CVE-2026-83947

Executive Summary

CVE-2026-83947 is a missing authorization vulnerability (CWE-862) in Microsoft Azure Event Grid that allows an authenticated, low-privileged attacker to perform spoofing over a network. Because the flaw is a cloud-service control-plane/data-plane authorization gap, the affected population is any Azure tenant using Event Grid subscriptions, topics, or custom delivery endpoints. Successful exploitation yields high integrity impact within Event Grid's security scope, enabling forged event delivery, while confidentiality and availability remain unaffected.

Technical Analysis

The NVD description states, in full: "Missing authorization in Azure Event Grid allows an authorized attacker to perform spoofing over a network." The assigned weakness is CWE-862 (Missing Authorization), which covers cases where an application fails to perform an authorization check on a resource or action that requires one. In this CVE the defect is present in Azure Event Grid, a managed publish-subscribe service that routes events from publishers to subscribers.

The defining characteristic of CWE-862 is the absence of a permission check — not a flawed check, an injection flaw, or a memory-safety bug. The affected operation is reachable by a network client (AV:N) and requires only that the attacker hold some valid, low-privileged credential in the targeted environment (PR:L). No user interaction is required (UI:N) and the attack is rated Low complexity (AC:L), meaning the exploit path does not depend on race conditions or unusual preconditions.

Because the authorization decision is missing, a request that should be rejected — or scoped to the caller's own resources — is instead processed. The vulnerability class maps directly to the stated impact: spoofing. In an event-broker context, spoofing means an event or event source can be presented with an identity, origin, or scope that the caller is not authorized to assert. The CVSS vector confirms the blast radius: Scope is Changed (S:C), so the vulnerable component's authority can affect resources beyond its own security scope; Integrity impact is High (I:H), while Confidentiality and Availability are None (C:N/A:N).

No specific handler names, API paths, request parameters, topic or subscription identifiers, or code-level constructs are disclosed in the available description. Consequently, no implementation-level claims are made here. What can be stated with confidence is the vulnerability class (broken/missing authorization on a network-reachable Azure Event Grid operation), the required attacker position (an authorized principal with valid credentials), and the resulting security property loss (integrity/spoofing within and possibly beyond Event Grid's scope).

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template is available for this CVE, and none is referenced in the intelligence provided. The attack chain below is therefore inferred strictly from the vulnerability class (CWE-862) and the stated impact (spoofing over a network by an authorized attacker). No function names, endpoints, or parameters are asserted because none are present in the source description.

1. Reconnaissance / target identification. The attacker first identifies an Azure tenant and an Event Grid deployment that accepts events from principals the attacker can obtain or influence. Because the vuln requires PR:L, the attacker's realistic starting point is any low-privileged identity with access to Event Grid resources — for example a service principal, managed identity, application registration, or user account that has been granted some level of Event Grid access. Enumeration of Event Grid topics, subscriptions, and delivery configuration is the reconnaissance objective. No specific enumeration command or endpoint is documented in the source data.

2. Triggering the missing authorization. The attacker issues a request to the affected Event Grid operation using their legitimate low-privileged credential. The vulnerable code path performs authentication (the caller is known and valid) but does not perform the corresponding authorization check. As a result, the operation is executed for a resource, scope, or identity the caller should not be permitted to act upon. This is the classic CWE-862 failure mode: identity is verified, permission is not.

3. Payload / execution at the vulnerability-class level. Because the impact is spoofing, the payload is an event (or an event submission) carrying an asserted origin, source, subject, or publisher identity that the attacker is not authorized to claim. The missing authorization check allows that assertion to be accepted and propagated by the broker. No specific payload schema, event field name, or API path is documented, so the precise wire format cannot be stated.

4. Post-exploitation impact. The forged/spoofed event is delivered into Event Grid's routing fabric, where it can trigger downstream subscribers and automation. Per the CVSS vector, integrity is High (I:H) and scope is Changed (S:C) — meaning the spoof can affect systems and workflows outside Event Grid's own boundary that consume these events. Confidentiality and availability are not impacted. In practice, forged events in an event-driven architecture can cause unintended actions in downstream consumers (for example, triggering functions, queues, or automation that trusts Event Grid as an event source).

Impact Assessment

The CVSS 3.1 base score is 7.7 (High) with vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:N. Subscore interpretation:

• Attack Vector: Network — the affected Event Grid operation is reachable remotely.
• Attack Complexity: Low — no special conditions are required.
• Privileges Required: Low — the attacker must hold a valid, low-privileged identity; this is not an unauthenticated flaw.
• User Interaction: None — no victim action is needed.
• Scope: Changed — resources beyond the vulnerable component's security scope can be affected.
• Confidentiality: None — no data exposure is described.
• Integrity: High — the spoofing allows unauthorized modification/forgery of event data or event origin.
• Availability: None — no denial-of-service impact is described.

Business impact centers on trust in the event pipeline. Organizations that use Azure Event Grid to drive automation, workflows, orchestration, incident response, or data movement should treat forged events as a potential integrity compromise: a spoofed event that is accepted as genuine can cause downstream systems to take actions the operator never intended. The blast radius is bounded by (a) which identities the attacker can obtain with low privilege and (b) which downstream subscribers trust Event Grid as an authoritative source without additional verification. Because Scope is Changed, defenders should not assume the impact is confined to Event Grid itself. No EPSS score has been assigned by FIRST.org at the time of writing, and the CVE is not listed in CISA KEV.

Detection Guidance

Sigma Rule

title: Potential Azure Event Grid Spoofing via Missing Authorization (CVE-2026-83947)
id: 9f3c7a21-4b8e-4d2f-9c10-7a5e6d4b8f01
status: experimental
description: Detects Event Grid control-plane operations that may indicate abuse of missing authorization to spoof events. Tune to your tenant's expected principals.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-83947
author: Threat Intelligence
date: 2026/01/01
logsource:
  product: azure
  service: activitylogs
detection:
  selection_operation:
    operationName|contains:
      - 'Microsoft.EventGrid'
  selection_action:
    operationName|endswith:
      - '/topics/write'
      - '/topics/eventSubscriptions/write'
      - '/eventSubscriptions/write'
      - '/publish/action'
  filter_known_principals:
    caller|contains:
      - 'trusted-automation-'
  condition: selection_operation and selection_action and not filter_known_principals
falsepositives:
  - Legitimate automation and deployment pipelines
level: medium
tags:
  - attack.t1078
  - cve.2026.83947

YARA Rule

rule CVE_2026_83947_EventGrid_Spoof_Artifacts
{
    meta:
        description = "Detects suspicious references to Azure Event Grid spoofing activity patterns. No known PoC exists; rule targets textual artifacts only."
        author = "Threat Intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-83947"
        date = "2026-01-01"
        cve = "CVE-2026-83947"
    strings:
        $eg1 = "eventgrid" nocase
        $eg2 = "EventGrid" nocase
        $eg3 = "eventSubscriptions" nocase
        $eg4 = "Microsoft.EventGrid" nocase
    condition:
        any of ($eg*) and filesize < 1MB
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-83947 Possible Azure Event Grid Spoofing Activity (Event Grid URI)"; flow:established,to_server; http.host; content:"eventgrid.azure.net"; nocase; http.uri; content:"eventSubscriptions"; nocase; classtype:attempted-admin; reference:cve,2026-83947; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-83947; sid:90083947; rev:1; metadata:created_at 2026_01_01, updated_at 2026_01_01;)

Elastic Detection

sequence by user.name with maxspan=5m
  [ authentication where event.outcome == "success" and user.name != null ]
  [ any where
      event.dataset in ("azure.activitylogs", "azure.platformlogs") and
      process.name : "Microsoft.EventGrid*" and
      event.action in ("topics/write", "topics/eventSubscriptions/write", "eventSubscriptions/write", "publish/action")
  ]

Splunk SPL Query

index=azure sourcetype="azure:activity" operationName="Microsoft.EventGrid*"
| where match(operationName, "(?i)(eventSubscriptions|topics|publish)")
| stats count min(_time) as first_seen max(_time) as last_seen
    values(operationName) as operations values(caller) as callers
    by caller, resourceId, src_ip
| 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="azure,eventgrid,">
  <rule id="100847" level="10">
    <decoded_as>json</decoded_as>
    <field name="operationName">Microsoft\.EventGrid.*</field>
    <field name="operationName">(?i)(eventSubscriptions|publish|topics)</field>
    <description>CVE-2026-83947: Suspicious Azure Event Grid authorization-sensitive operation by low-privileged principal</description>
    <mitre>
      <id>T1078</id>
    </mitre>
    <group>azure_eventgrid_spoofing,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix version, affected version range, or official workaround is published in the available intelligence. Treat all Azure Event Grid deployments as potentially exposed until Microsoft publishes an advisory or fix.

1. Immediate mitigations:

• Enforce least privilege on all Azure identities that can interact with Event Grid. Because the flaw requires only a low-privileged authenticated principal, reducing the set of principals with Event Grid access directly reduces exposure.
• Audit Event Grid role assignments (Subscription, Resource Group, and Topic/Subscription scope) and remove any principal that does not strictly require publish or manage rights.
• Add downstream verification: do not treat Event Grid as an implicit trust boundary. Validate event origin, subject, and expected publisher identity in consumers before taking action.
• Monitor the detection rules above and alert on unexpected Event Grid write/publish operations, particularly from principals that normally do not perform them.

2. Patch/upgrade steps:

• No fixed version is available. Track the NVD entry at https://nvd.nist.gov/vuln/detail/CVE-2026-83947 and the Microsoft Security Response Center (MSRC) advisory feed for the first authoritative patch reference.
• Because Azure Event Grid is a managed cloud service, remediation is expected to be server-side; when Microsoft publishes a fix, validate that your tenant's Event Grid resource provider version is updated and re-test subscription/delivery behavior.

3. Long-term hardening:

• Adopt an explicit zero-trust posture for event delivery: authenticate and authorize each publisher-to-subscriber hop rather than relying on broker-level trust.
• Continuously inventory and review Azure role assignments (recommend periodic access reviews and just-in-time elevation) to keep the low-privileged-credential population small.
• Deploy centralized logging for Azure activity and Event Grid delivery, with anomaly baselining per principal, so authorization anomalies such as CWE-862 abuse are detected early.
• Track CWE-862 as a recurring class in cloud services and include it in threat modeling for event-driven and automation-driven architectures.

References

• NVD — CVE-2026-83947: https://nvd.nist.gov/vuln/detail/CVE-2026-83947
• CWE-862: Missing Authorization: https://cwe.mitre.org/data/definitions/862.html
• Microsoft Security Response Center (MSRC) security update guide: https://msrc.microsoft.com/update-guide
• MITRE ATT&CK T1078 — Valid Accounts: https://attack.mitre.org/techniques/T1078/
• CISA Known Exploited Vulnerabilities catalog (CVE-2026-83947 not listed at time of writing): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat actor campaigns and exploit-in-the-wild evidence: none identified. No public proof-of-concept, exploit code, or Nuclei template exists, the CVE is not in CISA KEV, and no EPSS score has been assigned.