CVE-2026-20051
Published: 25 February 2026
Summary
CVE-2026-20051 is a high-severity Use of Uninitialized Variable (CWE-457) vulnerability in Cisco Nexus (inferred from references). Its CVSS base score is 7.4 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Network Denial of Service (T1498); ranked at the 15.5th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SC-5 (Denial-of-service Protection) and SC-7 (Boundary Protection).
Deeper analysis
CVE-2026-20051 is a logic error in the Ethernet VPN (EVPN) Layer 2 ingress packet processing of Cisco Nexus 3600 Platform Switches and Cisco Nexus 9500-R Series Switching Platforms. This vulnerability arises when the affected devices process a crafted Layer 2 ingress frame, potentially leading to improper handling of Ethernet VPN traffic.
An unauthenticated, adjacent attacker who is Layer 2-adjacent to the targeted device can exploit this vulnerability by sending a stream of crafted Ethernet frames through the device. Successful exploitation triggers a Layer 2 Virtual eXtensible LAN (VXLAN) traffic loop, resulting in a denial-of-service (DoS) condition. The loop oversubscribes bandwidth on network interfaces, causing all data plane traffic to be dropped. The vulnerability has a CVSS v3.1 base score of 7.4 (AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H) and is associated with CWE-457 (Use of Uninitialized Variable).
The Cisco Security Advisory recommends manual intervention to mitigate active exploitation, including stopping the crafted traffic and flapping all involved network interfaces. For suspected Layer 2 loops related to this issue, organizations should contact the Cisco Technical Assistance Center (TAC) or their support provider. Additional details are available in the advisory at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-nxos-ether-dos-Kv8YNWZ4.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-8669
Vulnerability details
A vulnerability with the Ethernet VPN (EVPN) Layer 2 ingress packet processing of Cisco Nexus 3600 Platform Switches and Cisco Nexus 9500-R Series Switching Platforms could allow an unauthenticated, adjacent attacker to trigger a Layer 2 traffic loop. This vulnerability…
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is due to a logic error when processing a crafted Layer 2 ingress frame. An attacker could exploit this vulnerability by sending a stream of crafted Ethernet frames through the targeted device. A successful exploit could allow the attacker to cause a Layer 2 Virtual eXtensible LAN (VxLAN) traffic loop, which, in turn, could result in a denial of service (DoS) condition. This Layer 2 loop could oversubscribe the bandwidth on network interfaces, which would result in all data plane traffic being dropped. To exploit this vulnerability, the attacker must be Layer 2-adjacent to the affected device. Note: To stop active exploitation of this vulnerability, manual intervention is required to both stop the crafted traffic and flap all involved network interfaces. For additional assistance if a Layer 2 loop that is related to this vulnerability is suspected, contact the Cisco Technical Assistance Center (TAC) or the proper support provider.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Vulnerability in switch EVPN/VXLAN processing allows adjacent attacker to inject crafted frames that induce a traffic loop, directly enabling Network Denial of Service (T1498) by oversubscribing bandwidth and dropping all data-plane traffic.
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Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Directly counters the crafted-frame-induced VXLAN loop that produces the bandwidth oversubscription DoS on the affected Nexus platforms.
Boundary-protection rules can filter or rate-limit the specific L2 ingress frames an adjacent attacker must send to trigger the logic error.
Continuous monitoring of L2 traffic patterns and interface utilization can identify the onset of the VXLAN loop before full DoS occurs.