CVE-2026-19640

Executive Summary

CVE-2026-19640 is an improper authorization vulnerability in Arista EOS affecting the gNMI (gRPC Network Management Interface). An authenticated user with gNMI access may receive incorrect authorization results, allowing operations beyond their currently assigned role-based permissions. The vulnerability is rated CVSS 4.2 (Medium), requires network access and valid low-privilege credentials, and has no known public exploit, PoC, or in-the-wild activity as of this report.

Technical Analysis

CVE-2026-19640 is classified as CWE-863 (Incorrect Authorization). The defect resides in the authorization logic that Arista EOS applies to requests received over the gNMI (gRPC Network Management Interface) service. gNMI is a gRPC-based management and streaming telemetry interface used to retrieve and modify device configuration and state on network switches and routers. Access to gNMI operations is governed by the device's role-based authorization model; users are assigned roles that map to permitted read/write operations across configuration paths.

The flaw causes the authorization subsystem to return incorrect authorization results for authenticated gNMI callers. In practical terms, an authenticated user whose role should restrict them to a narrow subset of operations may instead be permitted to invoke gNMI operations (such as Set/Get RPCs against configuration paths) that fall outside their assigned permissions. This is an authorization-decision correctness problem, not an authentication bypass: the attacker must already possess valid credentials and gNMI access (PR:L). The attack is network-reachable (AV:N) but carries high complexity (AC:H), suggesting that a specific, non-default condition — a particular request pattern, sequencing, or configuration state — is required to elicit the incorrect authorization result.

Because the vendor description does not disclose the specific internal functions, gNMI paths, or configuration conditions involved, this report deliberately does not name implementation details such as handler functions or path strings. The observable mechanism class is: a privileged network management API (gNMI) whose authorization gate can return a false-positive permission decision for authenticated low-privilege users.

Confidentiality and Integrity impact are both Low (C:L/I:L) per CVSS. Availability is not impacted (A:N). No scope change is present (S:U), meaning the vulnerable component's privileges are not assumed to cross into other security authorities beyond what the device's own role model already governs.

Notably, the CVSS base of 4.2 reflects the combination of network accessibility, high attack complexity, and low-privilege authentication requirement — the vulnerability is not trivially weaponizable, but it violates a core security boundary (role separation) on a network control-plane device.

Exploitation Scenarios

Important caveat: No public PoC, exploit code, or Nuclei template exists for CVE-2026-19640. The attack chain below is inferred strictly from the vulnerability class (CWE-863 improper authorization in a gNMI-exposed management interface) and the CVSS vector. No function names, gNMI paths, or API parameters are asserted because none are provided in the source data.

Step 1 — Reconnaissance / target identification. An attacker identifies reachable Arista EOS devices exposing the gNMI interface. This typically involves scanning for the gNMI/gRPC service (commonly TLS-wrapped and reachable on management networks) and fingerprinting the device as Arista EOS. Because the vulnerability is only exploitable by an authenticated user, the attacker must either (a) already hold a low-privilege account on the device, or (b) have obtained credentials through a separate compromise (e.g., leaked management credentials, shared service accounts, or a compromised jump host with EOS access).

Step 2 — Triggering the improper authorization decision. Using their legitimate low-privilege gNMI credentials, the attacker issues a gNMI request that, under correct authorization logic, should be denied for their role. Due to the incorrect authorization result, the EOS authorization subsystem approves the request. The high attack-complexity (AC:H) element implies that a specific request form, timing, or device state is required for the faulty authorization result to manifest — meaning not every unauthorized request will necessarily succeed.

Step 3 — Execution / abuse at the vulnerability-class level. The successful authorization result grants the attacker the ability to perform operations beyond their assigned permissions through the same authenticated gNMI channel. Depending on which operations the faulty decision covers, this could include reading configuration/state data the role should not expose, or modifying aspects of device configuration the role should not be permitted to change. The exact operation set is bounded by what gNMI exposes and what the mis-evaluated authorization check governs.

Step 4 — Post-exploitation impact. Impact is constrained to Low Confidentiality and Low Integrity. An attacker could obtain configuration or telemetry data beyond their privilege level and/or make limited unauthorized configuration changes. On a network device, even limited unauthorized writes can have outsized operational consequences (routing, ACL, or interface changes), but the CVSS scoring reflects the constrained subset of operations the faulty authorization path actually exposes.

Impact Assessment

Affected systems: Arista EOS platforms running the Arista EOS operating system with gNMI enabled and reachable by authenticated users. Devices where gNMI is not enabled are not exposed via this vector.

Business impact: Unauthorized access beyond assigned role permissions violates the principle of least privilege on a network control-plane device. The blast radius depends on (a) which operations the faulty authorization decision exposes, and (b) how many low-privilege accounts have gNMI reach. Because the flaw affects authorization correctness, it can undermine compliance controls that assume role separation is enforced on network infrastructure.

CVSS subscores referenced:

• AV:N — exploitable over the network.
• AC:H — exploitation requires non-trivial conditions.
• PR:L — requires low-privilege authenticated access.
• UI:N — no user interaction.
• S:U — no scope change.
• C:L / I:L — limited confidentiality and integrity impact.
• A:N — no availability impact.
• Base score: 4.2 (Medium).

EPSS: 0.00192 (9.1th percentile) — very low predicted likelihood of exploitation in the wild in the near term. CISA KEV: Not listed. No known malicious use reported by the vendor.

Detection Guidance

Detection for this vulnerability is challenging because exploitation occurs over authenticated gNMI/gRPC channels and mimics legitimate management traffic. The strongest signals are authorization anomalies and anomalous gNMI operations from low-privilege accounts.

Sigma Rule

title: Arista EOS gNMI Improper Authorization Attempt (CVE-2026-19640)
id: 7b3f0c2a-2e9d-4c11-9a77-1f8a2c4e9d10
status: experimental
description: Detects abnormal gNMI authorization behavior or gNMI operations by low-privilege accounts on Arista EOS that may indicate exploitation of CVE-2026-19640.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-19640
author: valtersit.com
date: 2026-01-01
logsource:
  product: arista
  service: eos
detection:
  selection_gnmi:
    - message|contains: 'gNMI'
    - message|contains: 'gnmi'
    - message|contains: 'gRPC'
  selection_authz:
    - message|contains: 'authorization'
    - message|contains: 'authoriz'
    - message|contains: 'permission'
    - message|contains: 'role'
  selection_deny_anomaly:
    - message|contains: 'denied'
    - message|contains: 'not permitted'
    - message|contains: 'unauthorized'
  condition: selection_gnmi and selection_authz
falsepositives:
  - Legitimate gNMI role configuration changes by administrators
  - Routine gNMI telemetry collection
level: medium
tags:
  - attack.t1078
  - cve.2026-19640

YARA Rule

rule CVE_2026_19640_Arista_gNMI_Authz_Indicator
{
    meta:
        description = "Heuristic detection of artifacts referencing Arista EOS gNMI authorization activity tied to CVE-2026-19640"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-19640"
        date = "2026-01-01"
        cve = "CVE-2026-19640"

    strings:
        $gnmi1 = "gNMI" ascii wide nocase
        $gnmi2 = "gRPC Network Management Interface" ascii wide nocase
        $arista = "Arista EOS" ascii wide nocase
        $authz1 = "authorization" ascii wide nocase
        $authz2 = "not authorized" ascii wide nocase
        $authz3 = "permission denied" ascii wide nocase

    condition:
        uint16(0) == 0x5A4D or
        (($arista or $gnmi1 or $gnmi2) and 2 of ($authz*))
}

Suricata IDS Rule

alert tcp any any -> any 6030 ( \
  msg:"CVE-2026-19640 Possible Arista EOS gNMI Improper Authorization Activity"; \
  flow:established,to_server; \
  content:"|2f|gnmi|2e|"; nocase; \
  content:"gNMI"; nocase; \
  reference:cve,2026-19640; \
  reference:url,nvd.nist.gov/vuln/detail/CVE-2026-19640; \
  classtype:attempted-admin; \
  sid:2026196401; rev:1; \
  metadata:affected_product "Arista EOS", created_at "2026-01-01", \
  deployment Perimeter, confidence Medium; )

Note: Port 6030 is a representative gNMI/gRPC service port placeholder; adjust to the deployment's actual gNMI port. The rule matches gNMI service references within gRPC traffic and should be tuned against the environment's legitimate telemetry baseline.

Elastic Detection

// Detect anomalous gNMI administrative operations attributed to low-privilege
// accounts on Arista EOS, which may indicate CVE-2026-19640 improper authorization.
sequence by host.name, user.name with maxspan=1m
  [ authentication where event.action == "logon" and event.outcome == "success" ]
  [ network where
      (network.protocol == "grpc" or network.protocol == "gnmi") and
      process.name != "telemetry-agent" ]

Splunk SPL Query

index=network sourcetype=arista:eos (message="*gNMI*" OR message="*gnmi*" OR message="*gRPC*")
| search (message="*authoriz*" OR message="*permission*" OR message="*role*" OR message="*denied*")
| stats count min(_time) as first_seen max(_time) as last_seen
        values(message) as messages
        by host, user
| where count > 0
| 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="arista,eos,gnmi,cve-2026-19640">
  <rule id="100640" level="10">
    <decoded_as>json</decoded_as>
    <field name="program">gnmi</field>
    <match>authoriz|permission|role|denied|not permitted|unauthorized</match>
    <description>CVE-2026-19640: Arista EOS gNMI authorization anomaly detected</description>
    <mitre>
      <id>T1078</id>
    </mitre>
    <group>cve_2026_19640,gnmi_authz,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch, hotfix, or vendor fix has been identified from any tracked source. No workarounds are documented in the available intelligence. The following guidance is defense-in-depth and must be validated against an Arista Security Advisory before production changes.

1) Immediate mitigations

• Restrict gNMI reachability: place the gNMI/gRPC interface on a dedicated, tightly controlled management network segment. Ensure no gNMI exposure on data-plane or internet-facing interfaces.
• Enforce least privilege on gNMI accounts: eliminate shared accounts, audit roles, and remove gNMI access from accounts that do not require it.
• Apply strict firewall/ACL controls limiting gNMI service access to a small set of trusted management hosts.
• Monitor gNMI authorization events for anomalies (see Detection Guidance), particularly operations by low-privilege accounts.
• If gNMI is not operationally required, disable the service.

2) Patch/upgrade steps

• Monitor the Arista Security Advisory portal and Arista EOS release notes for a fix addressing gNMI authorization for CVE-2026-19640.
• When a fixed EOS release is published, upgrade affected platforms following Arista's standard EOS upgrade procedure and change-control process.
• Re-validate gNMI role/authorization behavior after upgrade.

3) Long-term hardening

• Adopt a zero-trust posture for network device management APIs: authenticated does not equal authorized.
• Implement continuous compliance checks that verify role-to-operation mappings on EOS devices.
• Centralize management-plane logging into a SIEM and alert on authorization anomalies and role changes.
• Conduct periodic access reviews of gNMI-capable accounts and their assigned roles.
• Track Arista advisories and maintain an inventory of EOS versions and exposed management services.

References

1. NVD — CVE-2026-19640: https://nvd.nist.gov/vuln/detail/CVE-2026-19640
2. CWE-863 — Incorrect Authorization: https://cwe.mitre.org/data/definitions/863.html
3. Arista Security Advisories portal (monitor for the advisory corresponding to CVE-2026-19640): https://www.arista.com/en/support/advisories-notices
4. MITRE ATT&CK T1078 — Valid Accounts: https://attack.mitre.org/techniques/T1078/
5. CISA Known Exploited Vulnerabilities Catalog (verify CVE-2026-19640 is NOT listed — as of this report it is not): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Exploit-in-the-wild and threat actor campaigns: None reported. Arista states the issue was discovered internally and that the company is not aware of any malicious use in customer networks. No public PoC, exploit code, or Nuclei template is available. EPSS is 0.00192 (9.1th percentile), indicating low near-term exploitation likelihood.