CVE-2026-20275: Cisco IOS XR Calculation Error Vulnerability

CVE-2026-20275 Overview

Cisco disclosed CVE-2026-20275 as part of an internal security hardening review of Cisco IOS XR Software. The vulnerability stems from an incorrect calculation weakness classified under [CWE-682]. An adjacent-network attacker without authentication can exploit this flaw to compromise confidentiality, integrity, and availability of affected devices. Cisco identified the issue through proactive internal review rather than external disclosure. The advisory groups this CVE with other internally discovered vulnerabilities addressed in the same hardening release.

Critical Impact
An unauthenticated adjacent attacker can trigger incorrect calculation logic in Cisco IOS XR to achieve high impact on confidentiality, integrity, and availability of network infrastructure.

Affected Products

Cisco IOS XR Software (versions addressed by the referenced hardening release)
Cisco routing platforms running vulnerable IOS XR builds
Refer to the Cisco Security Advisory on IOS XR Hardening for the fixed-release matrix

Discovery Timeline

2026-09-02 - CVE-2026-20275 published to NVD
2026-09-02 - Last updated in NVD database

Technical Details for CVE-2026-20275

Vulnerability Analysis

CVE-2026-20275 is an incorrect calculation vulnerability [CWE-682] affecting Cisco IOS XR Software. The flaw allows an unauthenticated attacker positioned on an adjacent network to send crafted input that triggers faulty arithmetic or logical computation within the software. Successful exploitation yields high impact across confidentiality, integrity, and availability. Cisco discovered the issue internally during a comprehensive security review of the IOS XR codebase.

Root Cause

The root cause is an incorrect calculation defect in IOS XR components. CWE-682 covers cases where a product performs arithmetic or logical operations that produce results not intended by the developer. Such flaws often surface in packet parsing, length validation, or state machine bookkeeping. Cisco has not published low-level internals in the public advisory. Consult the Cisco Security Advisory on IOS XR Hardening for component-level details.

Attack Vector

Exploitation requires access to a network adjacent to the target device, such as the same broadcast domain or a directly connected link. No authentication or user interaction is required. An attacker sends crafted protocol traffic that reaches a vulnerable calculation path in IOS XR. The impact can extend to control-plane disruption, traffic manipulation, or exposure of processed data.

No public proof-of-concept exploit is available at the time of publication, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2026-20275

Indicators of Compromise

Unexpected process restarts, crashes, or memory anomalies logged by IOS XR system processes
Anomalous control-plane traffic originating from adjacent-network hosts targeting routing or management protocols
Unexplained routing table changes or protocol adjacency flaps on IOS XR devices

Detection Strategies

Compare running IOS XR versions against the fixed-release list in the Cisco advisory and flag devices on vulnerable builds
Baseline adjacent-network protocol traffic and alert on malformed packets or protocol violations
Correlate device syslog and SNMP trap data in a SIEM to identify repeated process faults tied to specific source segments

Monitoring Recommendations

Forward IOS XR syslog to a centralized log platform and monitor for crash, traceback, and memory-related events
Enable Control Plane Policing (CoPP) telemetry and review drops for anomalies
Track NetFlow or IPFIX records for unusual adjacent-source flows toward router management interfaces

How to Mitigate CVE-2026-20275

Immediate Actions Required

Inventory all Cisco IOS XR devices and identify versions listed as vulnerable in the Cisco advisory
Schedule upgrades to a fixed IOS XR release identified by Cisco as part of the hardening bundle
Restrict layer-2 and layer-3 adjacency to trusted peers using infrastructure ACLs and port security

Patch Information

Cisco released fixed IOS XR software as part of a broader hardening release that addresses multiple internally discovered vulnerabilities, including CVE-2026-20275. Administrators should review the Cisco Security Advisory on IOS XR Hardening to identify the specific fixed train and rebuild targets for their platforms. Apply the patch during a maintenance window and validate routing convergence after upgrade.

Workarounds

Apply infrastructure ACLs (iACLs) to limit which adjacent hosts can send traffic to the router control plane
Enforce Control Plane Policing (CoPP) policies to rate-limit and filter unexpected protocol traffic
Segment management and routing peers into dedicated VLANs or VRFs to reduce adjacent-network exposure
Disable unused protocols and interfaces on IOS XR devices to shrink the attack surface

bash

# Example: apply an infrastructure ACL to restrict adjacent access to the control plane
configure
ipv4 access-list PROTECT-CONTROL-PLANE
 10 permit tcp host <trusted-peer> any eq bgp
 20 permit ospf host <trusted-peer> any
 30 deny ipv4 any any log
!
interface <uplink-interface>
 ipv4 access-group PROTECT-CONTROL-PLANE ingress
commit