CVE-2026-20349

Cisco Secure Firewall Adaptive Security Appliance (ASA) and Secure Firewall Threat Defense (FTD) Heap Inspection Vulnerability

Actively exploited (CISA KEV) High 8.6 CVSS 3.1

Description

A vulnerability in the Remote Access SSL VPN service for 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 device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to insufficient error checking when processing HTTP requests. An attacker could exploit this vulnerability by sending a crafted HTTP request to the Remote Access SSL VPN service on an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a DoS condition.

Description as published by the source, in English.

Key facts

Published: 2026-08-11
Last modified: 2026-09-16
Assigned by: psirt@cisco.com
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
Scored by: psirt@cisco.com
Fix available: Not registered in the sources

Actively exploited

CISA added this vulnerability to the Known Exploited Vulnerabilities catalogue on 2026-08-11, with a remediation deadline of 2026-08-14 for US federal agencies.

Required action: Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

Affected products

Vendor	Product	Versions	Status
Cisco	Adaptive Security Appliance Software	9.16.1	Affected
Cisco	Adaptive Security Appliance Software	9.16.1.28	Affected
Cisco	Adaptive Security Appliance Software	9.16.2	Affected
Cisco	Adaptive Security Appliance Software	9.16.2.11	Affected
Cisco	Adaptive Security Appliance Software	9.16.2.13	Affected
Cisco	Adaptive Security Appliance Software	9.16.2.14	Affected
Cisco	Adaptive Security Appliance Software	9.16.2.3	Affected
Cisco	Adaptive Security Appliance Software	9.16.2.7	Affected
Cisco	Adaptive Security Appliance Software	9.16.3	Affected
Cisco	Adaptive Security Appliance Software	9.16.3.14	Affected
Cisco	Adaptive Security Appliance Software	9.16.3.15	Affected
Cisco	Adaptive Security Appliance Software	9.16.3.19	Affected
Cisco	Adaptive Security Appliance Software	9.16.3.23	Affected
Cisco	Adaptive Security Appliance Software	9.16.3.3	Affected
Cisco	Adaptive Security Appliance Software	9.16.4	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.14	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.19	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.27	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.38	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.39	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.42	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.48	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.55	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.57	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.61	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.62	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.67	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.70	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.71	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.76	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.82	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.84	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.85	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.89	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.9	Affected
Cisco	Adaptive Security Appliance Software	9.16.4.92	Affected
Cisco	Adaptive Security Appliance Software	9.18.1	Affected
Cisco	Adaptive Security Appliance Software	9.18.1.3	Affected
Cisco	Adaptive Security Appliance Software	9.18.2	Affected
Cisco	Adaptive Security Appliance Software	9.18.2.5	Affected
Cisco	Adaptive Security Appliance Software	9.18.2.7	Affected
Cisco	Adaptive Security Appliance Software	9.18.2.8	Affected
Cisco	Adaptive Security Appliance Software	9.18.3	Affected
Cisco	Adaptive Security Appliance Software	9.18.3.39	Affected
Cisco	Adaptive Security Appliance Software	9.18.3.46	Affected
Cisco	Adaptive Security Appliance Software	9.18.3.53	Affected
Cisco	Adaptive Security Appliance Software	9.18.3.55	Affected
Cisco	Adaptive Security Appliance Software	9.18.3.56	Affected
Cisco	Adaptive Security Appliance Software	9.18.4	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.135	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.22	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.24	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.29	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.34	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.40	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.47	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.5	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.50	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.52	Affected
Cisco	Adaptive Security Appliance Software	9.18.4.53	Affected

Weakness type

CWE-244: Improper Clearing of Heap Memory Before Release ('Heap Inspection')
Using realloc() to resize buffers that store sensitive information can leave the sensitive information exposed to attack, because it is not removed from memory.

References

https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-as… Vendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-20349 US Government Resource

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CVE

What it is: A reference number naming one specific vulnerability, for example CVE-2024-9474.
What it tells you: Which vulnerability we are talking about, and in which product.
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Used with the others: The number is the key. NVD, KEV and EPSS all describe the same CVE.

NVD CWE CISA KEV EPSS

How to read this together

No single value should be used on its own. A high CVSS score does not mean a vulnerability is being exploited, and a low one does not mean it is harmless. The four sources answer four different questions.

CVE names the specific vulnerability.
NVD adds severity and technical detail about it.
CWE describes the type of mistake behind it.
CISA KEV shows that it is already being exploited.
EPSS estimates how likely exploitation is soon.

The order that decides what is urgent

Do you actually have the product?
Is the version you run affected?
Is it listed in CISA KEV?
Does it have a high EPSS score?
Is the system reachable from the internet or from others?
How important is the system to the business?
Are there compensating controls already in place?

The first two questions are not about the vulnerability at all. They are about you, and they remove most of the list before you start.

A short example

Fix this week

CVSS 6,5 (medium)

In CISA KEV: yes
EPSS: 94 per cent
A firewall facing the internet

Can be scheduled

CVSS 9,8 (critical)

In CISA KEV: no
EPSS: 0,04 per cent
A test server with no internet access

Sorted by CVSS alone, the one on the right comes first. It is the more serious flaw in theory. But the one on the left is being used against real organisations right now, and it sits where an attacker can reach it. That is the one to fix first.

The critical one still has to be fixed. It just does not have to be fixed today.

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