CVE-2026-76456

Executive Summary

CVE-2026-76456 is a high-severity (CVSS 8.6) vulnerability affecting Cisco NX-OS, the network operating system used across Cisco's data center switching and Nexus platform portfolio. The flaw stems from improper input validation of special elements used in a command (CWE-20), and according to the vendor, it was discovered during a comprehensive internal security review that produced a software hardening release addressing multiple related issues. The CVSS vector indicates network-reachable exploitation with low complexity and no authentication or user interaction required, and critically, a Scope change (S:C) with a High Availability impact (A:H) — meaning an attacker can potentially disrupt the availability of resources beyond the vulnerable component itself. No public PoC, exploit code, or Nuclei template is currently available, and no patch evidence exists from any tracked source, leaving affected deployments exposed.

Technical Analysis

CVE-2026-76456 is classified under CWE-20 (Improper Input Validation), specifically described in the NVD entry as related to "improper input validation of special elements used in a command." This description points to a command-injection or command-parsing class of vulnerability, where user-controllable or externally-supplied input containing special elements is processed by a command handler without adequate sanitization. The Cisco NX-OS engineering team identified this issue alongside "multiple internally discovered vulnerabilities" during a proactive internal security review, and the resulting fix is bundled into a "software hardening release."

The CVSS v3.1 vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H provides important context:

• AV:N (Network) — the vulnerability is exploitable over a network vector, not requiring local access or physical adjacency.
• AC:L (Low) — no specialized conditions or race windows are needed; the attack is straightforward to execute.
• PR:N (None) — no authentication is required. An unauthenticated network attacker can trigger the flaw.
• UI:N (None) — no user interaction is required, enabling fully automated exploitation.
• S:C (Scope Changed) — the impact extends beyond the vulnerable component's security authority. This is significant for a network OS: a compromise in one processing context may affect the management plane, control plane, or other system resources.
• C:N / I:N (No Confidentiality or Integrity impact) — the vulnerability does not directly result in data disclosure or modification.
• A:H (High Availability impact) — the primary impact is a complete loss of availability, consistent with a denial-of-service outcome (e.g., process crash, device reload, or service hang).

Taken together, the vector describes an unauthenticated, remotely triggerable denial-of-service condition against Cisco NX-OS devices, where the special-element input validation failure allows malformed input to reach a command-processing routine and cause availability loss that propagates beyond the isolated component.

Notably absent from the available data: specific affected version ranges, the exact NX-OS subsystem or command parser involved, and any CVE-specific Cisco Security Advisory ID. These details must be obtained from the vendor's official channel. The description provided is a generalized hardening-release statement rather than a granular technical advisory, so the analysis here is deliberately bounded to the vulnerability class and the CVSS-derived attack characteristics.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template currently exists for CVE-2026-76456, and the vendor description does not name the specific command, protocol, or interface involved. The following attack chain is therefore inferred from the vulnerability class (CWE-20, command-related input validation) and the CVSS vector — it does not assert specific endpoints, function names, or payloads that are not present in the source data.

1. Reconnaissance / Target Identification. An attacker first identifies reachable Cisco NX-OS devices. Nexus switches are commonly exposed on management networks and, in some deployments, on interfaces reachable from broader network segments. Shodan queries such as product:"Cisco NX-OS" or certificate-based pivots (ssl.cert.subject.cn:"cisco") can enumerate candidate hosts. Because the CVSS vector is AV:N/PR:N, the attacker needs only network reachability — no valid credentials.

2. Triggering the Flaw. The attacker sends crafted input containing special elements to a network-facing command-processing path in NX-OS. Per the CWE-20 description, the flaw is that these special elements are not properly validated before being used in a command. The attack does not require authentication or user interaction, so the crafted input can be delivered directly from an unauthenticated network position to the affected service.

3. Execution Mechanism (class-level). At the vulnerability-class level, improper validation of special elements in a command handling context typically means that attacker-supplied special characters or structural tokens are interpreted rather than sanitized. Depending on the subsystem, this can cause the command processor to malfunction, terminate, or enter an invalid state. The absence of specific advisory detail means the precise protocol and payload syntax cannot be stated; exploitation would be built by fuzzing or by reverse-engineering the affected parser once the fixed release is diffed against the vulnerable one.

4. Post-Exploitation Impact. Given the CVSS subscores (C:N/I:N/A:H, S:C), the realistic outcome is availability loss: a crash, hang, or reload of an NX-OS process or the device itself, potentially affecting adjacent resources due to the scope change. For a data-center switch, that translates to loss of forwarding for attached workloads, management-plane unavailability, and potential cascade effects across the fabric. Integrity and confidentiality are not directly impacted by this CVE, so it is best characterized as a remote denial-of-service / availability weapon rather than a data-exfiltration vector.

Because no PoC is public, defenders should assume the exploit is either private or yet to emerge, but plan for rapid weaponization given the low attack complexity and lack of authentication.

Impact Assessment

The business impact of CVE-2026-76456 is centered on availability of critical network infrastructure:

• Blast radius (S:C — Scope Changed): Because the scope is changed, the impact is not confined to the vulnerable component. In a network OS, a single disrupted process or parser can degrade management access, control-plane stability, or forwarding behavior for other resources, amplifying the outage.
• Availability (A:H): A total loss of availability for the affected component is achievable. In a Nexus deployment, this can mean a switch or its management interface becomes unreachable, interrupting data-plane or control-plane functions.
• Confidentiality/Integrity (C:N/I:N): The CVE does not directly enable data theft or tampering, which places it in the denial-of-service category rather than a breach category.
• Exposure surface (AV:N/AC:L/PR:N/UI:N): Any NX-OS device reachable over the network is potentially in scope. No credentials or user action are needed, so exposure scales with the number of reachable devices — a serious concern for large data-center fabrics.
• Affected systems: Cisco NX-OS platforms (Nexus switching family). Exact version ranges are not disclosed in the available data, so organizations must consult the vendor hardening release notes to determine whether their specific builds are affected.
• Patching posture: With patch confidence at 0/4 and no tracked patch evidence, affected organizations cannot rely on a published fix from the available intelligence alone. A vendor hardening release is referenced but not enumerated, so verification against Cisco's official advisory and release notes is mandatory.

Operationally, an unauthenticated remote DoS against data-center switches can halt tenant workloads, break storage and compute connectivity, and generate incident-response costs disproportionate to the simplicity of the attack.

Detection Guidance

Sigma Rule

title: Cisco NX-OS Improper Input Validation - Availability Disruption (CVE-2026-76456)
id: 8f3c1d2a-4b7e-4c9a-9d1f-2a6e5b8c7d40
status: experimental
description: Detects crash, reload, or abnormal termination events on Cisco NX-OS indicative of exploitation of CWE-20 improper input validation (CVE-2026-76456).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-76456
author: valtersit.com
date: 2026/01/01
logsource:
  product: cisco
  service: nxos
  definition: Requires NX-OS syslog forwarded to SIEM.
detection:
  selection_reload:
    - "Reload requested"
    - "System reloading"
    - "kernel panic"
    - "process crashed"
  selection_commands:
    - "validate"
    - "invalid command"
    - "input validation"
    - "malformed"
  condition: selection_reload or selection_commands
falsepositives:
  - Legitimate administrative reloads
  - Scheduled maintenance
level: high
tags:
  - attack.t1190
  - cve.2026.76456

YARA Rule

rule CVE_2026_76456_NXOS_Input_Validation
{
    meta:
        description = "Detects strings on Cisco NX-OS artifacts consistent with CWE-20 command input validation issues (CVE-2026-76456)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-76456"
        date = "2026-01-01"
        severity = "high"

    strings:
        $nxos1 = "NX-OS" ascii wide nocase
        $nxos2 = "Cisco Nexus" ascii wide nocase
        $err1  = "invalid command" ascii wide nocase
        $err2  = "input validation" ascii wide nocase
        $err3  = "malformed" ascii wide nocase
        $err4  = "process crashed" ascii wide nocase
        $err5  = "unexpected input" ascii wide nocase

    condition:
        (uint16(0) == 0x5A4D or uint32(0) == 0x7F454C46) and
        (any of ($nxos*)) and
        (2 of ($err*))
}

Suricata IDS Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"CVE-2026-76456 Cisco NX-OS Improper Input Validation Exploit Attempt"; flow:to_server,established; content:"NX-OS"; nocase; pcre:"/(invalid\s+command|malformed\s+input|input\s+validation)/i"; threshold:type both, track by_src, count 5, seconds 60; classtype:attempted-dos; reference:cve,CVE-2026-76456; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-76456; sid:90076456; rev:1; metadata:created_at 2026_01_01, updated_at 2026_01_01;)

Elastic Detection

sequence by host.id with maxspan=2m
  [ any where event.category == "network" and
      (stringContains(network.protocol, "tcp") or stringContains(network.protocol, "http")) and
      network.direction == "inbound" ]
  [ process where event.action in ("crash", "terminated-signal", "reload") and
      process.name != null and
      (stringContains(process.name, "nxos") or stringContains(process.name, "cisco")) ]

Splunk SPL Query

index=network sourcetype=cisco:nxos OR sourcetype=cisco:asa
| search ("invalid command" OR "malformed" OR "input validation" OR "process crashed" OR "reload")
| stats count min(_time) as first_seen max(_time) as last_seen by host src_ip
| where count > 3
| eval first_seen=strftime(first_seen,"%Y-%m-%d %H:%M:%S"), last_seen=strftime(last_seen,"%Y-%m-%d %H:%M:%S")
| sort - count
| table host src_ip count first_seen last_seen

Wazuh OSSEC Rule

<group name="cisco,nxos,">
  <rule id="100456" level="12">
    <decoded_as>cisco-nxos</decoded_as>
    <match>invalid command|input validation|malformed input|process crashed|system reloading</match>
    <description>CVE-2026-76456: Potential Cisco NX-OS improper input validation exploitation (DoS)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <options>no_full_log</options>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations.

   • Restrict network reachability to NX-OS management and control interfaces to trusted administrative networks only. An unauthenticated, network-reachable flaw (AV:N/PR:N) is most effectively contained by reducing exposure surface.
   • Apply management-plane ACLs and out-of-band management segregation so that NX-OS devices cannot be reached by untrusted or broad internal segments.
   • Monitor NX-OS syslog for crash, reload, and malformed-input events and alert on anomalies (see detection guidance above).
   • Because the vendor references a "software hardening release" addressing internally discovered vulnerabilities, confirm the exact fixed release with Cisco TAC and apply it as soon as it is verified for your platform.

2. Patch/upgrade steps.

   • Consult Cisco's official security advisory and NX-OS release notes to identify the specific fixed build for CVE-2026-76456 and confirm which of your device versions are affected. Do not assume a version range — none is provided in the available data.
   • Stage the hardening release in a lab against your running configuration and feature set before production rollout.
   • Maintain a current NX-OS baseline image inventory so remediation can be tracked per device.

3. Long-term hardening.

   • Formalize a recurring firmware/image review process for network operating systems, treating them as first-class patch targets rather than appliances.
   • Enforce authentication and authorization on all management interfaces; although this CVE requires no authentication, defense-in-depth reduces the impact of adjacent weaknesses.
   • Adopt configuration backup, integrity verification, and rapid rollback capabilities to shorten recovery time from availability-impacting events.
   • Track Cisco security advisories and internal hardening releases through a subscribed vulnerability feed to close the gap where CVE descriptions are generalized.

References

• NVD — CVE-2026-76456: https://nvd.nist.gov/vuln/detail/CVE-2026-76456
• MITRE CWE-20 — Improper Input Validation: https://cwe.mitre.org/data/definitions/20.html
• Cisco Security Advisories (vendor hardening / advisory portal): https://sec.cloudapps.cisco.com/security/center/publicationListing.x
• FIRST EPSS (not yet scored for this CVE at time of writing): https://www.first.org/epss/
• CISA Known Exploited Vulnerabilities Catalog (no KEV entry as of this report): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaign or in-the-wild exploitation evidence for CVE-2026-76456 is currently available in the provided data. No public PoC or exploit code exists.