CVE-2025-20189: Cisco IOS XE DoS Vulnerability

CVE-2025-20189 Overview

CVE-2025-20189 is a denial of service (DoS) vulnerability in the Cisco Express Forwarding (CEF) functionality of Cisco IOS XE Software running on Cisco ASR 903 Aggregation Services Routers equipped with the Route Switch Processor 3 (RSP3C). An unauthenticated, adjacent attacker can exhaust system resources by sending crafted Address Resolution Protocol (ARP) messages at a high rate, ultimately forcing the active route switch processor (RSP) to reload. Where no redundant RSP is installed, the router itself reloads, interrupting traffic across the platform. The flaw is tracked under [CWE-762: Mismatched Memory Management Routines].

Critical Impact
Sustained ARP flooding from an adjacent attacker can crash the active RSP and reload the router, disrupting service on carrier-grade aggregation infrastructure.

Affected Products

Cisco IOS XE Software (multiple 3.x, 16.x, and 17.x trains through 17.14.1a)
Cisco ASR 903 Aggregation Services Router
Cisco Route Switch Processor 3 (RSP3C)

Discovery Timeline

2025-05-07 - CVE-2025-20189 published to NVD
2025-08-05 - Last updated in NVD database

Technical Details for CVE-2025-20189

Vulnerability Analysis

The vulnerability resides in how Cisco IOS XE handles ARP message processing within the Cisco Express Forwarding subsystem on the RSP3C. CEF maintains adjacency tables that are populated and updated via ARP resolution. When ARP messages arrive at the device, memory structures are allocated to track resolution state and adjacency information. Improper memory management in this code path means that high-volume ARP processing does not return resources to the system in a balanced manner.

Over time, this leads to resource exhaustion on the active RSP. Once memory pressure exceeds operational thresholds, the active RSP reloads. On chassis configured with a redundant RSP, switchover occurs and the standby takes over. On single-RSP configurations, the entire router reloads, causing a full traffic outage.

Root Cause

The root cause is improper memory management ([CWE-762]) during ARP message processing in the CEF code path on the RSP3C. Allocated buffers and state objects are not properly released or reused under sustained high-rate ARP input, resulting in cumulative resource consumption rather than steady-state behavior.

Attack Vector

Exploitation requires adjacent network access — the attacker must be on the same Layer 2 broadcast domain as an interface of the affected ASR 903 router. No authentication or user interaction is required. The attacker sends crafted ARP messages at a high rate over a sustained period. Because ARP is an unauthenticated link-layer protocol, any device with Layer 2 reachability to a router interface can transmit the required traffic. No verified proof-of-concept code is publicly available at this time.

Detection Methods for CVE-2025-20189

Indicators of Compromise

Unexpected reloads of the active RSP3C on ASR 903 chassis, with crash logs referencing CEF or ARP-related processes.
Sustained high rates of ARP requests or replies arriving on one or more router interfaces from a single adjacent source.
Progressive growth of memory utilization on the active RSP without a corresponding change in routing table or session counts.
Repeated CEF adjacency churn or ARP table thrashing visible in show ip arp and show adjacency output.

Detection Strategies

Monitor ARP packet rates per interface using NetFlow, IPFIX, or SPAN-based traffic analysis and alert on anomalous spikes from a single MAC or IP.
Baseline RSP memory utilization with show platform resources and show memory summary and alert on sustained upward trends.
Correlate device syslog messages for RSP reload events, CEF process restarts, and memory allocation failures.

Monitoring Recommendations

Ingest router syslog and SNMP traps into a centralized telemetry platform to track RSP reload counts and memory exhaustion events.
Enable storm-control or ARP inspection counters on access-facing interfaces and review them on a defined cadence.
Track adjacency-table size and CEF entry churn over time to identify abnormal ARP-driven activity.

How to Mitigate CVE-2025-20189

Immediate Actions Required

Identify all ASR 903 routers equipped with RSP3C modules and inventory the IOS XE version running on each.
Apply the fixed Cisco IOS XE Software release referenced in the Cisco Security Advisory.
Where patching cannot be performed immediately, deploy redundant RSPs to preserve service continuity during a potential reload.
Restrict Layer 2 adjacency to router interfaces through physical segmentation and port security on connected switches.

Patch Information

Cisco has published a security advisory for this issue. Refer to the Cisco Security Advisory cisco-sa-asr903-rsp3-arp-dos-WmfzdvJZ for the list of fixed software releases and upgrade guidance specific to the RSP3C platform.

Workarounds

Apply ARP rate limiting and Control Plane Policing (CoPP) policies on the ASR 903 to cap the volume of ARP traffic punted to the RSP.
Enable Dynamic ARP Inspection (DAI) on adjacent Layer 2 switches to drop unauthorized ARP traffic before it reaches the router.
Limit physical and logical Layer 2 access to router interfaces, allowing only trusted, validated peers in the broadcast domain.

bash

# Example Control Plane Policing snippet to rate-limit ARP traffic
class-map match-any COPP-ARP
 match protocol arp
!
policy-map COPP-POLICY
 class COPP-ARP
  police rate 1000 pps conform-action transmit exceed-action drop
!
control-plane
 service-policy input COPP-POLICY