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.

Zimbra Collaboration contains a cross-site scripting (XSS) vulnerability in the CalendarInvite feature of the Zimbra webmail classic user interface. An attacker can exploit this vulnerability via an email message containing a crafted calendar header, leading to the execution of arbitrary JavaScript code.

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_Releases/8.8.15/P46#Security_Fixes
• wiki.zimbra.comwiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P39#Security_Fixes
• wiki.zimbra.comwiki.zimbra.com/wiki/Zimbra_Releases/10.0.7#Security_Fixes

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on May 19, 2025, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by June 9, 2025, 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 24% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 98% 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) 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.