--- id: CVE-2026-20248 title: >- 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 T… summary: >- 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 T… severity: medium cvss: 6.8 cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H' cwe: - CWE-195 vendor: Cisco product: Cisco Secure Firewall Adaptive Security Appliance (ASA) Software affected: - secure_firewall_adaptive_security_appliance_asa_software 9.16.1 - secure_firewall_adaptive_security_appliance_asa_software 9.16.1.28 - secure_firewall_adaptive_security_appliance_asa_software 9.16.2 - secure_firewall_adaptive_security_appliance_asa_software 9.16.2.3 - secure_firewall_adaptive_security_appliance_asa_software 9.16.2.7 - secure_firewall_adaptive_security_appliance_asa_software 9.16.2.11 - secure_firewall_adaptive_security_appliance_asa_software 9.16.2.13 - secure_firewall_adaptive_security_appliance_asa_software 9.16.2.14 - secure_firewall_adaptive_security_appliance_asa_software 9.16.3.3 - secure_firewall_adaptive_security_appliance_asa_software 9.16.3 - secure_firewall_adaptive_security_appliance_asa_software 9.16.3.14 - secure_firewall_adaptive_security_appliance_asa_software 9.18.1 - secure_firewall_adaptive_security_appliance_asa_software 9.16.3.15 - secure_firewall_adaptive_security_appliance_asa_software 9.18.1.3 - secure_firewall_adaptive_security_appliance_asa_software 9.18.2 - secure_firewall_adaptive_security_appliance_asa_software 9.16.3.19 - secure_firewall_adaptive_security_appliance_asa_software 9.16.3.23 - secure_firewall_adaptive_security_appliance_asa_software 9.18.2.5 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4 - secure_firewall_adaptive_security_appliance_asa_software 9.18.2.7 - secure_firewall_adaptive_security_appliance_asa_software 9.19.1 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.9 - secure_firewall_adaptive_security_appliance_asa_software 9.18.2.8 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.14 - secure_firewall_adaptive_security_appliance_asa_software 9.18.3 - secure_firewall_adaptive_security_appliance_asa_software 9.19.1.5 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.18 - secure_firewall_adaptive_security_appliance_asa_software 9.19.1.9 - secure_firewall_adaptive_security_appliance_asa_software 9.18.3.39 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.19 - secure_firewall_adaptive_security_appliance_asa_software 9.19.1.12 - secure_firewall_adaptive_security_appliance_asa_software 9.18.3.46 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.27 - secure_firewall_adaptive_security_appliance_asa_software 9.19.1.18 - secure_firewall_adaptive_security_appliance_asa_software 9.18.3.53 - secure_firewall_adaptive_security_appliance_asa_software 9.18.3.55 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.38 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.39 - secure_firewall_adaptive_security_appliance_asa_software 9.18.3.56 - secure_firewall_adaptive_security_appliance_asa_software 9.20.1 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.42 - secure_firewall_adaptive_security_appliance_asa_software 9.19.1.22 - secure_firewall_adaptive_security_appliance_asa_software 9.18.4 - secure_firewall_adaptive_security_appliance_asa_software 9.20.1.5 - secure_firewall_adaptive_security_appliance_asa_software 9.18.4.5 - secure_firewall_adaptive_security_appliance_asa_software 9.19.1.24 - secure_firewall_adaptive_security_appliance_asa_software 9.16.4.48 - secure_firewall_adaptive_security_appliance_asa_software 9.18.4.8 - secure_firewall_adaptive_security_appliance_asa_software 9.20.2 - secure_firewall_adaptive_security_appliance_asa_software 9.19.1.27 published: '2026-09-16' updated: '2026-09-18' sourceUpdated: '2026-09-18T13:28:28.567' source: NVD sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-20248' references: - url: >- https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-tcpdns-dos-p6dUnjr5 label: psirt@cisco.com - url: 'https://software.cisco.com' tags: - nvd - cve.org - csaf - vendor-advisory - cisco epss: 0.0034 epssPercentile: 0.24631 ssvc: exploitation: none automatable: 'no' technicalImpact: partial timestamp: '2026-09-17T13:32:41.718147Z' ingestedAt: '2026-09-16T16:37:52.166Z' --- ## Overview 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. ## Remediation Refer to the linked advisories for vendor-supplied fixes and affected version ranges. ## Vendor advisories - **cisco-sa-asaftd-tcpdns-dos-p6dUnjr5** · Cisco · affected: Cisco Firepower 2100 Series, Cisco Firepower 1000 Series, Cisco ASA 5500-X Series Firewalls, Cisco 3000 Series Industrial Security Appliances (ISA), Cisco Firepower 9000 Series, Cisco Firepower 4100 Series, … · updated 2026-09-18 · [advisory](https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-tcpdns-dos-p6dUnjr5)