CVE-2021-22896 in Mail
Summary
by MITRE • 06/11/2021
Nextcloud Mail before 1.9.5 suffers from improper access control due to a missing permission check allowing other authenticated users to create mail aliases for other users.
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Analysis
by VulDB Data Team • 06/14/2021
The vulnerability identified as CVE-2021-22896 affects Nextcloud Mail versions prior to 1.9.5 and represents a critical access control flaw that undermines the security posture of the email management system. This issue stems from a missing permission check within the application's alias creation functionality, which allows authenticated users to manipulate the email alias configuration for other users within the same Nextcloud instance. The flaw exists in the web application's authorization logic where proper user validation and privilege verification are not enforced during the alias creation process, creating an avenue for privilege escalation and unauthorized data manipulation.
The technical implementation of this vulnerability resides in the application's API endpoints responsible for managing email aliases within the Nextcloud Mail module. When an authenticated user attempts to create or modify email aliases through the system's interface or API, the application fails to verify whether the requesting user has the appropriate permissions to perform such actions on behalf of other users. This missing authorization check creates a path where any authenticated user can submit requests that modify another user's email alias configuration, effectively allowing them to impersonate or redirect emails intended for legitimate users. The flaw operates at the application layer and affects the core authentication and authorization mechanisms that should normally prevent cross-user privilege escalation.
The operational impact of this vulnerability extends beyond simple data manipulation to encompass potential security breaches and privacy violations within Nextcloud environments. An attacker with access to the Nextcloud instance can exploit this weakness to create malicious email aliases that redirect communications intended for legitimate users, potentially enabling phishing attacks, data interception, or unauthorized access to sensitive information. The vulnerability also compromises the integrity of the email system by allowing unauthorized modifications to user email configurations, which can disrupt normal communication flows and create confusion among users. This type of flaw particularly affects organizations that rely on Nextcloud for secure email management and collaboration, as it undermines the trust model that should exist between users and the system administrators.
Organizations implementing Nextcloud Mail should prioritize immediate patching of affected versions to address this vulnerability, as the risk of exploitation increases with the number of authenticated users within the system. The recommended mitigation strategy involves upgrading to Nextcloud Mail version 1.9.5 or later, which includes proper permission validation and authorization checks for alias creation operations. Additionally, system administrators should implement network-level monitoring to detect unusual alias creation patterns and establish regular audit procedures to identify unauthorized modifications to user email configurations. Security teams should also consider implementing additional access controls and privilege management policies to limit the scope of user capabilities within the Nextcloud environment, reducing the potential impact of similar authorization flaws.
This vulnerability aligns with CWE-284, which describes improper access control in software applications, and can be categorized under the ATT&CK technique T1078 for Valid Accounts and T1566 for Phishing, as it enables unauthorized users to manipulate email communications and potentially gain access to sensitive information. The flaw demonstrates how insufficient input validation and authorization checks can create dangerous attack vectors that bypass the intended security model of the application. Organizations should also consider implementing principle of least privilege controls and regular security assessments to identify and remediate similar access control vulnerabilities in their Nextcloud deployments and other collaborative platforms.