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Security researchers have uncovered active exploitation of a critical zero-day vulnerability, CVE-2025-20393, affecting Cisco Secure Email Gateway (SEG) and Cisco Secure Email and Web Manager (SEWM) devices. At least 120 internet-exposed devices have been confirmed as vulnerable, according to intelligence, out of more than 650 exposed Cisco email security appliances identified online. The lack of an available patch significantly increases risk, especially as these systems are widely deployed to defend organizations against phishing, malware, and other email-based threats.

The vulnerability directly targets Cisco’s email security infrastructure, which inspects inbound and outbound email traffic within enterprise environments. Although Cisco has withheld detailed technical exploitation information to prevent mass abuse, the confirmation of in-the-wild exploitation indicates that threat actors are already weaponizing the flaw. This raises serious concerns that attackers could bypass email security controls, compromise gateway devices, or leverage them as entry points into internal networks.

Cisco has officially acknowledged CVE-2025-20393 and issued a security advisory, urging customers to take immediate defensive action. While no permanent fix or patch timeline has been announced, Cisco recommends reviewing security configurations and implementing temporary mitigations outlined in its advisory. These mitigations are currently the only line of defense for affected organizations and are critical for reducing exposure during this active exploitation phase.

The incident highlights the broader challenge of zero-day vulnerabilities in critical infrastructure, particularly systems positioned at key network choke points like email gateways. A successful compromise could allow attackers to intercept sensitive communications, distribute malware or ransomware, and establish persistent access within enterprise environments. Security teams are strongly advised to monitor for suspicious activity, restrict unnecessary external access to affected devices, and apply all recommended countermeasures until an official patch becomes available.