Synacor · Zimbra Collaboration Suite (ZCS)
CVE-2025-66376
Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability
What it is
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.
CISA: 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.
What to do
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Deadline for US federal agencies: April 1, 2026
- Added to the CISA list
- March 18, 2026
- Published
- January 5, 2026
- Severity
- High 7.2 CVSS 3.1
- Chance of attacks in the next 30 days (EPSS)
- 20% in the top 2.5% of all known vulnerabilities
- Weakness
- CWE-79
- More details
- NVD · CVE.org · CISA
All references (5)
More exploited flaws from Synacor
| Added | Vulnerability | Product | Severity | Attack chance |
|---|---|---|---|---|
| Aug 21, 2026 | OS Command Injection | Zimbra Collaboration Suite (ZCS) | High 8.9 | 72% |
| Apr 20, 2026 | Cross-site Scripting | Zimbra Collaboration Suite (ZCS) | Medium 6.1 | 1.7% |
| Oct 7, 2025 | Cross-site Scripting | Zimbra Collaboration Suite (ZCS) | Medium 5.4 | 4.1% |
| Jul 7, 2025 | Server-Side Request Forgery (SSRF) | Zimbra Collaboration Suite (ZCS) | High 7.5 | 81% |
| May 19, 2025 | Cross-Site Scripting (XSS) | Zimbra Collaboration Suite (ZCS) | Medium 6.1 | 25% |
| Feb 25, 2025 | Cross-Site Scripting (XSS) | Zimbra Collaboration Suite (ZCS) | Critical 9.0 | 77% |
| Oct 3, 2024 | Command Execution | Zimbra Collaboration Suite (ZCS) | Critical 10.0 | 99.9% |
| Jul 27, 2023 | Cross-Site Scripting (XSS) | Zimbra Collaboration Suite (ZCS) | Medium 6.1 | 49% |
Sources: Known Exploited Vulnerabilities (CISA, public domain), CVE records (cve.org) and EPSS (FIRST). The chance of attacks is the EPSS estimate that someone will try to exploit the flaw in the next 30 days.