CVE-2019-9621

What is Zimbra Collaboration Suite?

Zimbra Collaboration Suite (ZCS) is an email and collaboration platform (webmail, calendaring, contacts) used as an on-premises alternative to hosted email services by enterprises, government agencies, and service providers worldwide — making it a frequent target for both opportunistic and targeted email-focused attacks.

Overview

CVE-2019-9621 is a server-side request forgery (SSRF) vulnerability (CWE-918) in Zimbra's ProxyServlet component, allowing an unauthenticated remote attacker to make the Zimbra server issue arbitrary outbound or internal requests. It is rated high severity (CVSS 7.5).

Technical Details

The ProxyServlet forwards requests on behalf of the Zimbra server without adequately validating or restricting the destination, letting an attacker coerce the server into making requests to internal-only services or other locations the attacker could not otherwise reach directly. On its own, SSRF is primarily an information-disclosure and internal-network reconnaissance tool, but in Zimbra's case it has been used as a stepping stone toward more damaging follow-on exploitation.

Discovery

Reported to Synacor (Zimbra's maintainer) and documented in Zimbra's public security advisory channel; NVD does not credit an individual external researcher.

Exploitation Context

This SSRF has been observed used in the wild in combination with Zimbra's XXE vulnerability (CVE-2019-9670) as part of multi-stage attack chains against internet-facing Zimbra webmail servers, giving attackers a path from an unauthenticated network request to remote code execution and mailbox compromise. CISA added this CVE to KEV in July 2025, reflecting continued real-world exploitation years after initial disclosure — consistent with the long tail of unpatched, internet-exposed Zimbra installations.

Remediation

1. Update Zimbra Collaboration Suite to the latest patched version.
2. Restrict network access to the Zimbra administration and proxy interfaces where possible.
3. Treat this vulnerability as part of a chain — ensure the related XXE (CVE-2019-9670) is also fully patched, since attackers have combined the two.
4. Review Zimbra logs for anomalous internal or outbound requests initiated by the proxy component.
5. Deploy network egress filtering to limit what internal resources a compromised Zimbra server could reach via SSRF.