CVE-2026-76486

Executive Summary

CVE-2026-76486 is a critical improper input validation vulnerability (CWE-121, stack-based buffer overflow class) in the VXLAN Operation, Administration, and Maintenance (OAM) feature of Cisco NX-OS Software, referred to as NGOAM. An unauthenticated, remote attacker can trigger the flaw by sending crafted IP packets to an IP interface on an affected device when NGOAM is enabled, potentially achieving arbitrary code execution with root privileges or causing repeated process crashes that force a device reload (Denial-of-Service). With a CVSS 3.1 base score of 9.8 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H), this is a network-reachable, pre-authentication flaw offering full confidentiality, integrity, and availability impact on impacted switches. Data center and campus networks running Cisco NX-OS platforms with the NGOAM feature active are in scope; no public PoC or exploit code is currently known, and no patch has been identified from tracked sources.

Technical Analysis

The vulnerability resides in Cisco NX-OS's implementation of VXLAN OAM — the operational tooling that provides fault management, performance monitoring, and connectivity verification for VXLAN overlay fabrics. Cisco packages this capability under the NGOAM feature name. The NVD description states plainly that the defect is improper input validation of IP traffic when the NGOAM feature is enabled, and that it is classified under CWE-121 (Stack-based Buffer Overflow).

That classification is the key architectural fact. CWE-121 errors occur when a program copies attacker-influenced data into a fixed-size stack buffer without enforcing length or content constraints, allowing data to overwrite adjacent stack memory — including saved return addresses and control data. Here, the input vector is IP traffic delivered to an IP interface on the device. Because the packets are processed by NGOAM code before any authentication check, the attacker requires no credentials and no user interaction: the vulnerable parser is reachable directly from the network.

The attack surface is therefore the management/data plane of any NX-OS device with NGOAM enabled and an IP-reachable interface. The pre-conditions are narrow but consequential: the attacker only needs IP reachability to the device plus the NGOAM feature being active. The description does not disclose the specific packet structure, header fields, or protocol encapsulation used to reach the vulnerable parser, nor the specific function or buffer involved, so the exact trigger syntax is not publicly documented. What is authoritative from the data available:

• Weakness class: Stack-based buffer overflow (CWE-121) from improper input validation.
• Trigger: Crafted IP packets sent to an IP interface, processed by NGOAM.
• Authentication: None required (PR:N in the CVSS vector).
• Complexity: Low (AC:L) — no race or special conditions, only feature enablement.
• Outcomes: Two distinct, both severe — arbitrary code execution as root, or process crash leading to device reload and DoS.

The dual outcome is characteristic of memory corruption in an embedded network OS: a carefully shaped payload can hijack control flow in a process running with root privileges, while a malformed variant simply crashes the process, and if the crashed process is critical to the supervisor, an automatic reload follows. The fact that exploitation yields root privileges directly — rather than requiring a post-exploitation privilege escalation — means the vulnerable parser executes in a highly privileged context, substantially raising the blast radius. Insufficient technical detail is available to name the exact function, buffer, or NX-OS subsystem component beyond "NGOAM."

Exploitation Scenarios

No public PoC or exploit code exists for CVE-2026-76486. The attack chain below is inferred strictly from the vulnerability class (CWE-121) and the NVD description's stated trigger and impact; no implementation-specific function names, endpoints, or parameters are asserted.

1. Reconnaissance / target identification. An attacker first identifies reachable Cisco NX-OS devices. Because the flaw lives in the NGOAM path, the attacker is scanning for Cisco data center and campus switches with IP-reachable management or routed interfaces. Device fingerprinting via Shodan or direct probing (product:"Cisco NX-OS", VXLAN port 4789 exposure, HTTPS management titles) narrows the candidate set. The attacker does not need credentials, so any internet- or adjacent-network-exposed IP interface is a candidate if NGOAM is enabled.

2. Triggering the flaw. Using the vulnerability's class — improper input validation causing a stack-based buffer overflow — the attacker sends crafted IP packets to an IP interface on the target. The NGOAM packet-handling path parses this input without adequate bounds/length validation, causing attacker-controlled data to exceed a fixed-size stack buffer. Because the CVSS vector records AC:L and PR:N, this trigger requires no unusual timing and no authentication.

3. Execution / payload delivery. For the code-execution outcome, the overflow must be shaped so that the overwritten stack data alters control flow in a privileged process. In CWE-121 exploitation generally, the attacker supplies an oversized field whose bytes land on saved control data, redirecting execution to attacker-chosen instructions or causing the parser to act on attacker-supplied structures. Because the resulting context is root, the attacker does not need a second-stage local privilege escalation. For the DoS outcome, no precision is required: any crash-inducing malformed packet suffices.

4. Post-exploitation impact. A successful code-execution exploit grants root on the NX-OS device. From there, an adversary can manipulate network configuration, intercept or redirect traffic traversing the switch, install persistence, pivot deeper into the fabric, or simply crash critical processes. The DoS variant forces a reload, disrupting all traffic through the device and any services dependent on it. The combination yields maximal CIA impact (C:H/I:H/A:H) on the affected device and significant downstream network availability risk.

Because no exploit is public, defenders should treat this as a latent, high-severity pre-auth network vector whose weaponization risk is real but currently unconfirmed.

Impact Assessment

CVE-2026-76486 carries a CVSS 3.1 base score of 9.8 (Critical) with the vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Every subscore reinforces severity:

• AV:N (Network) — exploitable across a network path; no local or adjacent-only restriction.
• AC:L (Low) — no special conditions beyond NGOAM being enabled.
• PR:N (None) — no authentication, so any reachable attacker qualifies.
• UI:N (None) — no victim interaction.
• S:U (Unchanged) — impact is scoped to the vulnerable component, but since that component runs as root on a network device, scoped impact is still total for the device.
• C:H / I:H / A:H — complete loss of confidentiality, integrity, and availability on the affected device.

Business impact is concentrated in network infrastructure. NX-OS platforms typically sit at the core or aggregation layer of data centers, campus fabrics, and service provider edge environments. A root-level compromise of such a device is a high-value foothold: the device is a trust anchor for traffic in transit and often holds configuration, secrets, and routing state. DoS-induced reloads cause fabric-wide outages affecting every workload and user behind the switch. Blast radius includes all networks transiting the device, plus any management planes reachable from it. EPSS has not yet scored this CVE, and Cisco's NGOAM feature is not universally enabled — organizations should confirm exposure. No CISA KEV listing, no public PoC, and no Nuclei template exist as of this writing, which tempers immediate urgency but does not reduce the severity should exploitation emerge.

Detection Guidance

Detecting CVE-2026-76486 requires both network-layer telemetry (crafted packets toward NX-OS interfaces) and host/device-layer telemetry (process crashes, reloads, unexpected root activity). The rules below are pattern-based starting points; because no public exploit exists, thresholds and payload signatures should be tuned against a controlled NGOAM test bed.

Sigma Rule

title: Cisco NX-OS NGOAM Process Crash and Unexpected Reload (CVE-2026-76486)
id: 7f3c2a91-6b1e-4d8a-9c2f-1e5a7b3c9d40
status: experimental
description: Detects NX-OS NGOAM process crashes, core dumps, and unexpected reloads consistent with exploitation of CVE-2026-76486.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-76486
author: Valters IT Threat Intelligence
date: 2026/01/01
logsource:
  product: cisco
  service: nxos
detection:
  selection_oam:
    message|contains:
      - 'NGOAM'
      - 'ngoam'
  selection_crash:
    message|contains:
      - 'core dump'
      - 'process crashed'
      - 'segmentation fault'
      - 'SIGSEGV'
      - 'unexpected reload'
      - 'system reload'
  condition: selection_oam and selection_crash
falsepositives:
  - Legitimate NGOAM fault-injection testing
  - Known software defects unrelated to CVE-2026-76486
level: high
tags:
  - attack.t1190
  - cve.2026.76486

YARA Rule

rule CVE_2026_76486_NXOS_NGOAM_Overflow
{
    meta:
        author = "Valters IT Threat Intelligence"
        description = "Heuristic detection of NGOAM-related artifacts or crash dumps referencing CVE-2026-76486 exploitation"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-76486"
        date = "2026-01-01"
        severity = "critical"

    strings:
        $ngoam1 = "NGOAM" ascii wide nocase
        $ngoam2 = "ngoam" ascii wide nocase
        $oam1   = "VXLAN" ascii wide nocase
        $crash1 = "segmentation fault" ascii nocase
        $crash2 = "core dump" ascii nocase
        $crash3 = "SIGSEGV" ascii nocase

    condition:
        (any of ($ngoam*)) and (2 of ($oam1, $crash*))
}

Suricata IDS Rule

alert ip any any -> any any (msg:"CVE-2026-76486 Cisco NX-OS NGOAM Crafted Packet Potential Buffer Overflow Attempt"; flow:to_server; dsize:>1200; content:"|00 50 56|"; depth:2; threshold:type limit, track by_src, count 5, seconds 10; classtype:attempted-admin; reference:cve,2026-76486; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-76486; sid:202676486; rev:1;)

Note: Because the exact NGOAM payload signature is not public, this rule intentionally targets the vulnerability class — large anomalous packets toward NX-OS VXLAN/OAM endpoints — and requires tuning. Deploy against traffic destined to NX-OS management/protected IP interfaces, ideally scoped by host group.

Elastic Detection

sequence by host.id with maxspan=5m
  [ any where process.name : "*ngoam*" or message : "*NGOAM*" ]
  [ any where
      (message : "*segmentation fault*" or message : "*core dump*" or
       message : "*SIGSEGV*" or message : "*unexpected reload*" or
       message : "*system reload*")
  ]

Splunk SPL Query

index=network sourcetype="cisco:nxos" ("NGOAM" OR "ngoam")
| search ("core dump" OR "segmentation fault" OR "SIGSEGV" OR "unexpected reload" OR "process crashed" OR "reload")
| stats count min(_time) as first_seen max(_time) as last_seen values(message) as evidence by host
| where count >= 1
| eval severity="critical", cve="CVE-2026-76486"
| sort - count

Wazuh OSSEC Rule

<group name="cisco,nxos,cve-2026-76486,">
  <rule id="202676486" level="14">
    <decoded_as>nxos</decoded_as>
    <match>NGOAM|ngoam</match>
    <match>core dump|segmentation fault|SIGSEGV|unexpected reload|process crashed|system reload</match>
    <description>CVE-2026-76486: Possible Cisco NX-OS NGOAM exploitation - NGOAM process crash/reload detected</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_76486,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch, fixed version, vendor advisory reference, workaround, or affected-version range is available from any tracked source for CVE-2026-76486.

1. Immediate mitigations (compensating controls).

   • Disable NGOAM if not operationally required. The vulnerability is only reachable when the NGOAM feature is enabled. Where VXLAN OAM is not actively used for fault management or performance monitoring, disable the feature to remove the attack surface entirely. Validate this against operational dependencies before applying.
   • Restrict IP reachability to device interfaces. Apply infrastructure ACLs and control-plane policing so that only trusted management subnets can send IP traffic to NX-OS interfaces. This reduces the unauthenticated remote exposure (PR:N, AV:N) that makes the flaw so severe.
   • Segment the management plane. Ensure management and routed interfaces are not exposed to untrusted networks, the internet, or lateral east-west paths from compromised workloads.
   • Monitor for crashes and reloads. Deploy the detection content above to catch NGOAM process crashes, core dumps, and unexpected reloads, which are the observable DoS outcome and may also precede or accompany code-execution attempts.
   • Rate-limit and filter anomalous packets toward NX-OS VXLAN/OAM endpoints as an interim network control (recognizing this is heuristic, given no public payload signature).

2. Patch / upgrade steps. No fixed release is identified. Track Cisco's official NX-OS security advisories directly and apply the vendor-supplied fixed release as soon as it is published. Because patch confidence is currently 0/4, do not assume any specific version is fixed based on this report. Maintain an accurate inventory of NGOAM-enabled devices so remediation can be applied rapidly when a patch lands.

3. Long-term hardening.

   • Maintain a hardware/software inventory of all NX-OS platforms and the enabled feature set (including NGOAM) so exposure is continuously visible.
   • Adopt a least-reachability posture for all network infrastructure: no unnecessary internet exposure of management or data-plane interfaces.
   • Subscribe to Cisco security advisories and integrate CVE feeds into vulnerability management for network devices, which are frequently under-instrumented relative to server fleets.
   • Establish configuration baselines and change control for optional features such as NGOAM, so risky features are only enabled where justified.
   • Prepare an incident response runbook for compromised network devices (root-level switch compromise), covering containment, configuration extraction, and hardware re-flashing.

References

1. NVD — CVE-2026-76486: https://nvd.nist.gov/vuln/detail/CVE-2026-76486
2. MITRE CWE-121 — Stack-based Buffer Overflow: https://cwe.mitre.org/data/definitions/121.html
3. MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
4. Cisco Security Advisories (vendor advisory portal — monitor for the CVE-2026-76486 fix): https://www.cisco.com/c/en/us/support/security-advisories.html
5. CISA Known Exploited Vulnerabilities Catalog (status as of this report — CVE-2026-76486 not listed; verify current state): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat actor / exploitation status: No public PoC, exploit code, or Nuclei template exists. CISA KEV listing is negative. No specific threat actor campaign or in-the-wild exploitation evidence is available from the provided data. The attack chain remains theoretical, inferred from the CWE-121 class and the NVD description; this status should be re-verified against Cisco advisories and CISA KEV on an ongoing basis.