Cisco warns customers of actively exploited zero-day in email gateways

The company confirmed the defect was exploited before it was disclosed and patched, but it did not describe the nature of the attacks or the scope of impact across its customer base.

Attackers of unknown origins and motivations are exploiting a critical zero-day vulnerability in Cisco Secure Email Gateway, authorities and researchers said Monday.

The vulnerability — CVE-2026-76461 —  was exploited before Cisco disclosed and patched the defect Monday and allows unauthenticated, remote attackers to execute commands with root privileges on vulnerable systems. “In practical terms, that gives the attacker control of the gateway itself,” Douglas McKee, director of vulnerability intelligence at Rapid7, told CyberScoop.

Cisco said its product security incident response team became aware of active exploitation of the defect affecting Cisco AsyncOS Software for Cisco Secure Email Gateway in September. When asked for further details, a company spokesperson pointed to the advisory and reiterated that the company is aware of active exploitation of the vulnerability.

The company did not say how many organizations are impacted for active exploitation thus far, but it indicated multiple customers were likely compromised prior to disclosure. 

“Cisco has conducted a thorough threat intelligence investigation on devices that belong to Cisco Secure Email Cloud. Cisco has directly contacted customers who own Cisco Secure Email Cloud devices where indicators of possible compromise were identified,” the company wrote in its security advisory. “Cisco is engaged in remediation and recovery operations. Cisco has already deployed mitigations that are within Cisco’s management.”

The Cybersecurity and Infrastructure Security Agency added the zero-day, which affects cloud-based and on-premises instances of Cisco Secure Email Gateway, to its known exploited vulnerabilities catalog shortly after Cisco’s disclosure. 

The tight timeline between Cisco’s public advisory and patch guidance, and CISA’s quick addition to the KEV catalog indicates the vulnerability deserves immediate attention, McKee said. 

“The combination here is pretty ugly. No authentication is required, an attacker can reach the vulnerable code by sending an email through the appliance, successful exploitation can result in root-level command execution, and Cisco has observed exploitation in the wild,” he added.

Researchers at Rapid7 and VulnCheck said they don’t yet know how many organizations are impacted by active exploits, but they encouraged Cisco customers to patch and hunt for potential signs of compromise as soon as possible. 

Spencer McIntyre, director of exploit development at VulnCheck, told CyberScoop the exploit could allow an attacker to maintain access to the email gateway and monitor communications. “Stealing or silently snooping on email comms is a common tactic for state-sponsored and other threat actors conducting espionage operations,” he said. 

“It’s going to be worse for organizations that have the appliance deployed on-premises. In this case, the attacker could pivot internally,” McIntyre added. “If, however, organizations use a cloud instance, the compromised gateway is less likely to have significant access to internal organizational resources.”

Cisco released indicators of compromise to help customers hunt for attempted exploitation in their environments, but the company added that attackers could remove or hide those traces with the level of access granted via exploitation.

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