that could enable privilege escalation or unauthorized data access. From an ATT&CK perspective, this vulnerability maps to techniques involving credential access and privilege escalation through logical flaws in application security controls. The vulnerability also relates to the broader category of insecure direct object references, where the system fails to properly validate access to calendar resources based on the legitimate token requirements.
Organizations should implement immediate mitigations including upgrading to Nextcloud Server version 11.0.3 or later, which contains the necessary patches to address the logical error in share token handling. Security administrators should also conduct thorough audits of existing public calendar shares to identify any potentially compromised resources and revoke access to calendars that may have been accessed through this vulnerability. Additionally, organizations should consider implementing additional monitoring and logging around calendar sharing activities to detect any suspicious access patterns that might indicate exploitation attempts. The patch for this vulnerability specifically addresses the validation logic that was allowing unauthorized access to public calendar resources, ensuring that share tokens are properly enforced and validated before granting access to calendar data.