CVE-2026-93627 in Collaboration Suite
Summary
by MITRE • 10/23/2026
An organizer authentication vulnerability in the email calendar processing of Zimbra Collaboration Suite (ZCS) allows an unauthenticated sender to cancel or modify an existing meeting in a recipient's calendar. Zimbra accepts iCalendar CANCEL and REQUEST components without verifying that the email sender is authorized to act as the supplied organizer. An attacker who knows the meeting UID and organizer address can supply a non-outdated sequence number to move the recipient's meeting to Trash or replace its details. Processing occurs automatically during email delivery without recipient interaction. ZCS 10.1.16 and 10.1.21 are affected.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified in Zimbra Collaboration Suite involves a critical flaw in the authentication logic governing calendar event processing, specifically within the handling of iCalendar components such as CANCEL and REQUEST messages. This issue stems from an insufficient verification mechanism where the system fails to validate that the email sender is authorized to act as the organizer specified in the meeting metadata. In standard collaborative scheduling protocols like iTIP (iTransfer Protocol), which underpins Zimbra's calendar functionality, there are strict rules regarding sequence numbers and authentication to prevent unauthorized modifications or cancellations of scheduled events. However, in this affected implementation, the server accepts these components without confirming that the originating email address matches the organizer identity embedded within the iCalendar object, nor does it adequately enforce constraints on sequence number progression relative to existing calendar entries.
From a technical perspective, the core defect lies in the lack of strict access control checks during the parsing and application of incoming calendar updates. When an email containing an iCalendar CANCEL or REQUEST component is received, Zimbra processes these instructions automatically as part of standard mail delivery operations without requiring any user interaction. This automation, while convenient for legitimate users syncing calendars across devices, becomes a significant attack vector when combined with the authentication bypass. An attacker who has knowledge of specific meeting identifiers (UIDs) and organizer email addresses can craft malicious iCalendar payloads that mimic valid organizational requests. By supplying a sequence number that is not outdated relative to the current state of the calendar entry, the attacker can successfully trick the server into applying these changes. This allows for the silent cancellation or modification of meetings directly in the recipient's calendar view.
The operational impact of this vulnerability is severe, primarily affecting confidentiality and integrity within collaborative environments. Because the processing occurs automatically during email delivery, victims are unaware that their schedules have been tampered with until they review their calendars later. An attacker can use this capability to disrupt business operations by canceling critical meetings, causing missed appointments and potential financial or reputational damage. Furthermore, an adversary could modify meeting details such as time, location, or attendees to facilitate social engineering attacks or physical security breaches if the modified event leads individuals to unexpected locations or times. The ability to move a meeting to Trash effectively removes it from immediate visibility without clear notification, adding a layer of stealth that complicates detection and incident response efforts for system administrators.
This vulnerability aligns with CWE-287, which describes Improper Authentication, as the system fails to properly verify the identity of the sender against the claimed organizer role. It also relates to CWE-913, specifically regarding improper control of dynamically-managed resources, where the calendar entries are manipulated without proper authorization checks. In terms of offensive security frameworks, this behavior maps to MITRE ATT&CK technique T1486, Data Encrypted for Impact, although in this context it is more accurately described as unauthorized modification or deletion of data (T1070 Indicator Removal on Host or T1539 Steal Web Session Cookie if session hijacking were involved, but here strictly T1486/T1070 analogs). The lack of user interaction places it firmly in the realm of automated exploitation via email delivery channels.
Mitigation strategies should focus on immediate patching and configuration hardening. Organizations running Zimbra Collaboration Suite versions 10.1.16 through 10.1.21 must apply the latest security patches provided by Zimbra to address this authentication bypass. Until patches are applied, administrators can implement network-level controls such as strict inbound email filtering rules that inspect iCalendar content for anomalies or restrict calendar-related MIME types from external sources if internal-only collaboration is sufficient. Additionally, enabling detailed logging of calendar modifications and setting up alerts for unusual patterns of CANCEL or REQUEST operations can aid in early detection. Users should also be educated to verify meeting status through secondary channels when critical appointments are unexpectedly canceled or altered, although this is a compensating control rather than a technical fix.