CVE-2026-20135 in Secure Firewall Threat Defense
Summary
by MITRE • 09/16/2026
A vulnerability in the TLS 1.3 implementation in Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition.
This vulnerability is due to improper buffer management during the TLS 1.3 connection. An attacker could exploit this vulnerability by sending a crafted TLS 1.3 packet to an affected system through a TLS 1.3-enabled listening socket. A successful exploit could allow the attacker to cause the LINA process to crash, which would cause the device to reload. The reload can happen before or after authentication of the connection.Note: TLS 1.3 connections include both data traffic and user-management traffic.
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Analysis
by VulDB Data Team • 09/16/2026
The Cisco Secure Firewall Threat Defense software contains a critical vulnerability within its TLS 1.3 implementation that poses a significant risk to network infrastructure stability. This flaw stems from improper buffer management during the handling of TLS 1.3 connection parameters, specifically affecting how memory is allocated and processed when receiving incoming packets. The technical root cause involves an error in validating or managing input data structures associated with the handshake process, leading to a state where the system fails to correctly handle crafted network traffic. This type of defect typically manifests as a buffer overflow or a logic error that corrupts internal memory states, ultimately triggering a fault within the core processing engine known as LINA. The presence of this vulnerability allows an unauthenticated remote attacker to interact with any TLS 1.3-enabled listening socket on the affected device without needing valid credentials first.
The operational impact of exploiting this flaw is severe and immediate, resulting in a denial of service condition for the firewall appliance itself rather than just the specific application layer services it protects. By sending specifically crafted TLS 1.3 packets to the target system, an attacker can trigger a crash in the LINA process. Because LINA is responsible for handling control plane functions including management traffic and data plane interactions related to security policies, its failure forces the entire device to reload unexpectedly. This reboot occurs regardless of whether the connection has been authenticated or not, meaning that even unauthenticated attempts are sufficient to disrupt service. The consequence is a complete interruption of network connectivity through the firewall, affecting both user-management traffic used for administrative access and data traffic passing through the security appliance, thereby creating a blind spot in the organization's defense perimeter during the downtime period.
From a classification perspective, this vulnerability aligns with CWE-120 Buffer Copy without Checking Size of Input Classic buffer overflow issues or potentially CWE-400 Uncontrolled Resource Consumption if the issue relates to memory exhaustion due to malformed inputs. In terms of adversary behavior as defined by MITRE ATT&CK, this exploit technique falls under T1498 Network Denial of Service, specifically reflecting indirect network service disruption via exploitation of software vulnerabilities. The ability to cause a device reload without authentication highlights a critical failure in input validation and state management within the TLS stack implementation. Such flaws are particularly dangerous because they do not require privilege escalation or prior access, making them attractive for automated scanning tools used by threat actors seeking to disrupt services quickly.
Mitigation strategies must focus on immediate patching of the Cisco Secure Firewall Threat Defense software to a version that addresses this buffer management flaw. Until patches can be applied, network administrators should consider implementing strict rate limiting and deep packet inspection at upstream perimeter devices to identify and block malformed TLS 1.3 packets before they reach the vulnerable firewall interface. Additionally, restricting access to management interfaces via IP allow-lists can reduce the attack surface for unauthenticated exploitation attempts. It is also advisable to monitor system logs for unexpected reload events or LINA process restarts as an indicator of potential compromise activity targeting this specific vulnerability. Regular updates and adherence to vendor security advisories remain the most effective defense against such implementation-level flaws in cryptographic protocols.