Medium · CVSS 5.3CVE-2026-66912

Stored Cross-Site Scripting in ZimbraCVE-2026-66912

Answer in brief

CVE-2026-66912 records a Medium severity (CVSS 5.3) vulnerability in Stored Cross-Site Scripting in Zimbra. The current sources do not mark it as known exploited. The current feed maps Synacor/Zimbra Collaboration (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 29, 2026Updated Oct 29, 2026Source checked Oct 29, 2026First seen by HOL Oct 29, 2026Material review Oct 29, 2026

Upstream Advisory

Key facts

Risk: Medium · CVSS 5.3
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 5.3. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps Synacor/Zimbra Collaboration (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
Synacor/Zimbra Collaborationgeneric	>=0 <10.1.21	10.1.21

Recommended response

Check inventory. Check lockfiles and deployed manifests for Synacor/Zimbra Collaboration.
Review the reported fix. Update Synacor/Zimbra Collaboration to 10.1.21 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Oct 29, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 29, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Oct 29, 2026

Sources and claim methodology

Upstream source description

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

Quoted source text, attributed separately from HOL analysis.