Vulnerability · Published Oct 29, 2026 · Assigned by CERT-PL
CVE-2026-66912Stored Cross-Site Scripting in Zimbra
Medium 5.3
In plain English
A medium-severity cross-site scripting in Zimbra Collaboration from Synacor. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 41% of all known vulnerabilities).
Official 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
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Synacor | Zimbra Collaboration | 0 – before 10.1.21 |
Weakness: CWE-79 · Vector: 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
References
- https://cert.pl/en/posts/2026/10/CVE-2026-66911 third-party-advisory
- https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories release-notes