CVE-2025-66376
MediumExploitedPublic exploitZimbra 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.
Published Jan 5, 2026 · Updated Oct 9, 2026
Key Facts
Patch immediately- CVE-2025-66376 is a medium-severity vulnerability (CVSS 6.1 v3.1) affecting Zimbra Collaboration Suite (Synacor), Collaboration.
- It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Mar 17, 2026. Public exploit code is available.
- EPSS estimates a 20% probability of exploitation in the next 30 days.
- Fixed versions are available, including Collaboration 10.0.18, Collaboration 10.1.13, Zimbra Collaboration Suite 10.0.18.
- Brandefense priority: patch immediately, because it is listed as exploited in the wild.
CVSS v3.1
6.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
EPSS (30-day exploit likelihood)
20%
Higher than 97.4% of all scored CVEs
Exploited in the wild
Yes
4 catalog listings
Public exploit code
Available
Proof-of-concept or weaponized module
Get the full CVE-2025-66376 brief as a PDF
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Description
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.
Known-Exploited Catalog Listings
| Catalog | Added | Remediation due | Ransomware use |
|---|---|---|---|
| CISA KEV | Mar 18, 2026 | Apr 1, 2026 | Not reported |
| Industry KEV feed | Mar 17, 2026 | Apr 1, 2026 | Not reported |
| CIRCL KEV | Mar 18, 2026 | Apr 1, 2026 | Not reported |
| ENISA EUVD | Mar 18, 2026 | n/a | Not reported |
Required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Affected Products
2 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| Zimbra Collaboration SuiteSynacor | See advisory | 10.0.18 |
| Collaboration | >= 10.0 | 10.0.18 |
Patched Releases
Collaboration
10.0.18, 10.1.13
Zimbra Collaboration Suite
10.0.18, 10.1.13
Timeline
Publicly disclosed
Jan 5, 2026
Added to Industry KEV feed
Mar 17, 2026
Added to CISA KEV
Mar 18, 2026
Added to CIRCL KEV
Mar 18, 2026
Added to ENISA EUVD
Mar 18, 2026
Last updated
Oct 9, 2026
Weakness Type
- CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
References
- advisoryhttps://github.com/CVEProject/cvelistV5/tree/main/cves/2025/66xxx/CVE-2025-66376.json
- advisoryhttps://nvd.nist.gov/vuln/detail/CVE-2025-66376
- advisoryhttps://wiki.zimbra.com/wiki/Security_Center
- advisoryhttps://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
- exploitVulnCheck exploitation report
FAQ
Common Questions.
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.
Affected: Zimbra Collaboration Suite (Synacor); Collaboration versions >= 10.0. The full list is in the CVE brief.
It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Mar 17, 2026.
Yes. It was added on Mar 18, 2026 with a federal remediation due date of Apr 1, 2026.
Public exploit code is available.
Fixed versions are available, including Collaboration 10.0.18, Collaboration 10.1.13, Zimbra Collaboration Suite 10.0.18.
Brandefense priority: patch immediately, because it is listed as exploited in the wild.
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