VulnerabilityPublished 4 Mar 2026

CVE-2026-20039: Cisco Secure Firewall Adaptive Security Appliance (ASA) Software · Heap Inspection

Affects Cisco Secure Firewall Adaptive Security Appliance (ASA) SoftwareSource CVE Database V5

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Not known to be exploited · CVSS 8.6 High.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 3.1 base score

8.6High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.3% chance of exploitation in 30 days – higher than 31% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Changed
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 4 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
CiscoCisco Secure Firewall Adaptive Security Appliance (ASA) Software	Affected 9.12.1 – 9.23.1 142 affected builds	Fixed in–
CiscoCisco Secure Firewall Threat Defense (FTD) Software	Affected 6.4.0 – 7.7.0 68 affected builds	Fixed in–

AI-suggested

Apply Cisco's official patches and software updates for ASA and FTD devices as soon as they become available to address the heap memory management flaw.
Restrict access to the VPN web server interface by implementing strict firewall rules, limiting exposure to trusted IP addresses or VPN users only.
Employ network segmentation to isolate management and VPN interfaces from general network traffic, reducing attack surface.
Monitor network traffic for unusual patterns of HTTP requests targeting the VPN web server, using intrusion detection/prevention systems (IDS/IPS) and security information and event management (SIEM) tools.
Implement rate limiting or connection throttling on the VPN web server to mitigate the impact of crafted request floods.
Regularly audit and review firewall and VPN configurations to ensure minimal exposure and adherence to security best practices.
Prepare incident response plans to quickly identify and recover from potential DoS attacks targeting this vulnerability.
Engage with Cisco support and subscribe to security advisories to stay informed about patches and mitigation guidance.

Description

A vulnerability in the VPN web server 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 a denial of service (DoS) condition on an affected device. This vulnerability is due to ineffective memory management of the VPN web server. An attacker could exploit this vulnerability by sending a large number of crafted HTTP requests to an affected device. A successful exploit could allow the attacker to cause the device to reload, resulting in a DoS condition.

Impact

The primary impact of CVE-2026-20039 is denial of service, which can cause Cisco ASA and FTD devices to reload unexpectedly, leading to temporary loss of firewall and VPN services. This disruption can affect network perimeter security, remote access capabilities, and overall business continuity. Organizations relying heavily on Cisco ASA/FTD for secure VPN access and firewall protection may experience outages, potentially impacting critical operations, especially in sectors like finance, healthcare, government, and telecommunications. The vulnerability does not directly compromise data confidentiality or integrity but severely affects availability, which can be exploited to cause operational downtime or as part of a larger attack strategy to distract or disable defenses. The ease of exploitation—requiring no authentication or user interaction—means attackers can launch attacks remotely over the network, increasing the threat surface. Given the widespread deployment of Cisco ASA/FTD devices globally, the potential scale of impact is significant. However, the absence of known exploits in the wild currently limits immediate widespread exploitation. Still, the vulnerability represents a high risk for targeted attacks against organizations with exposed VPN web servers.

References

sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-vpn-dos-SpOFF2Re (opens in a new tab)

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 11 Mar 2026, 19:46 UTC

Technical details

References

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