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

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 available at the following link: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-tcpdns-dos-p6dUnjr5 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 . 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 . Security Impact Rating: Medium CVE: CVE-2026-20248

What to do

Install the fixed versions listed in the official advisory. If you cannot update right away, apply the workarounds or mitigations it describes and limit access to the affected service.

Details

Vendor

Cisco

Product

Cisco ASA / FTD

CVE

CVE-2026-20248

Affected versions

Cisco Secure Firewall Adaptive Security Appliance (ASA) Soft 9.16: 37 versions, from 9.16.1 to 9.16.4.92
Cisco Secure Firewall Adaptive Security Appliance (ASA) Soft 9.18: 32 versions, from 9.18.1 to 9.18.4.135
Cisco Secure Firewall Adaptive Security Appliance (ASA) Soft 9.19: 13 versions, from 9.19.1 to 9.19.1.42
Cisco Secure Firewall Adaptive Security Appliance (ASA) Soft 9.20: 24 versions, from 9.20.1 to 9.20.4.235
Cisco Secure Firewall Adaptive Security Appliance (ASA) Soft 9.23: 9 versions, from 9.23.1 to 9.23.1.195
Cisco Secure Firewall Adaptive Security Appliance (ASA) Soft 9.22: 13 versions, from 9.22.1.1 to 9.22.3.5
Cisco Secure Firewall Adaptive Security Appliance (ASA) Soft 9.24: 5 versions, from 9.24.1 to 9.24.1.155
Cisco Secure Firewall Threat Defense (FTD) Software 7.0: 17 versions, from 7.0.0 to 7.0.9
Cisco Secure Firewall Threat Defense (FTD) Software 7.2: 18 versions, from 7.2.0 to 7.2.11
Cisco Secure Firewall Threat Defense (FTD) Software 7.3: 4 versions, from 7.3.0 to 7.3.1.2
Cisco Secure Firewall Threat Defense (FTD) Software 7.4: 11 versions, from 7.4.0 to 7.4.7
Cisco Secure Firewall Threat Defense (FTD) Software 7.6: 5 versions, from 7.6.0 to 7.6.4
Cisco Secure Firewall Threat Defense (FTD) Software 7.7: 4 versions, from 7.7.0 to 7.7.11
Cisco Secure Firewall Threat Defense (FTD) Software 10.0: 10.0.0 (from the advisory and NVD; check the exact builds in the official advisory)

CVSS score

6.8

Actively exploited

Not known at this time

Source

Cisco PSIRT – original advisory

Published

September 18, 2026

The description and the recommended action are quoted from the public advisory (the action is our generic advice); severity, exploitation status and affected versions are compiled automatically from public sources and may contain errors. The vendor's original advisory prevails: check it before taking action. This information is provided without warranty and does not replace a professional assessment.