Zimbra Collaboration Suite (ZCS) OS Command Injection Vulnerability

What it is

Zimbra Collaboration Suite (ZCS) contains an OS command injection vulnerability which could allow an unauthenticated attacker to send specially crafted SMTP requests that may result in execution of arbitrary operating system commands as the Zimbra user.

Who's affected

Anyone using Synacor Zimbra Collaboration Suite (ZCS).

Under attack?

Yes. CISA added this to its catalog of actively exploited vulnerabilities on Aug 21, 2026.

What to do

Install the vendor's security update. If there isn't one yet, apply the vendor's workaround, or stop using the product until it's fixed. CISA lists this as actively exploited, so treat it as urgent.

Fixed in

See the vendor's advisory

CISA deadline

Federal agencies must fix this by Aug 24, 2026. Everyone else should treat that as a ceiling, not a target.

CISA's instruction

Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.