Synacor · Zimbra Collaboration Suite (ZCS) · Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability
Verdict
Patch soon · actively exploited, reachable from the internet without credentials, and in the top 25% of CVEs by near-term exploitation probability
Added to CISA KEV 2026-04-20 · exploited in the wild for 160 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 0.0171, which places it in the top 25% 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 | 0.0171 |
|---|---|
| Percentile | 0.765 — top 25% |
| Added to KEV | 2026-04-20 |
|---|---|
| Days exploited | 160 |
| 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) 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.