---
id: CVE-2026-20074
title: "A vulnerability in the Intermediate System-to-Intermediate System (IS-IS) multi-instance routing feature of Cisco IOS XR Software could allow an unauthenticated, adjacent attacker to cause the IS-IS process to restart unexpectedly.\r\n\r\nTh…"
summary: "A vulnerability in the Intermediate System-to-Intermediate System (IS-IS) multi-instance routing feature of Cisco IOS XR Software could allow an unauthenticated, adjacent attacker to cause the IS-IS process to restart unexpectedly.\r\n\r\nTh…"
severity: high
cvss: 7.4
cvssVector: 'CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H'
cwe:
  - CWE-1287
vendor: cisco
product: ios_xr
affected:
  - 'ios_xr >= 7.8.0, < 25.2.2'
  - ios_xr = 25.3.0
patched:
  - ios_xr 25.2.2
published: '2026-03-11'
updated: '2026-07-08'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-20074'
references:
  - url: >-
      https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-isis-dos-kDMxpSzK
    label: psirt@cisco.com
tags:
  - nvd
epss: 0.0016
epssPercentile: 0.04352
ingestedAt: '2026-07-08T19:46:17.205Z'
---

## Overview

A vulnerability in the Intermediate System-to-Intermediate System (IS-IS) multi-instance routing feature of Cisco IOS XR Software could allow an unauthenticated, adjacent attacker to cause the IS-IS process to restart unexpectedly.

This vulnerability is due to insufficient input validation of ingress IS-IS packets. An attacker could exploit this vulnerability by sending crafted IS-IS packets to an affected device after forming an adjacency. A successful exploit could allow the attacker to cause the IS-IS process to restart unexpectedly, resulting in a temporary loss of connectivity to advertised networks and a denial of service (DoS) condition.
Note: The IS-IS protocol is a routing protocol. To exploit this vulnerability, an attacker must be Layer 2-adjacent to the affected device and must have formed an adjacency.&nbsp;&nbsp;

## Affected

- `ios_xr >= 7.8.0, < 25.2.2`
- `ios_xr = 25.3.0`

## Remediation

Upgrade past the affected range:

- `ios_xr 25.2.2`