CVE-2026-93628: Zimbra Collaboration Suite Calendar Processing Authentication Bypass via Forged iCalendar REPLY Attendee Response
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 An attacker who knows the meeting UID, organizer address and attendee address can send a non-outdated reply that changes the attendee's participation status in the organizer's calendar.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.
Affected Software
Event History
Frequently Asked Questions
What information does an attacker need to exploit this issue?
The attacker needs the meeting UID, the organizer's address, and the target attendee's address. They can then send a non-outdated forged iCalendar REPLY that changes that attendee's participation status.
Is user interaction required for the calendar change to occur?
No. The forged reply is processed automatically during email delivery, without interaction from either the organizer or the attendee.
Which Zimbra Collaboration Suite versions are identified as affected?
ZCS 10.1.16 and 10.1.21 are affected.