⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 4.3

How to Fix CVE-2026-20021: Cisco Secure Firewall Adaptive Security Appliance (ASA) Software (Bundle Sibling)

By Sai Kiran Pandrala. Last verified: 2026-05-25.

CVE-2026-20021 is a sibling vulnerability in the same vendor advisory as CVE-2026-20020. Apply the same patched build and you close both. The technical detail below is what differs.

⚡ At a glance
Severity4.3 (Medium)
Actively exploited?No public listing in CISA KEV
AffectedCisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.1.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.1.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2.5; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2.9; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.3.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.3.7; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.3.12; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.3.9; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.7; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.10; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.13; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.8; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.18; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.24; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.26; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.29; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.30; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.35; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.37; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.38; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.39; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.40; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.41; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.47; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.48; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.50; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.52; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.54; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.55; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.56; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.58; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.62; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.65; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.4.67; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.1.28; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.7; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.11; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.13; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.2.14; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.14; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.15; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.19; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.3.23; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.9; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.14; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.19; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.27; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.38; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.39; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.42; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.48; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.55; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.57; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.61; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.62; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.67; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.70; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.71; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.76; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.82; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.84; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.16.4.85; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.7; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.9; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.10; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.11; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.13; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.15; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.20; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.30; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.33; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.39; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.45; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.17.1.46; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.1.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.2.5; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.2.7; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.2.8; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.3.39; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.3.46; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.3.53; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.3.55; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.3.56; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.5; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.8; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.22; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.24; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.29; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.34; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.40; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.47; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.50; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.52; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.53; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.57; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.66; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.67; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.18.4.68; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.5; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.9; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.12; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.18; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.22; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.24; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.27; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.28; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.31; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.37; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.38; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.19.1.42; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.1.5; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.2.10; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.2.21; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.2.22; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.3.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.3.7; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.3.9; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.3.10; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.3.13; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.3.16; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.3.20; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.4.7; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.4.10; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.20.4.14; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.1.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.1.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.1.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.1.6; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.2.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.2.9; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.2.13; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.22.2.14; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.23.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.23.1.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.23.1.7; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.23.1.13; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.23.1.19; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.23.1.22; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.1; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.3; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.2; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.4; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.5; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.6; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.7; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.8; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.9; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.10; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.11; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.12; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.13; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.14; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.15; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.16; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.17; Cisco Secure Firewall Threat Defense (FTD) Software 6.4.0.18; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.0; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.0.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.1.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.2.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.3; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.4; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.5; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.6; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.6.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.6.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.6.3; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.7; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.8; Cisco Secure Firewall Threat Defense (FTD) Software 7.0.8.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.1.0; Cisco Secure Firewall Threat Defense (FTD) Software 7.1.0.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.1.0.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.1.0.3; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.0; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.0.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.3; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.4; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.4.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.5; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.5.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.6; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.7; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.5.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.8; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.8.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.9; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.10; Cisco Secure Firewall Threat Defense (FTD) Software 7.2.10.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.3.0; Cisco Secure Firewall Threat Defense (FTD) Software 7.3.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.3.1.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.3.1.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.0; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.1.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.2.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.2.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.2.3; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.2.4; Cisco Secure Firewall Threat Defense (FTD) Software 7.4.3; Cisco Secure Firewall Threat Defense (FTD) Software 7.6.0; Cisco Secure Firewall Threat Defense (FTD) Software 7.6.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.6.2; Cisco Secure Firewall Threat Defense (FTD) Software 7.6.2.1; Cisco Secure Firewall Threat Defense (FTD) Software 7.7.0; Cisco Secure Firewall Threat Defense (FTD) Software 7.7.10; Cisco Secure Firewall Threat Defense (FTD) Software 7.7.10.1
Fixed inSame patched build as CVE-2026-20020
Type (CWE)CWE-401: Missing Release of Memory after Effective Lifetime

What's different about CVE-2026-20021?

A vulnerability in the OSPF protocol of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software could allow an authenticated, adjacent attacker to exhaust memory on an affected device, resulting in a denial of service (DoS) condition.

This vulnerability is due to improperly validating input by the OSPF protocol when parsing packets. An attacker could exploit this vulnerability by by sending crafted OSPF packets to an affected device. A successful exploit could allow the attacker to exhaust memory on the affected device, resulting in a DoS condition.

How to fix CVE-2026-20021

Apply the patched build per the primary write-up: How to Fix CVE-2026-20020. All commands, verification steps, and rollback notes for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software are listed there.

Frequently asked questions

Does the CVE-2026-20020 patch close CVE-2026-20021?

Yes. Both CVEs are addressed by the same vendor patch. Applying the patched build closes the full bundle.

Is CVE-2026-20021 listed in CISA KEV?

No public KEV listing at the time of this writing.

Where is the official advisory?

See https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-ospf-ZH8PhbSW

References

*Written by Sai Kiran Pandrala. Part of the Cisco Secure Firewall Adaptive Security Appliance (ASA) Software bundle. Full procedure at how-to-fix-cve-2026-20020.*