Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability

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.

CheckOps priority index

5.9/10 Patch within 7 days

Severity7.2

Probability2.0

Exploitation4/5

Severity = CVSS. Probability = EPSS (FIRST). Exploitation = public signals: KEV, EUVD, ransomware, EPSS above 10 %, no account required.

EPSS · today 20.2% higher than 97.5% of CVEs

CVSS 3.1 7.2 AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:

Exploited for 230 days 18 Mar 2026

CISA due date 01 Apr 2026 for US federal agencies

Exploitation probability EPSS · FIRST

EPSS probability (%)Percentile among all CVEs

Exploitation signals 4 / 5

KEV catalog (CISA)18 Mar 2026
Exploited in EUVD (ENISA)18 Mar 2026
Used by ransomwareno
EPSS above 10 %20.2%
No account, over the networkyes

What to do

Fix

Apply the vendor’s fix as soon as it is available.

CISA guidance: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Meanwhile

Take the service off the internet or restrict access to it, watch the logs and look for signs of compromise.

Manufacturers (CRA)

Exploited vulnerability in your product: early warning to ENISA within 24 h, then notification within 72 h (Article 14 of the Cyber Resilience Act). The CRA explained

Affected products

CollaborationZimbra Collaboration Suite (ZCS)

Affected versions

10.1 <10.1.13
10.0 <10.0.18

Weaknesses (CWE)

CWE-79

Open source packages OSV

Affected libraries according to OSV, with fixed versions.

Timeline

05 Jan 2026
Published
18 Mar 2026
Exploitation reported (ENISA)
18 Mar 2026
Added to the KEV catalog
01 Apr 2026
CISA remediation due date
02 Oct 2026
EPSS peak: 20.2%

References

Other exploited vulnerabilities from the same vendor

CVE-2026-73570Zimbra Collaboration Suite (ZCS) OS Command Injection Vulnerability
8.1 · Actively exploited
CVE-2025-48700Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
4.3 · Background exploitation
CVE-2020-7796Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery Vulnerability
9.1 · Background exploitation
CVE-2025-68645Synacor Zimbra Collaboration Suite (ZCS) PHP Remote File Inclusion Vulnerability
7.4 · Background exploitation
CVE-2025-27915Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
3.5 · Background exploitation
CVE-2019-9621Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery (SSRF) Vulnerability
7.8 · Background exploitation