---
id: CVE-2026-66912
title: Stored Cross-Site Scripting in Zimbra
summary: >-
  Zimbra is vulnerable to Stored Cross-Site Scripting (XSS) via the From email
  header during HTML reply composition in Classic UI. Due to insufficient
  sanitization of the From header value, a remote attacker can send a crafted
  email contai…
severity: medium
cvss: 5.3
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N'
cvssSource: cna
cwe:
  - CWE-79
vendor: Synacor
product: Zimbra Collaboration
affected:
  - zimbra_collaboration < 10.1.21
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-11-05T20:46:54.626005Z'
published: '2026-10-29'
updated: '2026-11-05'
sourceUpdated: '2026-11-05T20:47:10.647Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-66912'
references:
  - url: 'https://cert.pl/en/posts/2026/10/CVE-2026-66911'
  - url: 'https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories'
tags:
  - cve.org
epss: 0.00467
epssPercentile: 0.406
ingestedAt: '2026-11-01T20:00:16.962Z'
---

## Overview

Zimbra is vulnerable to Stored Cross-Site Scripting (XSS) via the From email header during HTML reply composition in Classic UI. Due to insufficient sanitization of the From header value, a remote attacker can send a crafted email containing a malicious From header with an embedded JavaScript payload. When the victim opens the message in Classic UI and clicks Reply while composing in HTML mode with reply headers disabled, the unsanitized From header content is rendered as HTML and the JavaScript payload executes in the victim's browser, potentially allowing the attacker to steal session information or perform actions on behalf of the victim. This issue was fixed in version 10.1.21

## Affected

- `zimbra_collaboration < 10.1.21`

## Remediation

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