---
id: "CVE-2026-66912"
kind: "vulnerability"
aliases: ["CVE-2026-66912"]
title: "Zimbra Classic UI: crafted From header runs script on reply"
severity: "medium"
cvss: 5.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["email", "package", "self-hosted", "zimbra"]
not_affected_if: ["managed-hosting", "modern-ui-only"]
published_at: "2026-10-29T16:18:15+00:00"
first_seen_at: "2026-10-29T16:26:01+00:00"
updated_at: "2026-10-29T16:31:49+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-66912", "first_seen_at": "2026-10-29T16:26:01+00:00"}]
---

# Zimbra Classic UI: crafted From header runs script on reply

A crafted From header in an email can carry JavaScript that runs in the browser of a user who opens the message in Zimbra Classic UI and clicks Reply in HTML mode with reply headers disabled. An attacker could steal the victim's session or act as the victim inside Zimbra.

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

## How to check

- Check the installed Zimbra version and compare it with the fixed version 10.1.21.
- Check whether Classic UI is in use and whether users compose replies in HTML mode with reply headers disabled.

## Fix

- Zimbra `10.1.21` or later (per the NVD description; the affected range is not stated in the sources).

## Timeline

- 2026-10-29T16:26:01+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-66912

## From the sources

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

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