CVE-2026-20349: Cisco ASA and FTD Firewalls Face Remote DoS in SSL VPN, Already Exploited

Cisco has disclosed a high-severity denial-of-service vulnerability affecting its Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense products. The flaw, tracked as CVE-2026-20349, resides in the SSL VPN component and allows unauthenticated remote attackers to trigger device reboots by sending specially crafted HTTP requests. The issue also impacts FTD deployments using Zero Trust Network Access. With a CVSS v3.1 base score of 8.6, the vulnerability has already been observed in active exploitation campaigns since August 2026. Cisco released hotfixes for both affected platforms and strongly urges immediate updates, while confirming that Secure Firewall Management Center remains unaffected.

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Cisco Systems has published a security advisory detailing a denial-of-service vulnerability in its Cisco Secure Firewall Adaptive Security Appliance (ASA) and Cisco Secure Firewall Threat Defense (FTD) products. The flaw, identified as CVE-2026-20349, affects the SSL VPN functionality used for remote access.

Attackers can exploit insufficient validation of HTTP requests to send a maliciously crafted packet that forces the device to reboot without requiring authentication. This results in a complete service denial for VPN users and any connected sessions. The same issue extends to FTD configurations running Zero Trust Network Access.

The vulnerability received a CVSS v3.1 base score of 8.6 and is rated High severity. Cisco confirmed that exploitation attempts were observed in the wild during August 2026, making this a confirmed zero-day case prior to the advisory release on 11 August 2026.

Hotfixes addressing the issue are now available for both ASA and FTD. Administrators are strongly advised to apply the updates immediately. Cisco Secure Firewall Management Center (FMC) is not impacted by this vulnerability.

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