CVE-2025-20343 is a vulnerability identified in Cisco Identity Services Engine (ISE) software versions 3.4.0 through 3.4 Patch 3, related to the RADIUS setting that rejects requests from clients with repeated failures. The flaw stems from an incorrect comparison logic when processing RADIUS access requests for MAC addresses already flagged as rejected endpoints. An unauthenticated, remote attacker can exploit this by sending a specific sequence of crafted RADIUS access request messages targeting these rejected MAC addresses. This triggers a logic error that causes the Cisco ISE to restart unexpectedly, leading to a denial of service (DoS) condition. The vulnerability affects availability but does not compromise confidentiality or integrity. Exploitation requires no privileges or user interaction and can be performed remotely over the network, increasing the attack surface. Although no known exploits have been reported in the wild, the high CVSS score of 8.6 reflects the potential impact and ease of exploitation. Cisco ISE is widely used for network access control, authentication, and policy enforcement, making this vulnerability significant for organizations relying on it to secure their networks. The vulnerability could disrupt network access services, causing operational downtime and potential security policy enforcement gaps.

For European organizations, the impact of CVE-2025-20343 is primarily a denial of service against Cisco ISE, which is critical for network access control and authentication. A successful exploit can cause Cisco ISE to restart repeatedly, disrupting authentication services and potentially allowing unauthorized network access or causing legitimate users to lose connectivity. This can affect enterprise networks, government agencies, and critical infrastructure sectors that depend on Cisco ISE for enforcing security policies. The resulting downtime can lead to operational disruptions, increased risk of security breaches due to lapses in access control, and potential regulatory compliance issues under frameworks like GDPR if network security is compromised. The lack of confidentiality or integrity impact limits data breach risks, but availability loss in security infrastructure is a significant concern. Organizations with large-scale deployments of Cisco ISE or those in sectors requiring high availability and strict access control are at elevated risk.

1. Apply Cisco's official patches or updates for Cisco ISE versions 3.4.0 through 3.4 Patch 3 as soon as they become available to address the logic error in RADIUS request processing. 2. Monitor RADIUS traffic for unusual patterns, such as repeated access requests from the same MAC addresses that are already rejected, which could indicate exploitation attempts. 3. Implement network segmentation to isolate Cisco ISE servers from untrusted or less secure network segments, reducing exposure to remote attacks. 4. Employ rate limiting or filtering on RADIUS requests to mitigate the impact of repeated crafted requests targeting rejected endpoints. 5. Maintain robust logging and alerting on Cisco ISE to detect unexpected restarts or service disruptions promptly. 6. Conduct regular security assessments and penetration testing focusing on network access control infrastructure to identify potential weaknesses. 7. Educate network administrators about this vulnerability and ensure incident response plans include steps for handling Cisco ISE service disruptions.