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

CheckOps priority index

5.6/10 Patch within 7 days

Severity6.1

Probability2.5

Exploitation4/5

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

EPSS · today 24.6% higher than 97.9% of CVEs

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

Exploited for 533 days 19 May 2025

CISA due date 09 Jun 2025 for US federal agencies

Exploitation probability EPSS · FIRST

EPSS probability (%)Percentile among all CVEs

Exploitation signals 4 / 5

KEV catalog (CISA)19 May 2025
Exploited in EUVD (ENISA)19 May 2025
Used by ransomwareno
EPSS above 10 %24.6%
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) 9.0 and 10.0. A Cross-Site Scripting (XSS) vulnerability exists in the CalendarInvite feature of the Zimbra webmail classic user interface, because of improper input validation in the handling of the calendar header. An attacker can exploit this via an email message containing a crafted calendar header with an embedded XSS payload. When a victim views this message in the Zimbra webmail classic interface, the payload is executed in the context of the victim's session, potentially leading to execution of arbitrary JavaScript code.

Affected products

Zimbra Collaboration Suite (ZCS)

Weaknesses (CWE)

CWE-79

Open source packages OSV

Affected libraries according to OSV, with fixed versions.

Timeline

12 Aug 2024
Published
19 May 2025
Exploitation reported (ENISA)
19 May 2025
Added to the KEV catalog
09 Jun 2025
CISA remediation due date
14 Oct 2026
EPSS peak: 24.6%

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-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