CISA Warns of Zimbra Collaboration Suite (ZCS) Vulnerability Exploited in Attacks

CISA has issued an urgent warning regarding a critical vulnerability in Synacor’s Zimbra Collaboration Suite (ZCS) that is being actively exploited in cyberattacks. 

The vulnerability, tracked as CVE-2019-9621, poses significant risks to organizations using the popular email and collaboration platform.

Key Takeaways
1. CISA alerts on an SSRF flaw (CVE-2019-9621) in Zimbra ZCS, actively exploited by attackers.
2. Flaw allows unauthorized access to sensitive internal or cloud data via ProxyServlet.
3. Urgent fixes or product discontinuation required by July 28, 2025.
4. Follow Zimbra advisories and CISA guidance to protect systems.

Zimbra SSRF Vulnerability (CVE-2019-9621)

The vulnerability centers on a server-side request forgery (SSRF) flaw within the ProxyServlet component of Zimbra Collaboration Suite. 

This security weakness allows attackers to manipulate the server into making unauthorized requests to internal or external resources, potentially exposing sensitive data and compromising network security. 

The vulnerability has been classified under CWE-918 (Server-Side Request Forgery) and CWE-807 (Reliance on Untrusted Inputs in a Security Decision), indicating the severity of the trust boundary violations involved.

CISA added this vulnerability to its Known Exploited Vulnerabilities (KEV) catalog on July 7, 2025, signaling active exploitation in the wild. 

The agency’s decision to include CVE-2019-9621 in the KEV catalog reflects credible evidence that threat actors are leveraging this vulnerability to compromise targeted systems. 

While the connection to ransomware campaigns remains unknown, the SSRF nature of the vulnerability makes it particularly attractive to attackers seeking to establish initial footholds in enterprise environments.

Technical analysis reveals that the ProxyServlet component vulnerability allows malicious actors to craft specially designed requests that bypass security controls and access internal services. 

Through SSRF exploitation, attackers can potentially scan internal networks, access metadata services, and interact with backend systems that should be protected from external access. 

This type of vulnerability is particularly dangerous in cloud environments where metadata services often contain sensitive authentication tokens and configuration data.

The vulnerability’s classification under CWE-918 highlights how attackers can abuse the server’s functionality to make requests on their behalf, effectively using the compromised system as a proxy to reach otherwise inaccessible resources. 

Mitigations

CISA has established a compliance deadline of July 28, 2025, requiring federal agencies to implement necessary mitigations or discontinue use of affected Zimbra systems. 

Organizations are directed to apply vendor-provided mitigations immediately and follow applicable BOD 22-01 guidance for cloud services. 

For systems where effective mitigations are unavailable, CISA recommends discontinuing use of the product entirely.

System administrators should consult Zimbra’s official security advisories and the National Vulnerability Database for comprehensive remediation guidance. 

Organizations using Zimbra Collaboration Suite must prioritize immediate assessment and remediation efforts to prevent potential compromise through this actively exploited vulnerability.

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