CVE-2026-93628 in Collaboration Suite
Summary
by MITRE • 10/23/2026
An attendee authentication vulnerability in the email calendar processing of Zimbra Collaboration Suite (ZCS) allows an unauthenticated sender to forge another attendee's response to an existing meeting. Zimbra passes a null sender identity when processing iCalendar REPLY components, causing the attendee authorization filter to be skipped. An attacker who knows the meeting UID, organizer address and attendee address can send a non-outdated reply that changes the attendee's participation status in the organizer's calendar. The change occurs automatically during delivery without organizer or attendee interaction. ZCS 10.1.16 and 10.1.21 are affected.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 10/24/2026
The vulnerability identified in Zimbra Collaboration Suite involves a critical flaw within its email calendar processing engine, specifically concerning the handling of iCalendar REPLY components. This authentication weakness permits an unauthenticated external sender to forge responses on behalf of legitimate attendees without possessing their credentials or requiring any interactive action from them. The core technical issue stems from how Zimbra processes incoming calendar updates; when a REPLY component is received, the system fails to validate the identity of the sender against the expected attendee for that specific meeting instance. Instead of enforcing strict authorization checks based on the email address listed in the attendee field, the software passes a null or invalid sender identity during this processing phase. This design oversight causes the internal attendee authorization filter to be bypassed entirely, allowing any external party with knowledge of basic meeting metadata to inject false status updates directly into the organizer's calendar system.
From an operational perspective, the impact of this vulnerability is significant for organizational integrity and scheduling accuracy. An attacker who possesses three specific pieces of information—the unique identifier (UID) of the target meeting, the email address of the organizer, and the email address of a targeted attendee—can manipulate the perceived availability or attendance status of that attendee. By sending a crafted iCalendar REPLY message with an outdated timestamp to ensure it is not rejected as stale data, the attacker can change the attendee's participation status from accepted to declined, or vice versa, without any notification or confirmation from either party. This automatic processing during delivery means the modification occurs silently in the background, potentially leading to miscommunication, scheduling conflicts, and a loss of trust in the collaboration platform’s reliability. The vulnerability affects specific versions including Zimbra Collaboration Suite 10.1.16 and 10.1.21, highlighting that even relatively recent releases may contain legacy processing logic flaws if not properly patched.
This flaw aligns with CWE-345, which describes the lack of verification for expected data authenticity or integrity in a protocol implementation, as well as CWE-862 regarding missing authorization checks. In terms of adversary tactics, this vulnerability facilitates actions consistent with MITRE ATT&CK technique T1071 Application Layer Protocol, where attackers use standard application protocols to exfiltrate data or manipulate systems without triggering security controls. The ability to forge calendar responses can be leveraged for social engineering attacks, such as creating false alibis by marking an employee as absent from a critical meeting they actually attended, thereby disrupting business operations and potentially aiding in insider threat scenarios where malicious actors attempt to obscure their activities or frame colleagues.
To mitigate this risk, organizations running affected versions of Zimbra Collaboration Suite must apply the latest security patches provided by the vendor immediately. Administrators should verify that all mail servers are updated beyond version 10.1.21 if available, as earlier fixes may not fully address the root cause in the calendar processing module. In addition to patching, it is advisable to review access control policies for external email submissions and consider implementing stricter validation rules at the network perimeter or within a dedicated mail gateway that inspects iCalendar payloads before they reach the core Zimbra services. Monitoring logs for unusual spikes in calendar-related API calls or REPLY processing errors can also help detect potential exploitation attempts while mitigation measures are being deployed.