A vulnerability in the email message filtering feature of Cisco AsyncOS Software for Cisco Email Security Appliances (ESA) could allow an unauthenticated, remote attacker to cause the CPU utilization to increase to 100 percent, causing a denial of service (DoS) condition on an affected device. The vulnerability is due to improper filtering of email messages that contain references to whitelisted URLs. An attacker could exploit this vulnerability by sending a malicious email message that contains a large number of whitelisting URLs. Cisco has released free software updates that address this vulnerability.”]
Source: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20190109-esa-url-dos