Is CVE-2026-66912 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-66912 being exploited in the wild.

How severe is CVE-2026-66912?

Informational severity, published 29 October 2026.

Which products and versions are affected?

Synacor Zimbra Collaboration before 10.1.21.

Is there a fix for CVE-2026-66912?

Yes: fixed in 10.1.21. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-66912 attacked?

It is attacked over the network, in a web application.

Am I affected by CVE-2026-66912?

Check whether you run Synacor Zimbra Collaboration at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-66912: Zimbra Collaboration stored XSS in From header during reply composition

CVE-2026-66912 · Severity: info · Published 2026-10-29

Zimbra Collaboration, a web-based email and groupware platform used by organizations for messaging and calendaring, contains a stored cross-site scripting flaw in its Classic UI. An attacker can send a malicious email with a crafted From header; when a victim opens it and replies in HTML mode, JavaScript code executes in the victim's browser, potentially stealing session credentials or performing unauthorized actions.

Vendors: Synacor.

Quick answers

Is CVE-2026-66912 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-66912 being exploited in the wild.
How severe is CVE-2026-66912?: Informational severity, published 29 October 2026.
Which products and versions are affected?: Synacor Zimbra Collaboration before 10.1.21.
Is there a fix for CVE-2026-66912?: Yes: fixed in 10.1.21. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-66912 attacked?: It is attacked over the network, in a web application.
Am I affected by CVE-2026-66912?: Check whether you run Synacor Zimbra Collaboration at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

Fixed in: 10.1.21

Technical details

A stored XSS vulnerability in the Classic Web Client fails to properly sanitize the From email header when rendering reply composition in HTML mode with reply headers disabled. An unauthenticated remote attacker sends a crafted email with a malicious JavaScript payload in the From header; upon victim interaction (opening the message and clicking Reply), the unsanitized header content is rendered as HTML and executes in the victim's browser context, allowing session theft or account compromise.

Affected products

Synacor Zimbra Collaboration before 10.1.21

Timeline

2026-10-29: disclosed

References

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