CVE-2026-20248 in Secure Firewall Adaptive Security Appliance Software
Summary
by MITRE • 09/16/2026
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.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in Cisco Secure Firewall Adaptive Security Appliance and Threat Defense software represents a critical denial of service risk stemming from flawed logic within the Domain Name System over TCP implementation. Specifically, the defect resides in how the device handles DNS responses received via TCP connections. The core technical flaw involves an error in parsing incoming DNS queries while simultaneously tracking the size of associated buffers. This logical inconsistency allows for a state mismatch where the system fails to correctly validate or manage buffer boundaries during the processing of specific crafted replies. When an attacker submits a specially formatted response that exploits this discrepancy, it triggers an unexpected restart of the TCP DNS response handler within the operating environment.
The operational impact of this vulnerability is severe and immediate, resulting in a complete device reload. Because the firewall appliance reboots to recover from the internal error state caused by the malformed input, all network traffic passing through the device is interrupted during the reboot process. This constitutes a significant denial of service condition that can disrupt critical business operations dependent on continuous connectivity and security policy enforcement. The vulnerability does not allow for arbitrary code execution or data exfiltration but serves as an effective tool for attackers seeking to degrade availability by forcing repeated reboots, thereby exhausting administrative resources needed to restore services and potentially masking other malicious activities during the downtime periods.
Exploitation of this flaw requires specific network positioning conditions that limit its applicability in some scenarios but make it highly dangerous when those conditions are met. The attacker must possess the ability to respond directly to DNS queries initiated by the targeted firewall device. This typically implies that the adversary controls a legitimate DNS server or has achieved a man-in-the-middle position within the network path between the firewall and its configured upstream DNS resolvers. By intercepting or controlling these responses, an unauthenticated remote attacker can inject the crafted payload designed to trigger the buffer tracking logic error. The requirement for bidirectional communication capability means that this vulnerability is most pertinent in environments where internal threats exist or where network segmentation has been compromised allowing lateral movement and traffic interception capabilities.
From a classification perspective, this issue aligns with CWE-20 Improper Input Validation, as the system fails to adequately sanitize or verify the structure of incoming DNS data before processing it into memory buffers. Furthermore, in terms of tactical behavior within an attack lifecycle, this vulnerability facilitates actions consistent with ATT&CK technique T1498 Network Denial of Service, specifically through resource exhaustion via service disruption rather than bandwidth saturation. The mechanism relies on crashing a specific subsystem to force a full system restart, which is a classic availability-focused exploitation pattern targeting network infrastructure devices.
Mitigation strategies should prioritize immediate patching with the latest software versions released by Cisco that address this logic error in the DNS over TCP handler. Until patches are applied, organizations can implement mitigating controls at the perimeter or internal segmentation layers to restrict access to the firewall management interfaces and limit which hosts can initiate DNS queries through the appliance if possible. Additionally, deploying intrusion prevention systems with signatures capable of detecting anomalous DNS response sizes or malformed packet structures may help block exploit attempts before they reach the vulnerable component. Network administrators should also monitor for unexpected device reboots as an indicator of compromise, ensuring that logging and alerting mechanisms are configured to detect such events rapidly so that incident response teams can investigate potential exploitation activity promptly.