CVE-2020-8633 in Zimbra Collaboration Suite
Summary
by MITRE
An issue was discovered in Zimbra Collaboration Suite (ZCS) before 8.8.15 Patch 7. When grantors revoked a shared calendar in Outlook, the calendar stayed mounted and accessible.
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Analysis
by VulDB Data Team • 02/19/2020
The vulnerability identified as CVE-2020-8633 represents a critical access control flaw within the Zimbra Collaboration Suite (ZCS) platform that affects versions prior to 8.8.15 Patch 7. This security weakness manifests specifically in the calendar sharing functionality where the system fails to properly synchronize revocation of shared calendar access with the actual mounting state of those calendars within the Outlook client interface. The issue stems from a fundamental disconnect between the administrative actions taken to revoke calendar permissions and the client-side behavior that continues to display and allow access to previously shared calendar resources.
The technical implementation flaw occurs at the synchronization layer between the Zimbra server and Microsoft Outlook clients, where the calendar mounting process does not properly validate the current sharing permissions status. When an administrator or user with appropriate privileges revokes access to a shared calendar, the system correctly updates the permission database but fails to communicate this change effectively to the Outlook client application. This results in a persistent state where the calendar remains visible and accessible to the revoked users through the mounted calendar interface, creating a scenario where unauthorized individuals can continue to view calendar data even after explicit revocation of their access rights.
This vulnerability directly impacts the principle of least privilege and access control mechanisms within the Zimbra platform, potentially allowing unauthorized information disclosure and access to sensitive calendar data. The operational implications extend beyond simple calendar access, as calendar data often contains confidential information about meetings, schedules, and business activities that could be exploited for social engineering attacks or corporate intelligence gathering. The flaw creates a window of opportunity for attackers who may have previously gained access to shared calendars through legitimate means but can continue to access them after their access has been formally revoked by administrators.
The vulnerability aligns with CWE-284, which addresses improper access control, and demonstrates weaknesses in the authorization validation process within the Zimbra collaboration platform. From an ATT&CK framework perspective, this issue enables techniques such as privilege escalation and credential access through the exploitation of permission management flaws. The persistent mounting of revoked calendars also creates potential for data exfiltration scenarios where unauthorized users can continue to monitor calendar activities over extended periods. Organizations utilizing Zimbra Collaboration Suite must implement immediate mitigations including mandatory patch updates to version 8.8.15 Patch 7 or later, along with enhanced monitoring of calendar sharing activities and access logs to detect potential exploitation attempts.
Security teams should consider implementing additional controls such as regular calendar access audits, automated alerts for sharing permission changes, and network monitoring to detect unusual calendar access patterns that may indicate exploitation of this vulnerability. The flaw also highlights the importance of proper session management and real-time synchronization between administrative actions and client-side state updates in enterprise collaboration platforms. Organizations should also review their overall calendar sharing policies and implement more granular access controls to minimize the potential impact of such authorization bypass vulnerabilities.