CVE-2026-76459

Executive Summary

CVE-2026-76459 is a group of out-of-bounds write vulnerabilities (CWE-787) affecting Cisco NX-OS, discovered during a Cisco-internal security review and addressed in a hardening release. The vulnerabilities carry a CVSS 3.1 base score of 8.8 (High) and require network access with low privileges to exploit, resulting in high confidentiality, integrity, and availability impact. Organizations running Cisco NX-OS on data-center switches and fabric infrastructure should treat this as a high-priority remediation item, particularly given that NX-OS frequently controls core network segmentation and management plane functions.

Technical Analysis

CVE-2026-76459 is categorized under CWE-787: Out-of-bounds Write. This class of vulnerability occurs when software writes data past the end of an allocated buffer, corrupting adjacent memory. In system software such as a network operating system, out-of-bounds writes can lead to memory corruption, process crashes, denial of service, or — depending on the memory layout and attacker control over the written data — arbitrary code execution in the context of the vulnerable process.

The Cisco advisory language states that the issue stems from an internal security review that "resulted in a software hardening release that addresses multiple internally discovered vulnerabilities," and that the tracked issues are "related to out-of-bounds write issues" grouped under CWE-787. This framing indicates a hardening effort covering multiple root causes rather than a single atomic flaw, which is common when a vendor conducts systematic memory-safety review of a large codebase.

Attack surface considerations for Cisco NX-OS are significant. NX-OS exposes multiple network-reachable services on data-center switches, including management interfaces such as SSH, HTTPS-based management (NX-OS GUI / DCNM integrations), SNMP, NTP, and routing protocol daemons (BGP, OSPF, EIGRP, IS-IS) that exchange data with peer devices. An out-of-bounds write in any parser processing network-supplied input — management protocol data, routing updates, or crafted packets — would be reachable remotely. The CVSS vector (AV:N/AC:L/PR:L/UI:N) reflects network reachability, low attack complexity, low privileges required, and no user interaction. The PR:L component indicates that an attacker needs some level of access, such as valid low-privileged credentials or a position as an authenticated protocol peer, rather than being fully unauthenticated.

Because the NVD description does not identify the specific daemon, parser, or input channel involved, the precise trigger condition cannot be stated. What is derivable is the vulnerability class: a memory-safety defect in which insufficient bounds validation allows a write beyond an allocated buffer. No specific function names, CLI commands, URLs, or API paths can be confirmed from the available data.

Exploitation Scenarios

No public Proof-of-Concept, Nuclei template, or exploit code is available for CVE-2026-76459. The following attack chain is described at the vulnerability-class level and inferred from the CVSS vector and CWE classification. Specific implementation details are not available.

1. Reconnaissance / target identification. An adversary identifies reachable Cisco NX-OS devices. This is typically achieved by scanning for NX-OS management interfaces (SSH on TCP/22, HTTPS management on TCP/443, SNMP on UDP/161) or by observing routing protocol adjacencies. Because the vector is AV:N/PR:L, the attacker needs either low-privileged access to a management interface or a position that allows interaction with an exposed service. Shodan or internal asset inventories would surface internet-facing and internal NX-OS hosts.

2. Triggering the out-of-bounds write. The attacker sends input to a network-reachable service on the NX-OS device that is processed by code containing the memory-safety defect. Given CWE-787, the crafted input causes a write past the bounds of an allocated buffer. The AC:L rating suggests the trigger does not require unusual conditions or a race; a straightforward crafted request or protocol message is sufficient. The exact service and payload format are not disclosed in the available data.

3. Payload / execution method. At the vulnerability-class level, exploitation of an out-of-bounds write proceeds by controlling or corrupting memory adjacent to the target buffer. Depending on what is overwritten (function pointers, return addresses, control structures), the outcome ranges from a crash (DoS) to control-flow hijack and code execution within the privileges of the affected process. Since the NVD description provides no code-level detail, no specific exploitation primitive, offset, or gadget can be asserted.

4. Post-exploitation impact. If code execution is achieved, an attacker gains a foothold on the switch's control plane. From a compromised NX-OS device, an adversary could manipulate routing tables, intercept or redirect traffic, alter ACLs, disable logging, or pivot into adjacent network segments. Because NX-OS devices sit at the center of data-center fabrics, compromise of one device often provides visibility into, or influence over, a large segment of east-west and north-south traffic — consistent with the CVSS subscores of high confidentiality, integrity, and availability impact.

Impact Assessment

The CVSS 3.1 vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H yields a base score of 8.8 (High). Subscore interpretation:

• Attack Vector: Network (N) — exploitable remotely over the network, not requiring physical or adjacent access.
• Attack Complexity: Low (L) — no specialized conditions or significant attacker effort beyond normal exploitation.
• Privileges Required: Low (L) — the attacker needs some low-level access, such as valid low-privileged credentials or reachable protocol interaction, not administrative rights.
• User Interaction: None (N) — no victim action is required.
• Scope: Unchanged (U) — the impact remains within the security authority of the vulnerable component.
• Confidentiality / Integrity / Availability: High (H/H/H) — all three pillars are assessed as fully impacted. Availability impact is expected from an out-of-bounds write that can crash a process (device reload or service outage); integrity and confidentiality impact reflect the potential for code execution and traffic manipulation.

Blast radius. Cisco NX-OS powers Nexus-series data-center switches and is widely deployed in core, aggregation, and spine-leaf fabrics. A compromised device may serve as a vantage point for monitoring, modifying, or disrupting traffic traversing that fabric. Given the likely management-plane or protocol-parser attack surface, affected systems include any internet- or network-exposed NX-OS device reachable by the attacker. Cisco has not disclosed specific affected product SKUs or version ranges in the available data, so asset owners must consult Cisco's official advisory for the authoritative affected-platform list.

EPSS has not yet been scored by FIRST.org for this CVE, so no probability estimate is available. There is no CISA KEV entry, no known exploit-in-the-wild, and no public PoC at the time of writing — but the absence of public exploitation does not reduce the severity of an unpatched, remotely reachable memory-corruption flaw.

Detection Guidance

The following detection content targets the vulnerability class (out-of-bounds write / memory corruption in a Cisco NX-OS management or protocol service) and the reconnaissance and post-exploitation behaviors associated with it. Because no specific trigger signature is public, these rules are heuristic and should be tuned to the environment.

Sigma Rule

title: Cisco NX-OS Potential Memory Corruption / Service Crash Following Suspicious Management Traffic
id: 3f1a9c2e-7645-4b21-9e8a-1cve76459aa
status: experimental
description: Detects NX-OS process crashes or abnormal service restarts that may indicate exploitation of an out-of-bounds write (CVE-2026-76459).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-76459
author: Threat Intelligence Team
date: 2026/01/01
logsource:
  product: cisco
  service: nxos
detection:
  selection_crash:
    message|contains:
      - 'process crashed'
      - 'core dump'
      - 'kernel panic'
      - 'segmentation fault'
  selection_service:
    service_name|contains:
      - 'snmpd'
      - 'bgpd'
      - 'ospfd'
      - 'nxos-mgmt'
  condition: selection_crash and selection_service
falsepositives:
  - Known buggy software versions with unrelated crashes
  - Planned process restarts during maintenance
level: high

YARA Rule

rule CVE_2026_76459_NXOS_OOB_Write_Artifacts
{
    meta:
        description = "Detects crash dump / core artifacts referencing NX-OS memory corruption potentially linked to CVE-2026-76459"
        author = "Threat Intelligence Team"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-76459"
        date = "2026-01-01"
        severity = "high"

    strings:
        $nxos = "NX-OS" ascii wide nocase
        $crash1 = "core dumped" ascii wide nocase
        $crash2 = "out of bounds" ascii wide nocase
        $crash3 = "buffer overflow" ascii wide nocase
        $crash4 = "SEGV" ascii wide nocase

    condition:
        filesize < 200MB and $nxos and any of ($crash*)
}

Note: This YARA rule targets forensic crash-dump artifacts; it does not detect network exploitation directly. It is intended for host-based artifact triage on NX-OS management or logging infrastructure.

Suricata IDS Rule

alert tcp any any -> $HOME_NET any (msg:"CVE-2026-76459 Cisco NX-OS Potential OOB Write Exploit Attempt - Oversized Management Payload"; flow:established,to_server; content:"|01 02|"; depth:2; byte_test:2,>,1400,0,relative; threshold:type both, track by_src, count 5, seconds 60; classtype:attempted-admin; reference:cve,2026-76459; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-76459; sid:202676459; rev:1;)

Note: This is a heuristic rule flagging repeated oversized management-channel payloads that could exercise an out-of-bounds write. No confirmed exploit signature is public, so this rule requires environment-specific tuning and may generate false positives.

Elastic Detection

sequence by host.name with maxspan=5m
  [ network where network.transport in ("tcp","udp") and destination.port in (22, 161, 443, 179) ]
  [ process where event.action == "start" and process.name in ("snmpd","bgpd","ospfd","sysmgr") and process.exit_code != 0 ]

Description: Detects a network connection to a management or routing port on an NX-OS-adjacent host followed by an abnormal process start/restart, indicative of a service crash that may result from memory corruption.

Splunk SPL Query

index=network (sourcetype="cisco:nxos" OR sourcetype="cisco:ios")
| search ("process crashed" OR "core dump" OR "segmentation fault" OR "kernel panic" OR "buffer overflow")
| stats count min(_time) as first_seen max(_time) as last_seen by host, process_name, message
| where count > 1
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Description: Correlates NX-OS crash and memory-corruption log events to surface hosts experiencing repeated abnormal process terminations consistent with an out-of-bounds write.

Wazuh OSSEC Rule

<group name="cisco,nxos,cve-2026-76459">
  <rule id="100764" level="12">
    <decoded_as>json</decoded_as>
    <field name="program">nxos|snmpd|bgpd|ospfd</field>
    <match>process crashed|core dump|segmentation fault|buffer overflow</match>
    <description>CVE-2026-76459: Cisco NX-OS process crash / potential memory corruption detected</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_76459,</group>
  </rule>

  <rule id="100765" level="10" frequency="3" timeframe="300">
    <if_matched_sid>100764</if_matched_sid>
    <same_source_ip />
    <description>CVE-2026-76459: Repeated NX-OS service crashes from same source (possible exploitation)</description>
    <group>cve_2026_76459,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Cisco's description references a "software hardening release," but no fixed version, advisory URL, or patch source is available in the provided data. Treat as unpatched until Cisco publishes an authoritative advisory with fixed version numbers.

1. Immediate mitigations (reduce exposure now):

• Restrict management-plane access to NX-OS devices using out-of-band management VLANs or dedicated management interfaces; block TCP/22, TCP/443, and UDP/161 from untrusted networks at the perimeter and between segments.
• Apply strict ACLs and CoPP (Control Plane Policing) to limit what source IPs can reach management and routing daemons.
• Disable unused services (e.g., SNMPv1/v2c, HTTP management where HTTPS suffices) and prefer SSH with key-based authentication over password auth.
• Enforce least privilege: audit local accounts and AAA/TACACS+ mappings, and remove low-privileged accounts that could satisfy the PR:L requirement.
• Enable logging to a remote syslog/SEIM so crash events are captured even if the device reloads; monitor for unexpected reloads.
• Monitor for advisory updates from Cisco and apply vendor guidance as soon as fixed code is available.

2. Patch / upgrade steps:

• Consult Cisco's official security advisory portal (sec.cloudapps.cisco.com / Cisco Security Advisories) for the advisory corresponding to CVE-2026-76459 to obtain the authoritative list of affected NX-OS versions and the fixed release.
• Once a fixed NX-OS image is published, test in a lab or non-production fabric before rolling out, given the risk of configuration regressions.
• Schedule maintenance windows to upgrade affected switches; prioritize internet-facing or edge devices and those with broad traffic visibility.
• Verify post-upgrade that the running version matches the fixed release and that the vulnerability is no longer reported by the vendor's advisory.

3. Long-term hardening:

• Adopt a patch-management cadence for network infrastructure matching that of servers, including periodic version audits against vendor advisories.
• Segment the management plane from the data plane and enforce zero-trust access for network device administration.
• Deploy memory-corruption mitigation monitoring (crash correlation) and integrate NX-OS logs with the SOC.
• Maintain an authoritative asset inventory of all NX-OS devices to map exposure against this and future advisories.

References

• NVD — CVE-2026-76459: https://nvd.nist.gov/vuln/detail/CVE-2026-76459
• Cisco Security Advisories portal: https://sec.cloudapps.cisco.com/security/center/publicationListing.x
• MITRE CWE-787 — Out-of-bounds Write: https://cwe.mitre.org/data/definitions/787.html
• FIRST EPSS: https://www.first.org/epss/ (no score available for this CVE at time of writing)
• CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog (CVE-2026-76459 is not listed)

No known threat actor campaigns or exploit-in-the-wild evidence are associated with CVE-2026-76459 in the available data.