CVE-2025-20340: Cisco IOS XR Software DoS Vulnerability

CVE-2025-20340 Overview

CVE-2025-20340 is a denial of service (DoS) vulnerability in the Address Resolution Protocol (ARP) implementation of Cisco IOS XR Software. An unauthenticated, adjacent attacker can trigger a broadcast storm by sending sustained, high-rate ARP traffic to the management interface of an affected device. Successful exploitation degrades device performance, severs management connectivity, and can render the system completely unresponsive. The flaw is classified under [CWE-400] (Uncontrolled Resource Consumption) and stems from how IOS XR processes excessive ARP traffic on the management plane.

Critical Impact

An adjacent attacker can exhaust ARP processing resources on Cisco IOS XR devices, causing loss of management connectivity and full system unresponsiveness.

Affected Products

• Cisco IOS XR Software (management interface ARP processing)
• Cisco routing platforms running vulnerable IOS XR releases
• Refer to the Cisco Security Advisory for the complete list of fixed releases

Discovery Timeline

• 2025-09-10 - CVE-2025-20340 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-20340

Vulnerability Analysis

The vulnerability resides in the ARP handling logic of Cisco IOS XR Software on the management interface. When the device receives a sustained, high rate of ARP packets, the ARP processing subsystem cannot rate-limit or shed the load effectively. The result is a broadcast storm condition that consumes CPU and memory resources allocated to control-plane traffic.

As the ARP queue saturates, legitimate management traffic is starved. Operators lose SSH, NETCONF, and SNMP access to the device. Continued exploitation drives the system into complete unresponsiveness, which can disrupt routing operations on production networks.

The scope change in the CVSS vector reflects that impact extends beyond the management plane, affecting overall device availability. Because the attack requires adjacent network access, the attacker must reside on the same Layer 2 broadcast domain as the management interface.

Root Cause

The root cause is insufficient input rate limiting and resource control [CWE-400] in the ARP processing path for the management interface. IOS XR does not enforce adequate throttling on inbound ARP traffic, allowing a flood to monopolize the resources required to maintain the management plane.

Attack Vector

The attacker must be on the adjacent network segment of the management interface. No authentication or user interaction is required. The attacker generates and sustains an excessive volume of ARP packets directed at the management interface. The device's ARP subsystem becomes overwhelmed, producing the DoS condition. No code is required beyond a standard packet generator capable of producing ARP requests at high rates.

See the Cisco Security Advisory for vendor technical details.

Detection Methods for CVE-2025-20340

Indicators of Compromise

• Sudden spike in ARP request rates on the management network segment
• Loss of SSH, SNMP, or NETCONF responsiveness on IOS XR devices
• Elevated CPU usage attributable to ARP processing on the route processor
• Syslog messages indicating control-plane resource exhaustion or interface flapping

Detection Strategies

• Baseline normal ARP traffic volumes on management VLANs and alert on deviations
• Monitor IOS XR control-plane CPU and memory utilization with thresholds for sustained spikes
• Correlate management plane reachability loss with ARP traffic anomalies on adjacent switches
• Enable and review NetFlow or sFlow on management segments to identify ARP flood sources

Monitoring Recommendations

• Configure SNMP traps for ARP table churn and control-plane CPU thresholds
• Aggregate device syslogs in a central SIEM and create rules for management interface unreachability
• Track Layer 2 ARP rates per source MAC on switches adjacent to IOS XR management ports

How to Mitigate CVE-2025-20340

Immediate Actions Required

• Apply the fixed IOS XR Software release identified in the Cisco Security Advisory
• Restrict the management interface to a dedicated out-of-band network with strict Layer 2 access controls
• Audit which hosts share the management broadcast domain and remove untrusted devices
• Implement port security and ARP inspection on switches connecting to IOS XR management ports

Patch Information

Cisco has released fixed software versions for IOS XR. Consult the Cisco Security Advisory cisco-sa-iosxr-arp-storm-EjUU55yM for the specific fixed releases that apply to your platform and train. Upgrade promptly using Cisco's recommended migration path.

Workarounds

• Deploy Dynamic ARP Inspection (DAI) on adjacent switches to validate ARP packets and rate-limit floods
• Apply storm control on switch ports facing the IOS XR management interface to cap broadcast and ARP traffic
• Isolate the management interface on a physically separate out-of-band management network where feasible
• Configure Control Plane Policing (CoPP) and management plane protection policies on supported platforms

# Configuration example: storm control on upstream switch port facing IOS XR mgmt
interface GigabitEthernet1/0/24
 description Uplink to IOS XR management port
 storm-control broadcast level pps 100
 storm-control action shutdown
 ip arp inspection limit rate 50 burst interval 1