Exploited vulnerabilities / CVE-2025-66376
Confirmed exploited · cross-site scripting · Synacor Zimbra Collaboration Suite (ZCS)CVE-2025-66376
Synacor Zimbra Collaboration Suite (ZCS) cross-site scripting, confirmed exploited in the wild.
Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability in the Classic UI where attackers could abuse Cascading Style Sheets (CSS) @import directives in email HTML.
| CISA's name for it | Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability |
|---|---|
| Vendor and product | Synacor · Zimbra Collaboration Suite (ZCS) |
| Type | cross-site scripting · CWE-79 |
| Exploited | Confirmed by CISA listed 18 March 2026 |
| Likelihood | one of the most likely to be exploited of every vulnerability tracked |
| Deadline | 1 April 2026 · already passed |
| Used in ransomware | Not reported |
| Advisories | wiki.zimbra.com ↗ · nvd.nist.gov ↗ |
Three steps, in this order
Apply the vendor's fix or mitigation; CISA lists only vulnerabilities seen exploited.
If you cannot patch now, restrict network access to the affected product.
Check for signs of compromise since the date it was added to the list.
| CISA's 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. |
|---|
Know before someone else finds it
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CVE-2025-66376 in short
Is CVE-2025-66376 being exploited?
Yes. CISA added CVE-2025-66376 to its Known Exploited Vulnerabilities catalogue on 18 March 2026, which it does only on evidence of exploitation in the wild.
What is the CISA deadline for CVE-2025-66376?
US federal agencies must remediate it by 1 April 2026. Everyone else should treat that date as the latest safe point to patch an internet-facing system.
How likely is CVE-2025-66376 to be exploited in the next 30 days?
FIRST's Exploit Prediction Scoring System gives it 20%, the 97th percentile of every CVE it scores.
How do I fix CVE-2025-66376?
Apply the vendor's fix or mitigation; CISA lists only vulnerabilities seen exploited. If you cannot patch now, restrict network access to the affected product. Check for signs of compromise since the date it was added to the list.
Other Synacor vulnerabilities being exploited
| CVE | Product | Type | Listed |
|---|---|---|---|
| CVE-2026-73570 | Zimbra Collaboration Suite (ZCS) | command injection | 21 August 2026 |
| CVE-2025-48700 | Zimbra Collaboration Suite (ZCS) | cross-site scripting | 20 April 2026 |
| CVE-2020-7796 | Zimbra Collaboration Suite | server-side request forgery | 17 February 2026 |
| CVE-2025-68645 | Zimbra Collaboration Suite (ZCS) | exploited vulnerability | 22 January 2026 |
Sources: CISA Known Exploited Vulnerabilities catalogue (released 10 November 2026) and FIRST EPSS. Page compiled automatically by SAK T.L.M, the threat-intelligence tool of SAK T.E.C, on 13 November 2026.