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

What is affected

Synacor Zimbra Collaborate 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 (XSS) vulnerability in the Calendar feature that allows an attacker to execute arbitrary 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

CISA records this vulnerability as known to be used in ransomware campaigns: attackers use it to get in, then encrypt files and demand payment.

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 Collaborate 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 updates per vendor instructions.

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on February 25, 2022, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by March 11, 2022, 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 31% 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 the Calendar feature in Zimbra Collaboration Suite 8.8.x before 8.8.15 patch 30 (update 1), as exploited in the wild starting in December 2021. An attacker could place HTML containing executable JavaScript inside element attributes. This markup becomes unescaped, causing arbitrary markup to be injected into the document.