Synacor · Zimbra Collaboration Suite (ZCS) · Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability
Verdict
Patch first · actively exploited, reachable from the internet without credentials, and in the top 5% of CVEs by near-term exploitation probability
Added to CISA KEV 2025-05-19 · exploited in the wild for 496 days
CVSS 6.1 MEDIUM. The score is a summary; these three components are the part that decides whether something scanning the internet can use it.
| Attack vector | networkreachable over the network, with no local access needed |
|---|---|
| Privileges required | noneneeds no credentials at all |
| User interaction | requiredneeds a user to click, load or submit something first |
| Base vector | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N |
| Weakness | CWE-79 |
This describes the vulnerability. It says nothing about whether your systems run it.
EPSS puts the probability of exploitation in the next 30 days at 0.2363, which places it in the top 5% of all tracked CVEs. The percentile is the more useful half: 0.18 in absolute terms sounds low until you know most of the internet-facing software in the world scores under that.
| EPSS probability | 0.2363 |
|---|---|
| Percentile | 0.977 — top 5% |
| Added to KEV | 2025-05-19 |
|---|---|
| Days exploited | 496 |
| Federal remediation due | not set |
| Ransomware use | no known ransomware campaign use |
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
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.