CVE-2025-20191 is a high-severity vulnerability affecting multiple Cisco software products, including Cisco IOS, IOS XE, NX-OS, and AireOS used in Wireless LAN Controllers. The vulnerability arises from improper handling of DHCPv6 packets within the Switch Integrated Security Features (SISF) component. Specifically, the flaw involves a buffer access with an incorrect length value, which can be triggered by an unauthenticated attacker located on an adjacent network segment. By sending a specially crafted DHCPv6 packet, the attacker can cause the affected device to reload unexpectedly, resulting in a denial of service (DoS) condition. The vulnerability does not impact confidentiality or integrity but severely affects availability by causing device restarts. The CVSS 3.1 base score is 7.4, reflecting a high severity due to the ease of exploitation (no privileges or user interaction required) and the potential for widespread disruption. The vulnerability affects a broad range of Cisco NX-OS versions (from 7.0(3) to 10.4(3) and beyond), indicating that many deployed Cisco network devices are at risk. No known exploits are currently reported in the wild, but the vulnerability’s nature and impact make it a significant concern for network infrastructure security.

For European organizations, the impact of this vulnerability could be substantial, especially for enterprises, service providers, and government agencies relying on Cisco network infrastructure. A successful exploit could cause critical network devices such as switches and wireless controllers to reboot, leading to network outages, loss of connectivity, and disruption of business operations. This is particularly critical for sectors requiring high availability and real-time communications, such as finance, healthcare, transportation, and critical infrastructure. The DoS condition could also affect network segmentation and security controls, potentially exposing networks to further attacks during downtime. Given the widespread use of Cisco products in Europe’s enterprise and carrier networks, the vulnerability could impact a large number of organizations, causing operational and reputational damage. Additionally, the lack of authentication and user interaction requirements lowers the barrier for attackers, increasing the likelihood of exploitation in adjacent network environments such as corporate LANs or data center segments.

To mitigate this vulnerability effectively, European organizations should: 1) Prioritize patching by applying Cisco’s security updates for affected NX-OS and related software versions as soon as they become available. 2) Implement network segmentation to restrict DHCPv6 traffic only to trusted devices and interfaces, minimizing exposure to potential attackers on adjacent networks. 3) Deploy DHCPv6 filtering or rate limiting on network devices to detect and block malformed or suspicious DHCPv6 packets. 4) Monitor network traffic for unusual DHCPv6 activity and device reload events to enable rapid detection and response. 5) Use Cisco’s recommended configuration best practices for SISF and DHCPv6 handling to reduce attack surface. 6) Consider deploying intrusion detection/prevention systems (IDS/IPS) with signatures targeting DHCPv6 anomalies. 7) Conduct regular security audits and vulnerability assessments on network infrastructure to identify and remediate similar issues proactively. These measures, combined with timely patching, will reduce the risk of successful exploitation and minimize operational impact.