Cisco email security boxes can be rooted by... an email

Criminals are exploiting a critical Cisco Secure Email Gateway flaw that can turn a malicious email into root access. The vulnerability, tracked as CVE-2026-76461, carries a 9.8 CVSS score and affects physical and virtual Secure Email Gateway appliances regardless of their configuration. Cisco says there are no workarounds, so patching is the only fix. The bug lies in how Cisco's AsyncOS software handles incoming email. An attacker doesn't need to log in: they can send a booby-trapped message through a vulnerable gateway and, if the exploit works, run commands as root. Which is not exactly what you want from the box tasked with keeping nasty emails out. Cisco's Product Security Incident Response Team said it became aware of active exploitation in September, although the networking giant hasn't said who is behind the attacks, how long they have been going on, or how many organizations have been compromised. Cisco uncovered the bug while resolving a Technical Assistance Center support case. Signs suggest at least some Cisco cloud customers were caught up in the attacks. Cisco said it investigated devices belonging to its Secure Email Cloud service and directly contacted customers whose appliances showed indicators of possible compromise. It is now carrying out remediation and recovery work and says all of its Secure Email Cloud devices have been upgraded to AsyncOS 16.5.0-780. Admins running their own appliances have a little more work to do. Cisco recommends checking logs for signs of suspicious activity, but warns that finding nothing doesn't necessarily mean the system is clean. Once attackers have root access, Cisco says they could tamper with the logs and cover their tracks. Admins are also being told to check network and firewall logs for anything unusual, rather than relying on the gateway itself for answers. For virtual appliances suspected of being compromised, Cisco's recovery advice is fairly drastic: preserve the forensic evidence, deploy a fresh VM running fixed software, rebuild the configuration, and rotate credentials and cryptographic material. Cisco has fixed the bug in AsyncOS releases 15.5.5-014, 16.0.4-302 and 16.5.0-780, with customers strongly encouraged to move to the latter. There's still a decent-sized target pool out there. The Shadowserver Foundation was tracking more than 400 Cisco Secure Email Gateway appliances exposed to the internet as of Monday The flaw has also landed in CISA's Known Exploited Vulnerabilities catalog, with US federal civilian agencies ordered to remediate it by September 17. CVE-2026-76461 comes less than a year after attackers exploited another critical AsyncOS flaw, CVE-2025-20393, to break into Cisco Secure Email Gateway appliances and install persistence mechanisms. That bug eventually scored a perfect 10. For anyone still running an affected gateway, the message is fairly simple: the box designed to inspect hostile email can itself be pwned by one; attackers are already doing it, and there is no workaround to hide behind. ®