---
id: "CVE-2026-93628"
kind: "vulnerability"
aliases: ["CVE-2026-93628"]
title: "Zimbra Collaboration: forged calendar replies change attendee meeting status"
severity: "medium"
cvss: 5.3
epss: 0.00376
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["calendar", "email", "infrastructure", "self-hosted", "zimbra"]
not_affected_if: ["unknown"]
published_at: "2026-10-23T14:17:56+00:00"
first_seen_at: "2026-10-23T21:57:34+00:00"
updated_at: "2026-10-25T12:38:49+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-93628", "first_seen_at": "2026-10-23T21:57:34+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-93628", "first_seen_at": "2026-10-25T12:38:49+00:00"}]
---

# Zimbra Collaboration: forged calendar replies change attendee meeting status

Zimbra Collaboration Suite skips its attendee authorization check when processing iCalendar REPLY messages, so an unauthenticated sender can forge another attendee's response to a meeting. An attacker who knows the meeting UID, organizer and attendee addresses can change that attendee's participation status in the organizer's calendar.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed Zimbra version with `su - zimbra -c "zmcontrol -v"`; the sources name ZCS `10.1.16` and `10.1.21` as affected
- Unknown whether other versions are affected; the sources do not say

## Fix

- Move Zimbra Collaboration Suite off the affected `10.1.16` and `10.1.21` builds to a fixed release; the fixed version is unknown from the sources
- Unknown whether a configuration workaround exists

## Timeline

- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-93628
- 2026-10-25T12:38:49+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-93628

## From the sources

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 An attacker who knows the meeting UID, organizer address and attendee address can send a non-outdated reply that changes the attendee's participation status in the organizer's calendar.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.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_