CVE-2026-20118: Cisco IOS XR Software DoS Vulnerability

CVE-2026-20118 Overview

A vulnerability in the handling of an Egress Packet Network Interface (EPNI) Aligner interrupt in Cisco IOS XR Software could allow an unauthenticated, remote attacker to cause the network processing unit (NPU) and ASIC to stop processing, preventing traffic from traversing the interface. This vulnerability affects Cisco Network Convergence System (NCS) 5500 Series with NC57 line cards, Cisco NCS 5700 Routers, and Cisco IOS XR Software for Third Party Software.

The vulnerability is due to the corruption of packets in specific cases when an EPNI Aligner interrupt is triggered while an affected device is experiencing heavy transit traffic. An attacker could exploit this vulnerability by sending a continuous flow of crafted packets to an interface of the affected device. A successful exploit could allow the attacker to cause persistent, heavy packet loss, resulting in a denial of service (DoS) condition.

Critical Impact
Successful exploitation causes the NPU and ASIC to stop processing traffic, leading to persistent packet loss and service disruption on critical network infrastructure. Cisco has elevated this vulnerability's Security Impact Rating to High due to the affected devices operating within critical network segments.

Affected Products

Cisco IOS XR Software for Cisco Network Convergence System (NCS) 5500 Series with NC57 line cards
Cisco NCS 5700 Routers
Cisco IOS XR Software for Third Party Software

Discovery Timeline

2026-03-11 - CVE CVE-2026-20118 published to NVD
2026-03-12 - Last updated in NVD database

Technical Details for CVE-2026-20118

Vulnerability Analysis

This vulnerability resides in the interrupt handling mechanism of the Egress Packet Network Interface (EPNI) Aligner component within Cisco IOS XR Software. The EPNI Aligner is responsible for managing packet alignment during egress processing on network interfaces. When the device is under heavy transit traffic conditions and an EPNI Aligner interrupt is triggered, a race condition can occur that leads to packet corruption.

The vulnerability is classified under CWE-460 (Improper Cleanup on Thrown Exception), indicating that the interrupt handling routine fails to properly clean up or manage resources when the exception condition occurs during high-traffic scenarios. This improper cleanup leads to a cascading failure where the NPU and ASIC components cease packet processing operations.

Root Cause

The root cause of this vulnerability is improper cleanup on thrown exception (CWE-460) within the EPNI Aligner interrupt handling code. When an EPNI Aligner interrupt fires during periods of heavy traffic load, the exception handling mechanism fails to properly restore the system state or release resources. This results in packet corruption that propagates through the processing pipeline, ultimately causing the NPU and ASIC to enter a non-functional state where they cannot process network traffic.

Attack Vector

The attack vector is network-based and does not require authentication. An attacker can exploit this vulnerability remotely by:

Identifying a target device running vulnerable Cisco IOS XR Software on NCS 5500 Series (with NC57 line cards) or NCS 5700 routers
Generating and sending a continuous flow of specially crafted packets to an interface on the affected device
Timing the attack to coincide with or create conditions of heavy transit traffic
Triggering the EPNI Aligner interrupt handling flaw, causing packet corruption
Achieving persistent packet loss and denial of service as the NPU and ASIC stop processing traffic

The attack requires high complexity as specific traffic conditions must be present for successful exploitation. The vulnerability has a changed scope, meaning the impact extends beyond the vulnerable component to affect other network segments that depend on the targeted infrastructure.

Detection Methods for CVE-2026-20118

Indicators of Compromise

Sudden and persistent packet loss on interfaces of affected NCS 5500 or NCS 5700 devices
NPU or ASIC hardware components showing error states or stopped processing in system logs
EPNI Aligner interrupt errors appearing in Cisco IOS XR system logs
Unexpected interface flapping or traffic blackholing during high traffic periods

Detection Strategies

Monitor for EPNI Aligner interrupt-related error messages in Cisco IOS XR system logs using syslog forwarding
Configure SNMP traps for NPU and ASIC health status changes on affected platforms
Implement network flow analysis to detect unusual packet loss patterns on critical interfaces
Deploy SentinelOne Singularity for network infrastructure monitoring to detect anomalous traffic patterns indicative of exploitation attempts

Monitoring Recommendations

Establish baseline metrics for normal traffic patterns and packet loss rates on affected devices
Configure alerting thresholds for packet loss exceeding normal operational parameters
Enable detailed hardware component logging on NCS 5500 and NCS 5700 platforms to capture EPNI Aligner events
Implement traffic analysis at network boundaries to identify potential attack traffic targeting affected infrastructure

How to Mitigate CVE-2026-20118

Immediate Actions Required

Review the Cisco Security Advisory for specific affected versions and available patches
If active exploitation is suspected, immediately contact Cisco Technical Assistance Center (TAC) or your contracted maintenance provider
Implement traffic filtering where possible to limit exposure of affected interfaces
Consider traffic engineering to reduce load on potentially affected devices until patches can be applied

Patch Information

Cisco has released security advisories and patches to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific version information, fixed software releases, and upgrade paths for affected Cisco IOS XR Software deployments.

Organizations running Cisco NCS 5500 Series with NC57 line cards or NCS 5700 Routers should prioritize reviewing their software versions and planning upgrades to patched versions as soon as operationally feasible.

Workarounds

Contact Cisco TAC for device-specific workaround recommendations while awaiting patch deployment
Implement rate limiting on exposed interfaces to reduce the likelihood of triggering the vulnerability under heavy traffic conditions
Consider network segmentation to limit the attack surface of affected devices
Monitor the Cisco Security Advisory for any interim mitigation guidance specific to your deployment