CVE-2026-20171

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

Published May 20, 2026 CVSS 3.1 · 6.8 Medium EPSS < 1% KEV · Not listed

Description

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.

Affected Products

Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).

Vendor Product Default status Versions

Cisco

Cisco NX-OS Software

unknown

Version	Status	Constraints
`10.2(1)`	affected	—
`10.2(1q)`	affected	—
`10.2(2)`	affected	—
`10.2(3)`	affected	—
`10.2(3t)`	affected	—
`10.2(2a)`	affected	—
`10.3(1)`	affected	—
`10.2(4)`	affected	—
`10.3(2)`	affected	—
`10.3(3)`	affected	—
`10.2(5)`	affected	—
`10.2(3v)`	affected	—
`10.4(1)`	affected	—
`10.3(99w)`	affected	—
`10.2(6)`	affected	—
`10.3(3w)`	affected	—
`10.3(99x)`	affected	—
`10.3(3o)`	affected	—
`10.3(4)`	affected	—
`10.3(3p)`	affected	—
`10.3(4a)`	affected	—
`10.4(2)`	affected	—
`10.3(3q)`	affected	—
`10.3(5)`	affected	—
`10.2(7)`	affected	—
`10.4(3)`	affected	—
`10.3(3x)`	affected	—
`10.3(4g)`	affected	—
`10.5(1)`	affected	—
`10.2(8)`	affected	—
`10.3(3r)`	affected	—
`10.3(6)`	affected	—
`10.4(4)`	affected	—
`10.3(4h)`	affected	—
`10.5(2)`	affected	—
`10.3(7)`	affected	—
`10.4(5)`	affected	—
`10.5(3)`	affected	—
`10.2(9)`	affected	—
`10.4(4g)`	affected	—
`10.6(1)`	affected	—
`10.5(3t)`	affected	—
`10.3(8)`	affected	—
`10.4(6)`	affected	—
`10.5(3s)`	affected	—
`10.5(3e)`	affected	—
`10.5(3o)`	affected	—
`10.6(1s)`	affected	—
`10.5(3p)`	affected	—

No data.

Vendor Product Versions

Cisco

Nx-os Software

Version	Status	Scheme	Platform
`10.2(1)`	affected	generic	—
`10.2(1q)`	affected	generic	—
`10.2(2)`	affected	generic	—
`10.2(3)`	affected	generic	—
`10.2(3t)`	affected	generic	—
`10.2(2a)`	affected	generic	—
`10.3(1)`	affected	generic	—
`10.2(4)`	affected	generic	—
`10.3(2)`	affected	generic	—
`10.3(3)`	affected	generic	—
`10.2(5)`	affected	generic	—
`10.2(3v)`	affected	generic	—
`10.4(1)`	affected	generic	—
`10.3(99w)`	affected	generic	—
`10.2(6)`	affected	generic	—
`10.3(3w)`	affected	generic	—
`10.3(99x)`	affected	generic	—
`10.3(3o)`	affected	generic	—
`10.3(4)`	affected	generic	—
`10.3(3p)`	affected	generic	—
`10.3(4a)`	affected	generic	—
`10.4(2)`	affected	generic	—
`10.3(3q)`	affected	generic	—
`10.3(5)`	affected	generic	—
`10.2(7)`	affected	generic	—
`10.4(3)`	affected	generic	—
`10.3(3x)`	affected	generic	—
`10.3(4g)`	affected	generic	—
`10.5(1)`	affected	generic	—
`10.2(8)`	affected	generic	—
`10.3(3r)`	affected	generic	—
`10.3(6)`	affected	generic	—
`10.4(4)`	affected	generic	—
`10.3(4h)`	affected	generic	—
`10.5(2)`	affected	generic	—
`10.3(7)`	affected	generic	—
`10.4(5)`	affected	generic	—
`10.5(3)`	affected	generic	—
`10.2(9)`	affected	generic	—
`10.4(4g)`	affected	generic	—
`10.6(1)`	affected	generic	—
`10.5(3t)`	affected	generic	—
`10.3(8)`	affected	generic	—
`10.4(6)`	affected	generic	—
`10.5(3s)`	affected	generic	—
`10.5(3e)`	affected	generic	—
`10.5(3o)`	affected	generic	—
`10.6(1s)`	affected	generic	—
`10.5(3p)`	affected	generic	—
`10.5(3e)`	affected	generic	—
`10.5(3o)`	affected	generic	—

Remediation

No solution or workaround provided in the CVE record

Tracking

Advisories

No advisories available

Metrics

No CVSS v4.0

Attack Vector Network

Attack Complexity High

Privileges Required None

Scope Changed

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

Exploit prediction score: < 1% (0.467% probability within 30 days)

Exploitation none

Automatable no

Technical Impact partial

References

Link	Providers
https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-bgp-iefab-3hb2pwtx

History

May 21, 2026 8:30 AM First appearance in the feed. Affected vendors updated View changes

Type	Values Removed	Values Added
First Time appeared		Cisco Cisco nx-os Software
Vendors & Products		Cisco Cisco nx-os Software

May 20, 2026 8:15 PM Scoring metrics updated View changes

Type	Values Removed	Values Added
Metrics		ssvc `{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}`

May 20, 2026 4:45 PM Description updated. Title updated. Weaknesses updated. Added references. Scoring metrics updated View changes

Type	Values Removed	Values Added
Description		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.
Title		Cisco Nexus 3000 and 9000 Series Border Gateway Protocol Denial of Service Vulnerability
Weaknesses		CWE-670
References		https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-bgp-iefab-3hb2pwtx
Metrics		cvssV3_1 `{'score': 6.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H'}`