CVE-2025-66376
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.
do firstApply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Can a WAF block it? Yes. The attack is delivered via HTML email messages with CSS @import directives, which are unusual but could resemble legitimate emails. A WAF could partially detect such unusual CSS usage in email content but might also block some legitimate emails. A custom WAF rule can filter HTML email messages containing CSS @import directives, but attackers can vary the request. No tracked WAF vendor has published a rule for this CVE yet.
What a custom rule would match: HTML email messages containing CSS @import directives
Also consider: patch · custom WAF rule · CSP · output encoding
Based on an AI reading of the CVE description, and no vendor rule found yet. We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.
Is CVE-2025-66376 being exploited?
Yes. CVE-2025-66376 appears in CISA's Known Exploited Vulnerabilities catalog, which means exploitation in the wild has been confirmed and reported. Treat it as active rather than theoretical, and patch it ahead of higher-scoring vulnerabilities that carry no exploitation evidence.
How severe is CVE-2025-66376?
CVE-2025-66376 is rated High with a CVSS score of 7.2. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2025-66376?
Yes. A fix has been recorded for CVE-2025-66376. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2025-66376 affect?
CVE-2025-66376 affects synacor:zimbra_collaboration_suite. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
- affected>= 10.0 and < 10.0.18
- affected>= 10.1 and < 10.1.13
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.
cisa ssvc
- exploitation
- active
- automatable
- no
- technical impact
- partial
Assessed by CISA 17 Mar 2026 (SSVC 2.0.3).
timeline
- 5 Jan 2026Published
- 17 Mar 2026CISA SSVC assessment: exploitation active · 70 days (about 2 months) later
- 18 Mar 2026Added to CISA KEV (exploited in the wild) · 1 day later
- 19 Mar 2026CVE record last updated · 1 day later
- 1 Apr 2026CISA remediation due date (US federal agencies) · 13 days later
- 2 Nov 2026Analysed by vulneraibility.com · 216 days (about 7 months) later
- Linkwiki.zimbra.com /wiki/Zimbra_Security_Advisories
- Linkwiki.zimbra.com /wiki/Security_Center
- Linkwiki.zimbra.com /wiki/Zimbra_Responsible_Disclosure_Policy
- Linkwiki.zimbra.com /wiki/Zimbra_Releases/10.1.13#Security_Fixes
- Linkwiki.zimbra.com /wiki/Zimbra_Releases/10.0.18#Security_Fixes
- Linkcisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376
sources
- CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
- NVD: CVSS and CPE data
- CISA KEV catalog: exploitation, date added, due date
- CISA Vulnrichment (ADP): SSVC decision points
- FIRST EPSS: exploit probability and percentile
- Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.