CVE-2025-20343 is a vulnerability identified in Cisco Identity Services Engine (ISE) software versions 3.4.0 through 3.4 Patch 3, specifically related to the RADIUS setting that rejects requests from clients exhibiting repeated failures. The root cause is a logic error in how Cisco ISE processes RADIUS access requests for MAC addresses that have already been rejected. An attacker can exploit this flaw by sending a carefully crafted sequence of RADIUS access request messages targeting these rejected endpoints. This triggers an unexpected restart of the Cisco ISE service, causing a denial of service (DoS) condition. The vulnerability is exploitable remotely without requiring any authentication or user interaction, increasing its risk profile. The CVSS v3.1 score of 8.6 (AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H) reflects that the attack vector is network-based, with low attack complexity, no privileges or user interaction needed, and a high impact on availability with a scope change. While confidentiality and integrity remain unaffected, the availability impact can disrupt network access control and policy enforcement, potentially affecting enterprise security posture. No public exploits have been reported yet, but the vulnerability's nature suggests it could be weaponized in targeted attacks or automated scanning campaigns. Cisco has not yet published patches or mitigation details, so organizations must monitor advisories closely.

For European organizations, the impact of CVE-2025-20343 can be significant, especially those relying heavily on Cisco ISE for network access control, authentication, and policy enforcement. A successful exploit causes the Cisco ISE service to restart unexpectedly, leading to temporary denial of service. This disruption can result in loss of network access control, potentially allowing unauthorized devices to connect or legitimate users to be denied access, impacting business continuity. Critical sectors such as finance, healthcare, government, and telecommunications that depend on robust network security and continuous availability are particularly vulnerable. The downtime caused by repeated restarts could also increase operational costs and complicate incident response efforts. Additionally, the scope change in the CVSS score indicates that the vulnerability could affect multiple components or connected systems, amplifying the disruption. Given the remote, unauthenticated nature of the exploit, attackers could launch DoS attacks from outside the network perimeter, increasing the threat surface for European enterprises.

1. Monitor Cisco security advisories closely for official patches addressing CVE-2025-20343 and apply them promptly once available. 2. In the interim, implement network-level filtering to restrict RADIUS traffic to trusted sources only, minimizing exposure to crafted malicious requests. 3. Enable detailed logging and monitoring of RADIUS access requests to detect unusual patterns indicative of exploitation attempts, such as repeated requests from the same MAC addresses. 4. Segment the network to isolate Cisco ISE servers from untrusted or less secure network segments, reducing the attack surface. 5. Consider deploying rate limiting or anomaly detection on RADIUS traffic to prevent flooding with crafted requests. 6. Review and tighten RADIUS client configurations to ensure only authorized devices can communicate with Cisco ISE. 7. Prepare incident response plans specifically for potential DoS scenarios affecting network access control infrastructure. 8. Engage with Cisco support for any available workarounds or temporary configuration changes that can mitigate the vulnerability until patches are released.