Assessment
Why it matters
CISA lists this vulnerability as known to be exploited; exploitation is confirmed; EPSS is 22.0%; technical severity is CVSS 6.1.
Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import directives in an HTML e-mail message.
Who should care
- Teams operating Zimbra Collaboration Suite, Zimbra Collaboration Suite (ZCS)
- Vulnerability and exposure management teams
- Security operations teams monitoring exploitation activity
Response plan
What I would do
- Confirm whether Zimbra Collaboration Suite, Zimbra Collaboration Suite (ZCS) is present in the environment.
- Identify affected versions and establish whether vulnerable services are exposed or reachable.
- Vendor advisory from wiki.zimbra.com
- Review relevant security telemetry for evidence of attempted or successful exploitation.
- Document the remediation decision and track it to verified completion.
Treatment intelligence
Remediation intelligence
Vendor sources are listed before government and third-party guidance. Confirm product applicability and change prerequisites before deployment.
Verification noteAutomated link verification has not completed for this guidance yet.
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Technical details
- CVSS vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- CWE
- CWE-79
- Affected versions
- Synacor Zimbra Collaboration Suite (Start Including 10.0.0, End Excluding 10.0.18 | Start Including 10.1.0, End Excluding 10.1.13); Synacor Zimbra Collaboration Suite (ZCS)
- Fixed versions
- Synacor Zimbra Collaboration Suite (Fixed from 10.0.18, 10.1.13)
- Published
- 5 January 2026
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Required
Signal timeline
- CVE published
- Added to CISA KEV
- Latest EPSS score: 22.0%
- Signal calculated at 71