---
id: CVE-2026-93647
title: "An unauthenticated calendar sender can place active markup in a COUNTER message's RFC\_From\_address"
summary: "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."
severity: critical
cvss: 9.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N'
cwe:
  - CWE-79
vendor: Zimbra
product: Zimbra Collaboration Suite (ZCS)
affected:
  - collaboration_suite_zcs < 10.1.21
published: '2026-09-25'
updated: '2026-09-26'
sourceUpdated: '2026-09-26T04:17:52.143'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-93647'
references:
  - url: 'https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories'
    label: cve@rapid7.com
tags:
  - nvd
  - cve.org
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-09-25T00:00:00+00:00'
ingestedAt: '2026-09-25T14:09:46.365Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.