CVE-2026-20248: Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software TCP DNS Denial of Service Vulnerability
A vulnerability in the DNS over TCP implementation of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause the TCP DNS response handler to unexpectedly restart, causing the device to reload. This vulnerability is due to a logic error when parsing a DNS query and tracking the size of the incoming buffers. An attacker could exploit this vulnerability by formatting a crafted reply to a DNS query sent from the targeted device. A successful exploit could allow the attacker to cause the device to reload, causing a denial of service (DoS) condition. Note: The attacker must be able to respond to DNS queries from the device, either by controlling the DNS service or through a machine-in-the-middle attack.
Affected Software
Event History
Frequently Asked Questions
What access or position does an attacker need to exploit this issue?
The attacker must be able to send a crafted reply to a DNS query originating from the affected device. This requires control of the DNS service used by the device or a machine-in-the-middle position that allows the attacker to respond to its DNS queries.
What is the operational impact of successful exploitation?
Successful exploitation causes the TCP DNS response handler to restart unexpectedly, which reloads the affected device. The resulting impact is a denial-of-service condition; no confidentiality or integrity impact is described.
Are devices affected merely by receiving inbound DNS traffic?
The described attack path depends on the device sending a DNS query and then receiving an attacker-crafted response over TCP. The provided information does not describe exploitation through unsolicited inbound DNS requests.