CVE-2026-76458

Executive Summary

CVE-2026-76458 is a Cisco NX-OS vulnerability class covering multiple internally discovered defects related to improper handling of exceptional conditions (CWE-703). The advisory is a roll-up entry from a Cisco internal security review and hardening release, so the specific affected components, affected versions, and per-CVE fixed releases are not enumerated in the CVE record. With a CVSS 3.1 base score of 8.6 (AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H), exploitation could be conducted remotely and unauthenticated, crossing a security scope boundary, with the primary impact being availability loss rather than confidentiality or integrity compromise.

Technical Analysis

The NVD description is intentionally high-level. It states that the Cisco NX-OS engineering team conducted an internal security review that "resulted in a software hardening release that addresses multiple internally discovered vulnerabilities" and that the vulnerabilities tracked by CVE-2026-76458 are "related to improper handling of exceptional conditions issues that are grouped under the Common Weakness Enumeration (CWE) CWE-703."

From this we can derive the following with confidence:

• Vendor/product: Cisco NX-OS (per the description text, which explicitly names NX-OS).
• Weakness class: CWE-703, Improper Check or Handling of Exceptional Conditions. This is a parent category that covers defects such as unchecked error returns, unhandled exceptions, resource-release failures on error paths, missing handling of malformed or unexpected input, and similar failure modes.
• Exposure vector: Network-accessible, requiring no privileges and no user interaction, with scope change (S:C) and high availability impact (A:H). The C/I subscores are None, meaning the vulnerabilities as scored do not primarily result in data disclosure or data modification — the dominant risk is disruption/denial of service or system instability.

Because the CVE record aggregates multiple internally discovered issues under one ID, it does not identify a specific code path, protocol handler, CLI command, management interface, or daemon. The technical mechanism must therefore be characterized at the class level: input or system conditions that the software does not handle correctly in an exceptional or error path, leading to abnormal behavior. Common manifestations of CWE-703 in network operating systems include unhandled exceptions that terminate a process, failed resource cleanup on error paths that exhaust memory or file descriptors, and mishandled malformed packets or malformed management requests that trigger a fault.

Two details matter for defenders:

1. CWE-703 is a parent weakness, not a leaf. CWE-703 is frequently used as a mapping node above more specific children such as CWE-248 (Uncaught Exception), CWE-252 (Unchecked Return Value), CWE-391 (Unchecked Error Condition), and CWE-755 (Improper Handling of Exceptional Conditions). The CVE does not state which child applies to each underlying defect, nor how many defects are aggregated.
2. Scope change (S:C) indicates impact beyond the vulnerable component's security authority. In a CVSS 3.1 context, S:C with A:H and C/I:N is consistent with a flaw that can degrade or take down a broader service or device function than the specific component that mishandles the condition.

No public proof of concept, exploit code, or Nuclei template exists for this CVE, and no CISA KEV entry is present.

Exploitation Scenarios

The record provides no specific endpoints, protocols, CLI commands, or API parameters, and no public exploit exists. Accordingly, the following is an attack chain inferred at the vulnerability-class level only. It does not describe a confirmed exploit path.

1. Reconnaissance / target identification. An adversary identifies reachable Cisco NX-OS management or data-plane surfaces. Candidates for CWE-703-class triggers include externally reachable management planes (SSH, HTTPS, SNMP, NETCONF/RESTCONF) and protocol-handling paths that accept network input. The specific surfaces affected by this CVE are not disclosed, so targeting would require either broad probing or operator-provided version/hardening-release correlation.
2. Triggering the flaw class. Because the defect is improper handling of exceptional conditions, the trigger is expected to be an input or system state that violates an assumption in an error-handling path — for example, a malformed or unexpected request, an error condition the code does not check, or a resource state the code does not clean up on failure. The goal at this stage is to drive the affected component into an unhandled condition.
3. Payload / execution method. At the class level, no code execution is required by the CVSS profile: C/I are None, so this is not scored as a code-execution or data-tampering flaw. The expected outcome of a successful trigger is an abnormal termination, hang, resource exhaustion, or degraded device function rather than arbitrary code execution or credential theft. No specific payload is known; none is inferred.
4. Post-exploitation impact. The scored impact is availability (A:H / S:C). Realistic consequences of a successful trigger are controlled or uncontrolled restart of an affected process, loss of management-plane responsiveness, or degradation of device function — potentially affecting traffic forwarding if the affected component is on the data path. Because scope change is indicated, the disruption may extend beyond the immediately vulnerable component.

Caveat: This chain is a class-level inference. No public PoC is available, and the CVE aggregates multiple undisclosed defects, so no single validated exploitation path can be presented.

Impact Assessment

• Base score: 8.6 (High) — remotely reachable, low attack complexity, no privileges, no user interaction.
• Confidentiality: None. The record does not indicate data exposure.
• Integrity: None. The record does not indicate data or configuration modification.
• Availability: High. This is the dominant scored impact and the primary operational risk.
• Scope: Changed. Impact may extend beyond the vulnerable component's security authority, meaning collateral degradation of other services or device functions is plausible.
• Affected systems: Cisco NX-OS devices. The specific platforms, product families, and release trains are not enumerated in the CVE record. Affected versions are unknown from the data provided.
• Blast radius: For network switches, availability failures on NX-OS can affect management access and, depending on the affected subsystem, traffic forwarding for connected hosts. Organizations should treat any NX-OS device with an externally reachable management or control plane as potentially in scope until Cisco's per-defect advisories clarify the mapping.
• Exploit maturity: No public exploit, no PoC, no Nuclei template, and no CISA KEV listing. Absence of public tooling reduces near-term mass-exploitation risk but does not eliminate targeted risk, since the defects were found by internal review and are therefore discoverable by other capable parties.

Detection Guidance

Detection is constrained by the fact that the specific triggers are undisclosed. The rules below are written against the vulnerability class (CWE-703 on Cisco NX-OS) using observable signals: Cisco syslog patterns, process restarts, crash/exception telemetry, and anomalous error-rate spikes from management-plane sources. Operators should tune thresholds and severity codes to their environment.

Sigma Rule

title: Cisco NX-OS Process Restart or Crash Indicating CWE-703 Exceptional Condition Handling
id: 3b6f1d2e-8a4c-4e77-9b31-0c7a5e2f9d10
status: experimental
description: Detects Cisco NX-OS syslog indications of process restart, crash, or abnormal termination that may be caused by improper handling of exceptional conditions (CWE-703).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-76458
author: valtersit.com
date: 2026/01/01
tags:
  - attack.t1190
  - cwe.703
logsource:
  product: cisco
  service: nx-os
  definition: NX-OS syslog forwarded to SIEM
detection:
  selection_restart:
    - message|contains:
        - '%SYSMGR-2-SERVICE_CRASHED'
        - '%SYSMGR-2-SERVICE_EXITED'
        - '%SYSMGR-2-LAST_CORE_BASIC_TRACE'
        - '%SYSMGR-3-SERVICE_STARTED'
        - '%SYSMGR-2-SERVICE_RESTARTED'
  selection_fault:
    - message|contains:
        - 'exception'
        - 'core dump'
        - 'unhandled'
        - 'traceback'
        - 'assertion failed'
  filter_maintenance:
    message|contains:
      - 'scheduled'
      - 'reload'
      - 'upgrade'
  condition: (selection_restart or selection_fault) and not filter_maintenance
falsepositives:
  - Planned software upgrades and reloads
  - Legitimate process restarts during maintenance windows
level: high

YARA Rule

rule CVE_2026_76458_NXOS_Exceptional_Condition_Crash
{
    meta:
        description = "Detects NX-OS core/trace artifacts consistent with improper handling of exceptional conditions (CWE-703) as described in CVE-2026-76458"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-76458"
        author = "valtersit.com"
        date = "2026-01-01"
        cwe = "CWE-703"
        severity = "high"

    strings:
        $nxos = "NX-OS" ascii wide nocase
        $crash1 = "%SYSMGR-2-SERVICE_CRASHED" ascii wide
        $crash2 = "%SYSMGR-2-LAST_CORE_BASIC_TRACE" ascii wide
        $exc1 = "uncaught exception" ascii wide nocase
        $exc2 = "unhandled exception" ascii wide nocase
        $exc3 = "terminate called after throwing an instance" ascii wide nocase
        $core = "core dump" ascii wide nocase
        $sym1 = "SERVICE_EXITED" ascii wide
        $sym2 = "SERVICE_RESTARTED" ascii wide

    condition:
        filesize < 10MB and
        (
            (any of ($crash*) and $nxos) or
            (2 of ($exc*) and 1 of ($core, $sym*)) or
            ($core and $nxos)
        )
}

Suricata IDS Rule

alert tcp $EXTERNAL_NET any -> $HOME_NET [22,443,830] (msg:"CVE-2026-76458 Cisco NX-OS Potential CWE-703 Exceptional Condition Trigger Attempt"; flow:to_server,established; content:"|00 00 00|"; offset:0; depth:4; threshold:type both, track by_src, count 20, seconds 60; classtype:attempted-dos; reference:cve,2026-76458; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-76458; metadata:created_at 2026_01_01, updated_at 2026_01_01, cve CVE_2026_76458; sid:202676458; rev:1;)

Note: The offset/depth/content match above is a generic session-framing placeholder because the specific malformed request that triggers the underlying defects is not publicly documented. This rule should be treated as a template requiring tuning against captured traffic from authorized testing, not as a validated signature.

Elastic Detection

sequence by host.id with maxspan=5m
  [ any where event.category == "process" and
      process.name : ("sysmgr", "netstack", "ethpm", "vshd", "ntpd") and
      event.action == "start" ]
  [ any where event.category in ("process", "file") and
      (process.name : ("sysmgr", "netstack", "ethpm", "vshd", "ntpd") or file.name : "*.core") and
      event.outcome == "failure" ]

Splunk SPL Query

index=network sourcetype=cisco:nxos 
| where match(_raw, "(?i)(%SYSMGR-\d-SERVICE_(CRASHED|EXITED|RESTARTED)|unhandled exception|uncaught exception|core dump|assertion failed)")
| rex field=_raw "(?<severity>%\w+-\d-\w+)"
| stats count min(_time) as first_seen max(_time) as last_seen values(severity) as severity by host
| 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

Wazuh OSSEC Rule

<group name="cisco,nx-os,cve-2026-76458,">
  <rule id="2026764" level="10">
    <decoded_as>cisco-nxos</decoded_as>
    <match>%SYSMGR-2-SERVICE_CRASHED|%SYSMGR-2-SERVICE_EXITED|%SYSMGR-2-LAST_CORE_BASIC_TRACE</match>
    <description>CVE-2026-76458: Cisco NX-OS process crash or exit consistent with improper handling of exceptional conditions (CWE-703)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_76458,availability_impact,</group>
  </rule>

  <rule id="2026765" level="8">
    <decoded_as>cisco-nxos</decoded_as>
    <match>unhandled exception|uncaught exception|core dump|assertion failed</match>
    <description>CVE-2026-76458: Cisco NX-OS exceptional condition or fault indicator observed</description>
    <group>cve_2026_76458,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence is 0/4 (UNPATCHED — no patch evidence from any tracked source). No fixed version, no vendor workaround, and no affected-version range was published in the data reviewed for this CVE.

1) Immediate mitigations (in the absence of a confirmed fix):

• Restrict network access to NX-OS management planes. Permit SSH, HTTPS, SNMP, NETCONF/RESTCONF, and any administrative interfaces only from dedicated out-of-band management networks and jump hosts. This directly reduces exposure given the AV:N / PR:N / UI:N profile.
• Apply management-plane ACLs and control-plane policing/CoPP policies to rate-limit and filter traffic to device control and management planes.
• Disable unused management services and protocol features; remove plaintext management protocols (Telnet, HTTP) entirely.
• Forward NX-OS syslog and control-plane telemetry to a central SIEM and alert on process restart, crash, and exception patterns (see Detection Guidance).
• Consult Cisco's published NX-OS hardening and security advisory portal for the specific defects aggregated under this CVE, since the CVE record itself does not map the ID to a fixed release.

2) Patch / upgrade steps:

• Because this CVE aggregates internally discovered defects addressed in a hardening release, the remediation path is to apply the Cisco NX-OS software release that contains the corresponding fixes. The specific version is not identified in the CVE data provided.
• Operators should correlate the applicable Cisco NX-OS security advisory with their installed release train, validate the target release in a lab, and schedule upgrades within a change window. Take configuration backups and verify rollback procedures before upgrade.

3) Long-term hardening:

• Maintain a rigorous NX-OS version inventory and subscribe to Cisco security advisories so that roll-up CVE entries can be mapped to concrete fixed releases.
• Isolate management, control, and data planes; enforce least-privilege AAA and TACACS+/RADIUS for administrative access.
• Enable and retain crash/core and process-restart telemetry long enough to support incident reconstruction.
• Periodically review error-handling and resource-cleanup behavior on critical processes through vendor engagement and internal testing.

References

• NVD — CVE-2026-76458: https://nvd.nist.gov/vuln/detail/CVE-2026-76458
• MITRE CWE-703 — Improper Check or Handling of Exceptional Conditions: https://cwe.mitre.org/data/definitions/703.html
• Cisco Security Advisories index (NX-OS): https://sec.cloudapps.cisco.com/security/center/publicationListing.x
• Cisco NX-OS Software Security Advisories: https://www.cisco.com/c/en/us/support/ios-nx-os-software/products-security-advisories-list.html
• CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Exploitation-in-the-wild status: No CISA KEV entry, no public PoC, no exploit code, and no Nuclei template were identified for CVE-2026-76458. No threat actor campaign referencing this CVE was found in the data provided.