CVE-2026-20082: 
Cisco Adaptive Security Appliance (ASA) vulnerability analysis and mitigation

CVE-2026-20082 is a denial-of-service (DoS) vulnerability in Cisco Secure Firewall Adaptive Security Appliance (ASA) Software caused by improper handling of embryonic connection limits during TCP SYN flood attacks. It was first published on March 4, 2026, as part of Cisco's March 2026 Semiannual Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software Security Advisory Bundled Publication. The vulnerability exclusively affects ASA Software Release 9.20.4.14, regardless of device configuration; FMC and FTD software are not affected. It carries a CVSS v3.1 base score of 8.6 (High) (Cisco Advisory).

The root cause is classified as CWE-772 (Missing Release of Resource after Effective Lifetime), specifically in the ASA's embryonic connection limit handling logic. When the device is subjected to a TCP SYN flood targeting management or data interfaces, it incorrectly drops legitimate incoming TCP SYN packets due to improper resource management of half-open (embryonic) connections. No authentication or special privileges are required; an unauthenticated remote attacker only needs network access to an affected interface to trigger the condition. The vulnerability was discovered internally during the resolution of a Cisco TAC support case, and no public PoC or exploit code has been disclosed (Cisco Advisory).

Successful exploitation causes a complete denial of service for all TCP-based traffic to the affected ASA device, including remote management access, Remote Access VPN (RAVPN) connections, and any other TCP-based network protocols. The scope is marked as "Changed" in the CVSS vector, reflecting that the impact extends beyond the vulnerable component itself to dependent services and users relying on the firewall. There is no confidentiality or integrity impact, but the availability impact is rated High, as all incoming TCP connections to the device can be prevented from being established (Cisco Advisory).

As of the advisory publication date, Cisco PSIRT is not aware of any public announcements or malicious use of this vulnerability in the wild. No exploit code or proof-of-concept has been publicly released. The EPSS score is approximately 0.0014 (0.14%), indicating a low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Detection plugins are available via Qualys (ID: 317777) and Nessus (ID: 311447) (Cisco Advisory, Feedly).

1. Reconnaissance: Identify internet-facing Cisco ASA devices running Software Release 9.20.4.14 using network scanning tools (e.g., Shodan, Censys) or banner grabbing against management and data interfaces.
2. Target selection: Confirm the target is running the specific vulnerable release (9.20.4.14), as only this version is affected regardless of configuration.
3. Launch TCP SYN flood: Send a high-volume stream of crafted TCP SYN packets to the management or data interfaces of the target ASA device using tools such as hping3, scapy, or purpose-built flood utilities.
4. Trigger embryonic connection limit mishandling: The flood causes the ASA to exhaust its embryonic connection tracking resources, leading it to incorrectly drop legitimate incoming TCP SYN packets.
5. Achieve DoS condition: All new TCP connections to the device — including RAVPN, SSH management, and other TCP-based services — are blocked, resulting in a denial of service for all affected features (Cisco Advisory).

• Network: Sudden spike in TCP SYN packets directed at ASA management or data interfaces from one or multiple source IPs; sustained high-volume half-open connection attempts visible in NetFlow or firewall logs.
• Logs: ASA syslog messages indicating embryonic connection limit thresholds being reached or exceeded; log entries showing legitimate TCP connections being dropped unexpectedly (e.g., %ASA-6-302013 or %ASA-6-302014 anomalies, or connection denied messages for known-good sources).
• Availability: Loss of remote management access (SSH/HTTPS) to the ASA console; RAVPN client connection failures reported by end users; TCP-based monitoring tools reporting the ASA as unreachable (Cisco Advisory).

Cisco has released ASA Software Release 9.20.4.19 as the fixed version for this vulnerability. There are no workarounds available; upgrading to the fixed release is the only remediation. Organizations running ASA Software Release 9.20.4.14 should prioritize upgrading to 9.20.4.19 immediately. Customers without a Cisco service contract should contact the Cisco TAC with their product serial number to obtain the fixed software (Cisco Advisory).

The vulnerability was covered by The Hacker Wire, which published an article on the Cisco ASA DoS via embryonic connection limit bypass (The Hacker Wire). The Center for Internet Security (CIS) included it in their advisory on multiple vulnerabilities in Cisco products (CIS Advisory). Belgium's Centre for Cybersecurity (CCB) issued a warning about multiple critical vulnerabilities in Cisco products including this one (CCB Advisory). Network World also reported on Cisco's emergency patches for critical firewall vulnerabilities in the same bundled publication (Network World).