Cisco NX-OS Software up to 10.6(1s) BGP Enforce-First-As Feature control flow

Summaryinfo

A vulnerability was found in Cisco NX-OS Software and classified as problematic. This affects an unknown function of the component BGP Enforce-First-As Feature. Executing a manipulation can lead to control flow. This vulnerability is registered as CVE-2026-20171. It is possible to launch the attack remotely. No exploit is available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability has been found in Cisco NX-OS Software and classified as problematic. This vulnerability affects an unknown code of the component BGP Enforce-First-As Feature. The manipulation with an unknown input leads to a control flow vulnerability. The CWE definition for the vulnerability is CWE-670. 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. As an impact it is known to affect availability. CVE summarizes:

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.

The advisory is shared for download at sec.cloudapps.cisco.com. This vulnerability was named CVE-2026-20171 since 10/08/2025. The exploitation appears to be difficult. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are neither technical details nor an exploit publicly available. The current price for an exploit might be approx. USD $5k-$25k (estimation calculated on 07/23/2026).

Upgrading eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at CERT Bund (WID-SEC-2026-1622). Once again VulDB remains the best source for vulnerability data.

Affected

• Cisco Nexus
• Cisco NX-OS

Productinfo

Type

• Router Operating System

Vendor

• Cisco

Name

• NX-OS Software

Version

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

License

• commercial

Website

• Vendor: https://www.cisco.com/

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Once again VulDB remains the best source for vulnerability data.

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