Synacor Zimbra Collaboration Suite (ZCS): file access 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 PHP remote file inclusion vulnerability that could allow for remote attackers to craft requests to the /h/rest endpoint to influence internal request dispatching, allowing inclusion of arbitrary files from the WebRoot directory.

What an attacker can do

This is a file access flaw. It lets an attacker read or write files they should never reach, such as configuration files that hold passwords, or upload a malicious file that the system then runs.

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/Security_Center

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on January 22, 2026, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by February 12, 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 49% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 99% of all scored vulnerabilities.

Its CVSS severity score is 8.8 out of 10 (high), as recorded in the US National Vulnerability Database.

Technical description

A Local File Inclusion (LFI) vulnerability exists in the Webmail Classic UI of Zimbra Collaboration (ZCS) 10.0 and 10.1 because of improper handling of user-supplied request parameters in the RestFilter servlet. An unauthenticated remote attacker can craft requests to the /h/rest endpoint to influence internal request dispatching, allowing inclusion of arbitrary files from the WebRoot directory.