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Actively exploited vulnerability

Synacor Zimbra Collaboration Suite (ZCS): server-side request forgery flaw under attack

CVE-2019-9621 · Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery (SSRF) Vulnerability

Business software7.5 high

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 server-side request forgery (SSRF) vulnerability via the ProxyServlet component.

CISA's description

What an attacker can do

This is a server-side request forgery flaw. It lets an attacker make the server send requests on their behalf and reach internal systems that are not exposed to the internet.

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

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.

Action required by CISA for US federal agencies

Vendor advisories and fixes

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on July 7, 2025, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by July 28, 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 81% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 99.6% 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 Suite before 8.6 patch 13, 8.7.x before 8.7.11 patch 10, and 8.8.x before 8.8.10 patch 7 or 8.8.x before 8.8.11 patch 3 allows SSRF via the ProxyServlet component.

NIST National Vulnerability Database
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