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

mediumCVE-2026-20171

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. Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability. This advisory is available at the following link: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-bgp-iefab-3hb2pwtx Security Impact Rating: Medium CVE: CVE-2026-20171

CSIRTS triage

vendor: Cisco product: Nexus 3000 and 9000 Series Switches Denial of service

What: A denial of service vulnerability exists in the BGP enforce-first-as feature.
Who is affected: Users of Cisco Nexus 3000 and 9000 Series Switches are affected.
Urgency: Remediation is important as exploitation can lead to service disruption.
Action: Apply the software updates released by Cisco.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Cisco Security Advisories (INTL · vendor-psirt · site)
Severity: medium
Published: 2026-05-20
Exploitation: Not in CISA KEV at last sync

Original advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-bgp-iefab-3hb2pwtx?vs_f=Cisco%20Security%20Advisory%26vs_cat=Security%20Intelligence%26vs_type=RSS%26vs_p=Cisco%20Nexus%203000%20and%209000%20Series%20Switches%20Border%20Gateway%20Protocol%20Denial%20of%20Service%20Vulnerability%26vs_k=1

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-201710.47% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 37% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-20171	coverage & exploitation status	NVD · CVE.org

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