CVE-2026-93627: Zimbra Collaboration Suite Calendar Processing Authentication Bypass via Forged iCalendar CANCEL/REQUEST Organizer
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 An attacker who knows the meeting UID and organizer address can supply a non-outdated sequence number to move the recipient's meeting to Trash or replace its details.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.
Affected Software
Event History
Frequently Asked Questions
What does an attacker need to exploit this issue?
The attacker needs to know the target meeting's UID and the organizer address. They can then send a forged iCalendar CANCEL or REQUEST with a sequence number that is not outdated.
Is recipient interaction required?
No. The forged calendar component is processed automatically during email delivery, so the recipient does not need to open, accept, or otherwise interact with the message.
What can a successful attack do to a recipient's calendar?
A forged CANCEL can move an existing meeting to Trash, while a forged REQUEST can replace the meeting's details. The vulnerability affects integrity of calendar entries rather than confidentiality or availability.
Which ZCS versions are identified as affected?
Zimbra Collaboration Suite versions 10.1.16 and 10.1.21 are affected.