CVE-2026-20171: Always-Incorrect Control Flow Implementation in Cisco Cisco NX-OS Software
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.
AI Analysis
Technical Summary
This vulnerability in Cisco NX-OS Software's Border Gateway Protocol (BGP) enforce-first-as feature is due to incorrect parsing of a transitive BGP attribute. An unauthenticated remote attacker can exploit this by sending a crafted BGP update through an established BGP peer session, which causes the affected Cisco Nexus 3000 and 9000 Series Switches to drop the BGP session and flap with the forwarding peer, resulting in a denial of service condition. The issue affects multiple versions of NX-OS software as listed. The CVSS v3.1 base score is 6.8, indicating medium severity. No official remediation or patch information is currently provided by the vendor.
Potential Impact
Successful exploitation results in denial of service by causing BGP peer flaps and session drops on affected Cisco Nexus switches. This disrupts network routing stability but does not impact confidentiality or integrity. The vulnerability requires network access to an established BGP peer session and has a high attack complexity.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, consider restricting BGP peer access to trusted sources and monitoring BGP session stability for anomalies related to enforce-first-as feature usage.
CVE-2026-20171: Always-Incorrect Control Flow Implementation in Cisco Cisco NX-OS Software
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.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
This vulnerability in Cisco NX-OS Software's Border Gateway Protocol (BGP) enforce-first-as feature is due to incorrect parsing of a transitive BGP attribute. An unauthenticated remote attacker can exploit this by sending a crafted BGP update through an established BGP peer session, which causes the affected Cisco Nexus 3000 and 9000 Series Switches to drop the BGP session and flap with the forwarding peer, resulting in a denial of service condition. The issue affects multiple versions of NX-OS software as listed. The CVSS v3.1 base score is 6.8, indicating medium severity. No official remediation or patch information is currently provided by the vendor.
Potential Impact
Successful exploitation results in denial of service by causing BGP peer flaps and session drops on affected Cisco Nexus switches. This disrupts network routing stability but does not impact confidentiality or integrity. The vulnerability requires network access to an established BGP peer session and has a high attack complexity.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, consider restricting BGP peer access to trusted sources and monitoring BGP session stability for anomalies related to enforce-first-as feature usage.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- cisco
- Date Reserved
- 2025-10-08T11:59:15.391Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a0de26bba1db473628f2733
Added to database: 5/20/2026, 4:33:47 PM
Last enriched: 5/20/2026, 4:49:53 PM
Last updated: 5/21/2026, 3:19:27 AM
Views: 31
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