CVE Alert: CVE-2025-20343 – Cisco – Cisco Identity Services Engine Software

**Risk verdict** High risk DoS vulnerability in Cisco ISE Radius handling could trigger an unexpected restart and service disruption; there is no publicly observed exploitation reported yet, but the exposure is real for networks relying on ISE for RADIUS.

**Why this matters** An attacker could disrupt network access and NAC enforcement, affecting wired and wireless authentication across campuses and remote sites. Recovery requires ISE restart and potential reconfiguration, impacting availability and user productivity during outages.

**Most likely attack path** An external attacker on the network can send crafted RADIUS requests with no authentication and low complexity. The flaw stems from logic handling a MAC address that is already rejected, so a rapid sequence of requests could force an ISE restart (scope change), leading to denial of service and potential knock-on outages beyond the initial target.

**Who is most exposed** Enterprise deployments using Cisco ISE 3.4.x for 802.1X, VPN, or other RADIUS-based access controls—especially in large campuses or distributed sites with multiple ISE nodes.

Detection ideas

• Alerts for unexpected ISE process restarts or crash events
• CPU/memory spikes correlated with bursts of RADIUS traffic
• Authentication outages or spike in RADIUS failure logs
• RADIUS logs showing repeated crafted access attempts or abnormal request patterns
• Correlation with maintenance windows or quick failover events

Mitigation and prioritisation

• Apply the Cisco advisory and upgrade to the fixed ISE release; review the vendor’s patch guidance.
• Implement RADIUS traffic controls: rate-limit or filter traffic from untrusted segments where feasible.
• Ensure high-availability deployment with validated failover and tested recovery procedures.
• Perform controlled patch testing in a lab and schedule a maintenance window for production upgrade.
• Monitor for early indicators post-patch and verify service stability; treat as a priority due to DoS potential when exposed.