---
id: CVE-2026-20171
title: >-
  A vulnerability in the Border Gateway Protocol (BGP) enforce-first-as
  feature of Cisco Nexus 3000 Series Switches and Cisco Nexus 9000 Series
  Switches in standalone NX-OS mode could allow an unauthenticated, remote
  attacker to …
summary: >-
  A vulnerability in the Border Gateway Protocol (BGP) enforce-first-as
  feature of Cisco Nexus 3000 Series Switches and Cisco Nexus 9000 Series
  Switches in standalone NX-OS mode could allow an unauthenticated, remote
  attacker to …
severity: medium
cvss: 6.8
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H'
cwe:
  - CWE-670
published: '2026-05-20'
updated: '2026-07-23'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-20171'
references:
  - url: >-
      https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-bgp-iefab-3hb2pwtx
    label: psirt@cisco.com
tags:
  - nvd
epss: 0.00467
epssPercentile: 0.39622
ingestedAt: '2026-07-23T12:17:55.317Z'
---

## Overview

A vulnerability in the Border Gateway Protocol (BGP) enforce-first-as feature of Cisco Nexus 3000 Series Switches and Cisco Nexus 9000 Series Switches in standalone NX-OS mode could allow an unauthenticated, remote attacker to trigger BGP peer flaps, resulting in a denial of service (DoS) condition.

This vulnerability is due to incorrect parsing of a transitive BGP attribute. An attacker could exploit this vulnerability by sending a crafted BGP update through an established BGP peer session. If the update propagates to an affected device, it could cause the device to drop the BGP session and flap with the BGP peer that is forwarding this update, resulting in a DoS condition.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.