Synacor Zimbra Collaboration Suite (ZCS): code execution 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) allows an attacker to inject memcache commands into a targeted instance which causes an overwrite of arbitrary cached entries.

What an attacker can do

This is a code execution flaw. It lets an attacker run their own commands or programs on the affected system. In practice that usually means taking it over: installing malware, stealing data or using it to reach other devices on the same network.

How it is reached

Over the internet or network

Access the attacker needs

No account needed

Does the victim have to do something?

No, works without the victim doing anything

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 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 updates per vendor instructions.

Vendor advisories and fixes

• wiki.zimbra.comwiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P24.1#Security_Fixes

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on August 4, 2022, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by August 25, 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 94% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 99.8% of all scored vulnerabilities.

Its CVSS severity score is 7.5 out of 10 (high), as recorded in the US National Vulnerability Database.

Technical description

Zimbra Collaboration (aka ZCS) 8.8.15 and 9.0 allows an unauthenticated attacker to inject arbitrary memcache commands into a targeted instance. These memcache commands becomes unescaped, causing an overwrite of arbitrary cached entries.