Cisco Nexus BGP Enforce-First-AS DoS via BGP Peer Flaps
CVE-2026-20171 Published on May 20, 2026
Cisco Nexus 3000 and 9000 Series Border Gateway Protocol Denial of Service Vulnerability
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.
Vulnerability Analysis
CVE-2026-20171 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is consided to have a high level of attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
Always-Incorrect Control Flow Implementation
The code contains a control flow path that does not reflect the algorithm that the path is intended to implement, leading to incorrect behavior any time this path is navigated. This weakness captures cases in which a particular code segment is always incorrect with respect to the algorithm that it is implementing. For example, if a C programmer intends to include multiple statements in a single block but does not include the enclosing braces (CWE-483), then the logic is always incorrect. This issue is in contrast to most weaknesses in which the code usually behaves correctly, except when it is externally manipulated in malicious ways.
Products Associated with CVE-2026-20171
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Affected Versions
Cisco NX-OS Software:- 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.