Synacor Zimbra Collaboration Suite (ZCS): cross-site scripting flaw under attack

What is affected

Synacor Zimbra Collaboration Suite (ZCS) is mostly run by companies, schools and public bodies, not on home computers. You are unlikely to have it yourself, but organisations that hold your data may. Breaches that start with flaws like this one are how personal data ends up leaked.

Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability that could allow attackers to execute arbitrary JavaScript within the user's session, potentially leading to unauthorized access to sensitive information.

What an attacker can do

This is a cross-site scripting flaw. It lets an attacker plant a script that runs in the browser of anyone who opens the affected page, which lets them steal that person's session or act on their behalf.

How it is reached

Over the internet or network

Access the attacker needs

No account needed

Does the victim have to do something?

Yes, the victim must open or click something

What to do

1. At home: nothing to install. Keep your own devices updated and use a different password for every service, so that a breach at one organisation does not open your other accounts.
2. If you administer Synacor Zimbra Collaboration Suite (ZCS) at work: apply the vendor's fix or mitigation now and review logs for signs of compromise, because the flaw was being exploited before it was listed.
3. If the product is past its support date and no fix exists, take it off the internet or retire it.

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

Vendor advisories and fixes

• wiki.zimbra.comwiki.zimbra.com/wiki/Zimbra_Security_Advisories

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on April 20, 2026, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by April 23, 2026, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.

The EPSS model estimates a 1.7% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 77% of all scored vulnerabilities.

Its CVSS severity score is 6.1 out of 10 (medium), as recorded in the US National Vulnerability Database.

Technical description

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.