CVE-2026-20352 in Identity Services Engine
Summary
by MITRE • 09/16/2026
A vulnerability in the RADIUS feature of Cisco Identity Services Engine (ISE) could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device.
This vulnerability is due to improper handling of certain RADIUS requests. An attacker could exploit this vulnerability by sending a crafted RADIUS request directly to an affected device. A successful exploit could allow the attacker to cause the ISE node to become unavailable. For single node deployments in that condition, endpoints that have not already authenticated would be unable to access the network until the node comes back up on its own.
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Analysis
by VulDB Data Team • 09/16/2026
The Cisco Identity Services Engine (ISE) serves as a critical component in modern enterprise networks by providing authentication, authorization, and accounting services for users and devices connecting to the infrastructure. Within this ecosystem, the Remote Authentication Dial-In User Service protocol plays a pivotal role in facilitating secure access control decisions based on credentials provided by endpoints. A significant security flaw has been identified within the RADIUS feature implementation of Cisco ISE, specifically concerning how certain incoming requests are processed. This vulnerability stems from an improper handling mechanism that fails to adequately validate or sanitize specific types of RADIUS packets before they trigger internal processing routines. The root cause lies in a logic error where the application does not correctly manage resource allocation or state transitions when encountering malformed or specially crafted input, leading to unstable behavior within the service process responsible for handling these requests.
From an operational perspective, this technical flaw enables an unauthenticated, remote attacker to execute a denial of service attack against the affected ISE node without requiring any prior credentials or network proximity beyond basic IP connectivity. By sending a specifically crafted RADIUS request directly to the vulnerable port on the target device, the attacker can trigger the improper handling routine that results in resource exhaustion or process crash. The immediate impact is severe for single-node deployments where high availability measures such as clustering are not implemented. In such scenarios, a successful exploitation causes the ISE node to become completely unavailable, effectively halting all authentication services provided by that specific server. This creates a critical bottleneck in network access control, preventing new endpoints from authenticating and gaining entry to protected network segments until the service is manually restarted or recovers automatically if possible.
The broader implications of this vulnerability extend beyond simple downtime for the ISE node itself. In environments relying on centralized authentication policies, such as those enforcing 802.1X port-based network access control, a compromised or downed identity server can lead to widespread connectivity issues across the organization. Endpoints that have not yet completed their initial authentication handshake will be denied access, potentially disrupting business operations and user productivity. Furthermore, in more complex deployments involving multiple nodes, while other healthy nodes might continue to serve requests, the loss of one node reduces overall capacity and resilience, making the remaining infrastructure more susceptible to further attacks or performance degradation under load. This scenario highlights the importance of robust input validation and state management within network security appliances that handle high volumes of authentication traffic.
To mitigate this risk, organizations should prioritize applying vendor-provided software updates that address the underlying code defects responsible for the improper request handling. Cisco has released patches designed to correct these logic errors and ensure that malformed packets are discarded safely without triggering system instability. In addition to patching, network architects should evaluate their deployment topology to include redundancy mechanisms such as ISE clustering or high-availability pairs, ensuring that a single point of failure does not result in total authentication service outage for the enterprise. Implementing strict access control lists on management interfaces can also limit exposure by restricting which IP addresses are permitted to send RADIUS requests to the identity server, thereby reducing the attack surface available to potential adversaries who might attempt to exploit this vulnerability remotely.
This incident aligns with common weakness classifications found in industry standards such as CWE-20 Improper Input Validation and CWE-400 Uncontrolled Resource Consumption, which describe flaws where software fails to properly check input data or manage resources during processing of unexpected inputs. From a threat intelligence perspective, the exploitation technique corresponds to ATT&CK tactic T1499 Endpoint Denial of Service, specifically subtechniques involving resource exhaustion through application layer attacks. Understanding these mappings helps security teams contextualize the severity and potential impact within their specific operational environments, allowing for more targeted defense strategies that focus on both immediate remediation via patching and long-term architectural improvements to enhance resilience against similar denial-of-service vectors targeting critical identity management infrastructure.