Cisco Secure Firewall Adaptive Security Appliance Software DNS over TCP Implementation buffer overflow

Summaryinfo

A vulnerability was found in Cisco Secure Firewall Adaptive Security Appliance Software and Secure Firewall Threat Defense Software. It has been classified as critical. Affected by this issue is some unknown functionality of the component DNS over TCP Implementation. The manipulation leads to buffer overflow. This vulnerability is uniquely identified as CVE-2026-20248. The attack is possible to be carried out remotely. No exploit exists. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability was found in Cisco Secure Firewall Adaptive Security Appliance Software and Secure Firewall Threat Defense Software (unknown version). It has been rated as critical. This issue affects an unknown code block of the component DNS over TCP Implementation. The manipulation with an unknown input leads to a buffer overflow vulnerability. Using CWE to declare the problem leads to CWE-120. The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow. Impacted is availability. The summary by CVE is:

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.

It is possible to read the advisory at sec.cloudapps.cisco.com. The identification of this vulnerability is CVE-2026-20248 since 10/08/2025. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. The technical details are unknown and an exploit is not publicly available. The pricing for an exploit might be around USD $5k-$25k at the moment (estimation calculated on 09/16/2026).

Upgrading eliminates this vulnerability.

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Productinfo

Type

• Firewall Software

Vendor

• Cisco

Name

• Secure Firewall Adaptive Security Appliance Software
• Secure Firewall Threat Defense Software

License

• commercial

Website

• Vendor: https://www.cisco.com/

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