Cisco Security Advisories guidance
Vendor revision: Sep 18, 2026Cisco has released software updates that address this vulnerability.
Overview
What is CVE-2026-20248?
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.
Source: NVD
Official advisory
Configuration and update guidance
Cisco Security Advisories · Updated Sep 18, 2026
Affected products and configuration
At the time of publication, this vulnerability affected Cisco Secure Firewall ASA Software and Secure FTD Software, regardless of device configuration.
For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.
Workarounds and temporary mitigations
There are no workarounds that address this vulnerability.
Fixed software
Cisco considers any workarounds and mitigations (if applicable) to be temporary solutions until an upgrade to a fixed software release is available. To remediate this vulnerability and avoid future exposure as described in this advisory, Cisco strongly recommends that customers upgrade to the fixed software indicated in this advisory.
Fixed Releases
In the following table, the left column lists Cisco software releases. The center column lists hot fixes that are affected by the vulnerabilities that are described in this advisory, and the right column indicates the first software release that includes the fix for these vulnerabilities. Customers who installed one of the listed hot fix releases must install the listed first fixed release.
Cisco Secure Firewall ASA Software Release | Affected Hot Fix Release | First Fixed Release |
|---|---|---|
9.16 and earlier | N/A | 9.16.4.103 |
9.18 | 9.18.4.135 | 9.18.4.94 |
9.20 | 9.20.4.235 | 9.20.4.49 |
9.22 | 9.22.3.191 | 9.22.3.26 |
9.23 | 9.23.1.195 9.23.1.211 | 9.23.1.47 |
9.24 | 9.24.1.155 9.24.1.221 | 9.24.1.26 |
In the following table, the left column lists Cisco software releases. The right column indicates whether a release is affected by the vulnerabilities that are described in this advisory and the first release that includes the fix for these vulnerabilities.
Cisco Secure FTD Software Release and Secure FMC Software Release | First Fixed Release |
|---|---|
7.0 and earlier | 7.0.10 |
7.2 | 7.2.12 |
7.4 | 7.4.8 |
7.6 | 7.6.6 |
7.7 | 7.7.13 |
10.0 | 10.0.2 |
10.1 | 10.1.0 |
Cisco strongly recommends that customers upgrade to the first fixed releases in the preceding tables because they include fixes for multiple vulnerabilities. For more information, see Cisco Secure Firewall Adaptive Security Appliance, Secure Firewall Threat Defense, and Secure Firewall Management Center Software Hardening Release: September 2026.
Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software
To help customers determine their exposure to vulnerabilities in Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software, Cisco provides the Cisco Software Checker. This tool identifies any Cisco security advisories that impact a specific software release and the earliest release that fixes the vulnerabilities that are described in each advisory (“First Fixed”).
If applicable, the tool also returns the earliest release that fixes all the vulnerabilities that are described in all the advisories that the Software Checker identifies (“Combined First Fixed”).
To use the tool, go to the Cisco Software Checker page and follow the instructions. Alternatively, use the following form to search for vulnerabilities that affect a specific software release. To use the form, follow these steps:
Choose which advisories the tool will search-all advisories, only advisories with a Critical or High Security Impact Rating (SIR), or only this advisory.
Choose the appropriate software.
Choose the appropriate platform.
Enter a release number-for example, 9.20.3.4 for Cisco Secure Firewall ASA Software or 7.4.2 for Cisco Secure FTD Software.
Click Check.
Additional Resources
For help determining the best Cisco Secure Firewall ASA, Secure FMC, or Secure FTD Software release, see the following Recommended Releases documents. If a security advisory recommends a later release, Cisco recommends following the advisory guidance.
Cisco Secure Firewall ASA Compatibility
Cisco Secure Firewall ASA Upgrade Guide
Cisco Secure Firewall Threat Defense Compatibility Guide
Exploitation status
The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.