CVE-2026-0400 is a format string vulnerability classified under CWE-134 found in SonicWall's SonicOS firmware. The flaw exists in how SonicOS handles externally-controlled format strings after an attacker has authenticated to the device. Format string vulnerabilities occur when user-supplied input is improperly used as a format string parameter in functions like printf, potentially allowing attackers to manipulate memory or cause crashes. In this case, the vulnerability enables a remote attacker with valid credentials to send crafted input that causes the firewall to crash, resulting in a denial-of-service (DoS). The affected versions include SonicOS 7.0.1-5169 and older, 7.3.1-7013 and older, and 8.1.0-8017 and older. Exploitation requires authenticated access with high privileges, no user interaction is needed, and the attack vector is network-based. The CVSS v3.1 score is 4.9, reflecting a medium severity primarily due to the impact on availability without confidentiality or integrity compromise. No public exploits or active exploitation have been reported as of the publication date. The vulnerability highlights the importance of secure coding practices in handling format strings and the risks posed by post-authentication flaws in network security devices.

The primary impact of CVE-2026-0400 is denial of service, where an attacker can crash SonicWall firewalls, causing network disruption and potential downtime. This can affect organizations relying on SonicWall devices for perimeter security, VPN access, and traffic filtering. The loss of firewall availability can expose internal networks to increased risk from other threats during downtime and disrupt business operations, especially for enterprises and service providers with critical uptime requirements. Since exploitation requires authenticated access, the risk is somewhat mitigated by strong access controls, but insider threats or compromised credentials could still lead to exploitation. The vulnerability does not allow data theft or modification, but the availability impact can indirectly affect confidentiality and integrity by disabling security controls. Organizations with large deployments of affected SonicOS versions may face operational challenges and increased incident response costs if exploited.

Organizations should immediately inventory SonicWall devices to identify affected SonicOS versions and plan for timely firmware upgrades once patches are released by SonicWall. Until patches are available, restrict administrative access to trusted networks and enforce strong authentication mechanisms such as multi-factor authentication to reduce the risk of credential compromise. Monitor firewall logs for unusual post-authentication activity that could indicate exploitation attempts. Implement network segmentation to limit access to management interfaces and consider deploying intrusion detection/prevention systems to detect anomalous traffic patterns. Regularly review and update firewall configurations to minimize attack surface. Engage with SonicWall support for guidance on interim mitigations or workarounds. Finally, maintain an incident response plan to quickly recover from potential denial-of-service events caused by exploitation of this vulnerability.