CVE-2026-20248

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

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Published Sep 16, 2026 CVSS 3.1 · 6.8 Medium EPSS < 1% KEV · Not listed
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&nbsp;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

No solution or workaround provided in the CVE record

Tracking

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Advisories

No advisories available
History
Type Values Removed Values Added
First Time appeared Cisco
Cisco adaptive Security Appliance Software
Cisco secure Firewall Threat Defense
Vendors & Products Cisco
Cisco adaptive Security Appliance Software
Cisco secure Firewall Threat Defense
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}
Type Values Removed Values Added
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&nbsp;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.
Title Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software TCP DNS Denial of Service Vulnerability
Weaknesses CWE-195
References
Metrics cvssV3_1

{'score': 6.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H'}
Weaknesses

Sources

Status
PUBLISHED
Assigner
cisco
Published
Updated
Sep 18, 2026, 7:08 PM
Reserved
Oct 8, 2025, 11:59 AM

View source JSON

Updated
Sep 17, 2026, 1:36 PM

JSON

Status
Awaiting Analysis
Published
Sep 16, 2026, 9:17 PM
Modified
Sep 18, 2026, 1:28 PM

View source JSON

No data.

Updated
Sep 18, 2026, 9:51 PM

JSON