Cisco Urges Customers to Patch Email Gateway Flaw and Check for Compromise

Attackers exploit a critical Cisco email gateway flaw to gain root access. Cisco urges customers to update scanner tools and check for compromise.

Cisco confirmed this week that threat actors are exploiting a critical vulnerability in Cisco Secure Email Gateway, tracked as CVE-2026-76519 with a CVSS score of 9.8. An attacker can exploit the flaw by sending a crafted email attachment through the gateway and, if it lands right, writing arbitrary files to the underlying operating system.

“A vulnerability in the Advanced Malware Protection (AMP) feature of Cisco Secure Email Gateway could allow an unauthenticated, remote attacker to write arbitrary files on the underlying operating system.” reads the advisory. “This vulnerability is due to improper handling of email attachments when the file reputation service is enabled. An attacker could exploit this vulnerability by sending an email that contains a crafted attachment through an affected device. A successful exploit could allow the attacker to write arbitrary files and execute commands with root privileges on the underlying operating system.”

The networking giant found this one in its own internal security scanning, which is at least some comfort, and became aware of active attacks in October.

The advisory highlights that cleanest that not every Cisco email gateway is vulnerable. The File Reputation service, part of the Advanced Malware Protection feature, has to be enabled and assigned to a mail policy, and the Content Scanner Tools version has to be earlier than 25.3.0.5525. If either condition doesn’t apply, the attack path doesn’t exist.

Checking is fast. To verify File Reputation status, open the web management interface and navigate to Mail Policies, then Advanced Malware Protection for either incoming or outgoing policies. To confirm the scanner version, run contentscannerstatus at the CLI. If the output shows anything older than 25.3.0.5525, the device is exposed.

The cleanest remediation is updating the Content Scanner Tools package.

“Updating the Content Scanner Tools does not require a software upgrade or a product restart. Customers who have enabled automatic Content Scanner Tools updates are not required to take any action once the update has been applied.” continues the advisory. “Customers who have disabled automatic updates should apply the Content Scanner Tools update manually.”

Gateways with automatic updates enabled should check that the update has been installed by verifying the scanner version. If automatic updates are disabled, administrators can run contentscannerupdate from the command line to install it manually.

Cisco has also released fixes in AsyncOS versions 15.5.5-014 and 16.5.0-780. Version 16.0 has no fixed release, so users must migrate to another version. Cisco recommends upgrading any version older than 16.5 directly to 16.5.0-780. Cisco-managed Secure Email Cloud customers are already protected and don’t need to take any action.

Cisco’s compromise check is a single command: grep "System Security" system_logs. It returns one of three outputs: a confirmed compromise indicator, a scan that failed to complete, or a clean result. The last one isn’t a guarantee, though.

“Upon successful exploitation of this vulnerability, attackers may obtain command execution with root privileges. With this level of access, attackers may remove or hide evidence of exploitation and indicators of compromise.” states the advisory. “Cisco strongly recommends that cross-checking the network logs and the firewall logs outside of the impacted device to identify any potential suspicious activity, including but not limited to unexpected uploads that were initiated from the affected device to external IP addresses or downloads from malicious IP addresses.”

That means a clean log result isn’t proof the device wasn’t compromised. Cisco strongly recommends also pulling firewall and network logs from outside the appliance to look for unexpected uploads to external addresses or downloads from suspicious ones.

For virtual appliances, Cisco recommends rebuilding the system if it has been compromised. First, save forensic data, as upgrading the affected system will destroy it. Then deploy a new virtual machine with a fixed software version, rebuild the configuration from scratch without importing old configuration files, and change all credentials and cryptographic keys. Clustered systems need extra care because SSH keys are used to authenticate between members. If one system is compromised, its private keys could put the others at risk.

This is the third Cisco email gateway flaw that attackers have exploited to gain root access since December 2025. Previous incidents involved a SQL injection flaw reported in September and a zero-day in the spam quarantine feature. Cisco has now confirmed that attackers are exploiting two vulnerabilities disclosed in a September security update.

Read the full Cisco advisory for indicators of compromise and details of the fixed releases.

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Pierluigi Paganini

(SecurityAffairs – hacking, CISCO)