CVE-2026-20171 PUBLISHED

Cisco Nexus 3000 and 9000 Series Border Gateway Protocol Denial of Service Vulnerability

Assigner: cisco
Reserved: 08.10.2025 Published: 20.05.2026 Updated: 20.05.2026

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.

Metrics

CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H
CVSS Score: 6.8

Product Status

Vendor Cisco
Product Cisco NX-OS Software
Versions Default: unknown
  • Version 10.2(1) is affected
  • Version 10.2(1q) is affected
  • Version 10.2(2) is affected
  • Version 10.2(3) is affected
  • Version 10.2(3t) is affected
  • Version 10.2(2a) is affected
  • Version 10.3(1) is affected
  • Version 10.2(4) is affected
  • Version 10.3(2) is affected
  • Version 10.3(3) is affected
  • Version 10.2(5) is affected
  • Version 10.2(3v) is affected
  • Version 10.4(1) is affected
  • Version 10.3(99w) is affected
  • Version 10.2(6) is affected
  • Version 10.3(3w) is affected
  • Version 10.3(99x) is affected
  • Version 10.3(3o) is affected
  • Version 10.3(4) is affected
  • Version 10.3(3p) is affected
  • Version 10.3(4a) is affected
  • Version 10.4(2) is affected
  • Version 10.3(3q) is affected
  • Version 10.3(5) is affected
  • Version 10.2(7) is affected
  • Version 10.4(3) is affected
  • Version 10.3(3x) is affected
  • Version 10.3(4g) is affected
  • Version 10.5(1) is affected
  • Version 10.2(8) is affected
  • Version 10.3(3r) is affected
  • Version 10.3(6) is affected
  • Version 10.4(4) is affected
  • Version 10.3(4h) is affected
  • Version 10.5(2) is affected
  • Version 10.3(7) is affected
  • Version 10.4(5) is affected
  • Version 10.5(3) is affected
  • Version 10.2(9) is affected
  • Version 10.4(4g) is affected
  • Version 10.6(1) is affected
  • Version 10.5(3t) is affected
  • Version 10.3(8) is affected
  • Version 10.4(6) is affected
  • Version 10.5(3s) is affected
  • Version 10.5(3e) is affected
  • Version 10.5(3o) is affected
  • Version 10.6(1s) is affected
  • Version 10.5(3p) is affected

Exploits

The Cisco Product Security Incident Response Team (PSIRT) is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.

References

Problem Types

  • Always-Incorrect Control Flow Implementation cwe