CVE-2026-20171

Vulnerability

A vulnerability in the Border Gateway Protocol (BGP) enforce-first-as feature of Cisco Nexus 3000 and 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. The root cause is incorrect parsing of a transitive BGP attribute by the affected feature [1].

Exploitation

An attacker can exploit this vulnerability by sending a crafted BGP update through an established BGP peer session. If the update propagates to an affected device, it causes the device to drop the BGP session and flap with the BGP peer forwarding the update [1]. No authentication is required, and the attacker only needs network access to a BGP peer.

Impact

Successful exploitation leads to repeated BGP session flaps, degrading network stability and potentially causing a denial of service for traffic relying on BGP routes. The affected device may become unreliable for routing updates [1].

Mitigation

Cisco has released software updates to address this vulnerability. Additionally, workarounds are available, such as disabling the enforce-first-as feature where possible. Users are advised to upgrade to a fixed software release as described in the Cisco Security Advisory [1].