CyberSecBulletin
CVE-2026-20039NVD status: Analyzed

Cisco Secure Firewall Adaptive Security…: Improper Clearing of Heap Memory Before Release

Published Updated By the CyberSecBulletin desk

CVE-2026-20039 is a high-severity improper clearing of heap memory before release vulnerability in Cisco Secure Firewall Adaptive Security Appliance (ASA) Software from Cisco, with a CVSS 3.1 base score of 8.6 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected Cisco Secure Firewall Adaptive Security Appliance (ASA) Software versions: 9.12.3; 9.12.1; 9.12.2, and others listed below. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.41% chance of exploitation within 30 days, which is low compared with other CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Adaptive Security Appliance Software to 9.16.4.84 or 9.18.4.57 or 9.20.3.16 or 9.22.2.4, whichever matches your release line, or a later version.
  2. Update Secure Firewall Threat Defense to 7.0.9 or 7.2.10 or 7.4.3 or 7.6.1, whichever matches your release line, or a later version.
  3. Read the advisory on sec.cloudapps.cisco.com for workarounds and exact fixed builds.
  4. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
CiscoCisco Secure Firewall Adaptive Security Appliance (ASA) Software9.12.3; 9.12.1; 9.12.2; 9.12.4; 9.12.3.2; 9.12.3.7; 9.12.2.5; 9.12.3.12; and 32 more rangesNot stated
CiscoCisco Secure Firewall Threat Defense (FTD) Software6.4.0.1; 6.4.0.2; 6.4.0.5; 6.4.0; 6.4.0.3; 6.4.0.4; 6.4.0.6; 6.4.0.7; and 32 more rangesNot stated
CiscoAdaptive Security Appliance Software9.12.1 up to, but not including, 9.16.4.84; 9.17.1 up to, but not including, 9.18.4.57; 9.19.1 up to, but not including, 9.20.3.16; 9.22.1.1 up to, but not including, 9.22.2.4; 9.23.1 up to, but not including, 9.23.1.39.16.4.84, 9.18.4.57, 9.20.3.16, 9.22.2.4, 9.23.1.3
CiscoSecure Firewall Threat Defense6.4.0 up to, but not including, 7.0.9; 7.1.0 up to, but not including, 7.2.10; 7.3.0 up to, but not including, 7.4.3; 7.6.0 up to, but not including, 7.6.1; 7.7.0 up to, but not including, 7.7.107.0.9, 7.2.10, 7.4.3, 7.6.1, 7.7.10

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
ChangedThe attack can reach beyond the vulnerable component into other systems.
Confidentiality
NoneNo data is disclosed.
Integrity
NoneNothing can be modified.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H. Scored by CNA (cisco.com).

Official 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.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Improper Clearing of Heap Memory Before Release

The product uses realloc() to resize buffers that contain sensitive information.

More Improper Clearing of Heap Memory Before Release vulnerabilities (CWE-244)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Other vulnerabilities in Cisco Secure Firewall Adaptive Security Appliance (ASA) Software

All Cisco Secure Firewall Adaptive Security Appliance (ASA) Software vulnerabilities

Recent Improper Clearing of Heap Memory Before Release vulnerabilities

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