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

CheckOps priority index

4.3/10 Patch within 7 days

Severity6.1

Probability0.2

Exploitation3/5

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

EPSS · today 1.7% higher than 77.4% of CVEs

CVSS 3.1 6.1 AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:

Exploited for 197 days 20 Apr 2026

CISA due date 23 Apr 2026 for US federal agencies

Exploitation probability EPSS · FIRST

EPSS probability (%)Percentile among all CVEs

Exploitation signals 3 / 5

KEV catalog (CISA)20 Apr 2026
Exploited in EUVD (ENISA)20 Apr 2026
Used by ransomwareno
EPSS above 10 %1.7%
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

Description ENISA EUVD

An issue was discovered in Zimbra Collaboration (ZCS) 8.8.15 and 9.0 and 10.0 and 10.1. A Cross-Site Scripting (XSS) vulnerability in the Zimbra Classic UI allows attackers to execute arbitrary JavaScript within the user's session, potentially leading to unauthorized access to sensitive information. This issue arises from insufficient sanitization of HTML content, specifically involving crafted tag structures and attribute values that include an @import directive and other script injection vectors. The vulnerability is triggered when a user views a crafted e-mail message in the Classic UI, requiring no additional user interaction.

Affected products

Zimbra Collaboration Suite (ZCS)

Weaknesses (CWE)

CWE-79

Open source packages OSV

Affected libraries according to OSV, with fixed versions.

Timeline

23 Jun 2025
Published
20 Apr 2026
Exploitation reported (ENISA)
20 Apr 2026
Added to the KEV catalog
23 Apr 2026
CISA remediation due date

References

Other exploited vulnerabilities from the same vendor

CVE-2026-73570Zimbra Collaboration Suite (ZCS) OS Command Injection Vulnerability
8.1 · Actively exploited
CVE-2025-66376Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability
5.9 · 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