cisco-sa-asaftd-tcpdns-dos-p6dUnjr5MediumCVSS 6.8

Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software TCP DNS Denial of Service Vulnerability

Published
September 16, 2026
Last Modified
September 18, 2026

🔗 CVE IDs covered (1)

CVE-2026-20248 →

📋 Description

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.

Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.

This advisory is part of a group of advisories. For a complete list of the advisories and links to them, see Cisco Advance Notification for Publication of September 16, 2026, Security Advisories ["https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-notice-jfxK98ZP"]. In addition, for further documentation of improvements and fixes in Cisco Secure Firewall products, see Cisco Secure Firewall Adaptive Security Appliance Software, Secure Firewall Threat Defense Software, and Secure Firewall Management Center Software Hardening Release: September 2026 ["https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-hardening-asaftdfmc-uvpPROhN"].

CVE-2026-20248 — Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software TCP DNS Denial of Service Vulnerability (CVSS 3.1 6.8, CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H)

🎯 Affected products208

  • Cisco 3000 Series Industrial Security Appliances (ISA)
  • Cisco ASA 5500-X Series Firewalls
  • Cisco Adaptive Security Virtual Appliance (ASAv)
  • Cisco Firepower 1000 Series
  • Cisco Firepower 2100 Series
  • Cisco Firepower 4100 Series
  • Cisco Firepower 9000 Series
  • Cisco Secure Firewall 1200 Series
  • Cisco Secure Firewall 200 Series
  • Cisco Secure Firewall 3100 Series
  • Cisco Secure Firewall 4200 Series
  • Cisco Secure Firewall 6100 Series
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
  • Cisco Secure Firewall Threat Defense (FTD) Software
  • Cisco Secure Firewall Threat Defense Virtual
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.1
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.1.28
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.11
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.13
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.14
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.3
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.7
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.14
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.15
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.19
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.23
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.3
  • Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4
  • +178 more not shown

✅ Remediation

Cisco has released software updates that address this vulnerability. https://software.cisco.com Workarounds: There are no workarounds that address this vulnerability.

🔗 References (7)