CVE-2026-20135: Cisco Secure Firewall Threat Defense Software TLS 1.3 Denial of Service Vulnerability

Quick Answer

Cisco published a High-rated advisory that includes CVE-2026-20135, affecting Cisco Secure Firewall ASA / FTD under the live advisory's conditions. Cisco does not separately state a CVE-specific Security Impact Rating (SIR); Common Vulnerability Scoring System (CVSS) base 8.6 (High). Confirm the exact platform, release, feature, and exposure; preserve relevant evidence; then follow Cisco's CVE-specific First Fixed guidance. Cisco reported no public announcement or malicious use for this CVE at the last check. That status and absence from CISA KEV are time-bounded and do not prove exploitation has never occurred.

Last verified: 2026-09-17 UTC from Cisco's advisory and CSAF record. Recheck both the affected-products and fixed-software sections before a production change.

Context

Status At A Glance

Product scope	Cisco Secure Firewall ASA / FTD
CVE	CVE-2026-20135
TechGeeks response tier	P1 — urgent
Publication	New final Cisco advisory
First published	2026-09-16T16:00:00+00:00
Last updated	2026-09-16T16:00:00+00:00
Cisco advisory SIR	High
CVE-specific Cisco SIR	Not separately stated
CVSS base score / severity	8.6 — High
CVSS vector	CVSS v3.1 — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
CISA KEV at last check	No — this is separate from Cisco severity and is not proof of no exploitation.
Cisco exploitation statement	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.

What Happened

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. Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability. This advisory is part of a group of advisories. For a complete list of the advisories and links to them, see Cisco Advance Notification for Publication of September 16, 2026, Security Advisories. In addition, for further documentation of improvements and fixes in Cisco Secure Firewall products, see Cisco Secure Firewall Adaptive Security Appliance Software, Secure Firewall Threat Defense Software, and Secure Firewall Management Center Software Hardening Release: September 2026.

Affected Scope

This vulnerability affects Cisco devices if they are running a vulnerable release of Cisco Secure FTD Software and have an SSL listening socket that is configured to allow TLS 1.3 connections. For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory. Determine Whether a Device Can Process TLS Packets To determine whether a device that is running Cisco Secure FTD Software can process TLS packets, use the show asp table socket | include SSL command and look for an SSL listening socket on any TCP port. The following example shows the output for SSL listening sockets on TCP ports 443 and 8443: ftd# show asp table socket | include SSL SSL 00185038 LISTEN 172.16.0.250:443 0.0.0.0:* SSL 00188638 LISTEN 10.0.0.250:8443 0.0.0.0:* Determine TLS Versions in Software Configuration To determine whether a device that is running Cisco Secure FTD Software is configured for TLS 1.3, use the show running-config all ssl CLI command. The output will show the maximum TLS version configured. If the version found is TLS 1.3, as shown in the following example, the device may be affected by this vulnerability: ftd# show running-config all ssl ssl server-max-version tlsv1.3 Another way to check the TLS 1.3 configuration on Cisco Secure FTD Software is to use the following steps on Cisco Secure Firewall Management Center (FMC) Software: Log in to Cisco Secure FMC Software. From the menu bar, choose Devices > Platform Settings. Add or edit the Platform Settings Policy that is applied to the affected device. From the navigation menu on the left, choose SSL. From the TLS version drop-down menu, see if TLSv1.3 is selected.

Match the exact hardware or virtual platform, installed train, patch level, enabled service, role, and management or data-plane exposure. A Cisco product-family name alone does not establish applicability.

Steps

What To Do Now

1. Confirm. Inventory Cisco Secure Firewall ASA / FTD and record platform, exact release, patch level, enabled feature, role, and exposure.
2. Preserve. Save UTC context plus off-device authentication, management, firewall, network, endpoint, and SIEM evidence before logs rotate or a disruptive change removes context.
3. Scope. Compare each asset with the live Cisco advisory, including configuration prerequisites and exclusions.
4. Reduce exposure. Evaluate and apply Cisco's temporary mitigation in the highlighted panel below; it does not replace the permanent fix.
5. Remediate. Confirm the affected product, release, and configuration. Apply Cisco's documented mitigation as temporary risk reduction. Enter the exact installed product, platform, and release in Cisco Software Checker and upgrade to the returned First Fixed release. The CVE-specific live advisory and Cisco Software Checker result control the First Fixed release; any combined hardening baseline is supplemental only.
6. Validate. Confirm the resulting version, service health, logging, expected access, an expected denial, failover or cluster state, and representative dependent workflows.
7. Escalate. If evidence or exposure creates concern, open incident response and Cisco TAC recovery work rather than treating patch completion as proof of trust.

Fixed Releases And Mitigation

Investigation And Recovery

Review management authentication, account and privilege changes, API and web requests, configuration changes, crashes or restarts, resource pressure, and relevant external network telemetry for the vulnerable period. Preserve original evidence and follow Cisco TAC or incident-response guidance for containment, credential rotation, trusted rebuild, or service restoration when findings warrant it.

Notes

Exploitation Evidence

Cisco's exact status at the last check: 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.

This CVE was not present in the TechGeeks CISA KEV snapshot at the last check. Cisco's 'not aware' statement and absence from KEV are time-bounded evidence, not claims that exploitation is impossible or has never happened. Severity and exploitation status remain separate signals.

Validation And Boundaries

This notice is documentation-backed. TechGeeks did not reproduce the exploit or independently test every fixed build. It does not prove that all members of the product family are affected, that the activity is widespread, that a clean indicator search excludes compromise, or that an update restored trust. Cisco's live advisory controls affected and fixed release information.

Related TechGeeks Resources

• TechGeeks Security Notice Center
• All Security Notices
• What to Do When a Device Hits CISA KEV
• Router CVE Exposure, Version, and Mitigation Checklist
• IoT Firmware Update and End-of-Support Checklist
• Live Protect and Runtime Vulnerability Shielding

References

• Cisco security advisory
• Cisco CSAF record
• Cisco September 16 publication notice
• NVD identifier record

Wrap-Up

Record whether CVE-2026-20135 applies. If it does, assign an owner and deadline appropriate to the High Cisco advisory SIR, preserve the evidence, complete Cisco's remediation, and document the validated result.

Correction policy: A material Cisco revision to scope, severity, exploitation status, workarounds, fixed software, or recovery guidance triggers an update and a new verification date.