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Is your network safe from the new Cisco AsyncOS zero-day vulnerability without a patch available?
Critical Advisory: Active Exploitation of Cisco AsyncOS Zero-Day (CVE-2025-20393)
Security administrators utilizing Cisco AsyncOS-based email solutions must immediately address a critical vulnerability currently under active exploitation. Cisco has confirmed a zero-day flaw, designated CVE-2025-20393, which carries the maximum severity CVSS score of 10.0.
The Threat Landscape
This vulnerability affects the Cisco Secure Email Gateway and the Cisco Secure Email and Web Manager (formerly the Content Security Management Appliance). The flaw allows remote attackers to execute arbitrary commands with full root privileges on the underlying operating system.
Current intelligence indicates that the Chinese Advanced Persistent Threat (APT) group, identified by Cisco Talos as UAT-9686, is orchestrating these attacks. The group has been actively targeting specific devices since at least November 2025.
Technical Mechanism and Exposure
For an adversary to successfully exploit this vulnerability, specific conditions must be met:
- The device must be running a vulnerable version of Cisco AsyncOS.
- The Spam Quarantine feature must be enabled.
- The Spam Quarantine interface must be accessible via the public internet.
The German Federal Office for Information Security (BSI) notes that exposing the Spam Quarantine interface to the internet is not a standard configuration. However, organizations that have modified their network architecture to allow external access to this feature are currently at high risk.
Incident Timeline and Impact
- November 2025: Initial attacks documented by security researchers.
- December 10, 2025: Cisco identifies the specific campaign targeting devices with open ports.
- December 17, 2025: Cisco publishes advisory [CIS25a], and the BSI issues a warning (Alert Level Yellow).
Forensic investigations reveal that UAT-9686 establishes persistence immediately upon compromise. The attackers deploy malware that allows them to maintain control over the device even after the initial intrusion. This persistence mechanism poses a significant challenge for remediation, as simple reboots or configuration changes may not remove the adversary’s foothold.
Immediate Action Required
As of December 19, 2025, no official patch is available to remediate CVE-2025-20393.
Administrators must prioritize mitigation over remediation. The primary defense is attack surface reduction. You must verify if the Spam Quarantine interface on your Cisco Secure Email Gateway is reachable from the internet. If it is, you must restrict access immediately. Access should be limited to trusted internal networks or management VPNs only.
Furthermore, security teams should review logs for unusual activity dating back to November 2025, specifically looking for unauthorized command execution or unexpected outbound traffic which may indicate the presence of the persistence mechanism described by Cisco Talos.