CVE-2021-22913 in Deck
Summary
by MITRE • 06/11/2021
Nextcloud Deck before 1.2.7, 1.4.1 suffers from an information disclosure vulnerability when searches for sharees utilize the lookup server by default instead of only the local Nextcloud server unless a global search has been explicitly chosen by the user.
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Analysis
by VulDB Data Team • 06/14/2021
The vulnerability described in CVE-2021-22913 affects Nextcloud Deck versions prior to 1.2.7 and 1.4.1, representing a significant information disclosure weakness that undermines the privacy and security of user data within the Nextcloud ecosystem. This flaw manifests specifically during the search operations for sharees, which are users or groups that can be granted access to shared deck cards and boards. The vulnerability stems from the default configuration where the system performs lookups through an external lookup server rather than restricting searches to the local Nextcloud instance. This design choice creates an unintended data exposure channel that allows external entities to potentially gather information about users, groups, and sharing relationships within the organization's Nextcloud environment.
The technical implementation of this vulnerability involves the search functionality's default behavior of utilizing a global lookup service when users attempt to share deck content with others. When a user initiates a search for a sharee, the system automatically queries an external server to find matching users or groups instead of limiting the search scope to the local Nextcloud installation. This external lookup mechanism bypasses the intended security boundaries and creates a pathway for information leakage that could reveal sensitive details about the organization's user base, sharing patterns, and internal collaboration structures. The vulnerability is particularly concerning because it operates automatically without explicit user consent or configuration changes, making it an insidious threat that affects all installations using affected versions.
The operational impact of this information disclosure vulnerability extends beyond simple data exposure to potentially enable more sophisticated attacks and reconnaissance activities. An attacker who can monitor or intercept the external lookup requests could gain insights into the organization's internal user structure, identify active users, understand sharing relationships, and potentially map out collaboration patterns within the Nextcloud environment. This intelligence gathering capability aligns with tactics described in the attack pattern framework where adversaries seek to understand target environments before launching more targeted attacks. The vulnerability essentially provides an information leak channel that could be exploited to build detailed profiles of the organization's Nextcloud usage patterns and user base. The implications are particularly severe in environments where Nextcloud is used for sensitive business operations, compliance requirements, or contains confidential project information.
Organizations can mitigate this vulnerability through several approaches that align with established security best practices and framework recommendations. The primary remediation involves upgrading to Nextcloud Deck versions 1.2.7 or 1.4.1, which include the necessary fixes to ensure that search operations for sharees are restricted to local Nextcloud servers only. Additionally, administrators should review and configure the sharing settings to explicitly disable external lookup services when such functionality is not required for legitimate business purposes. This approach follows the principle of least privilege and minimizes the attack surface by removing unnecessary external communication channels. Security teams should also implement network monitoring to detect unusual external lookup patterns and consider implementing firewall rules that restrict outbound connections to external lookup servers. The vulnerability's classification under CWE-200 (Information Exposure) and its alignment with ATT&CK techniques for reconnaissance and credential access highlights the importance of addressing this issue through both immediate patching and broader security configuration reviews. Organizations should also consider implementing additional monitoring and logging controls around sharing operations to detect potential unauthorized access attempts or unusual search patterns that might indicate exploitation attempts.