CVE-2026-26135: Azure Custom Locations SSRF Vulnerability

CVE-2026-26135 Overview

A Server-Side Request Forgery (SSRF) vulnerability has been identified in Microsoft Azure Custom Locations Resource Provider (RP). This security flaw allows an authorized attacker with low privileges to exploit the SSRF condition to elevate privileges over a network. The vulnerability affects the Azure Custom Locations Resource Provider, a critical component used for extending Azure Arc capabilities to on-premises, multi-cloud, and edge environments.

Critical Impact

An authenticated attacker can leverage this SSRF vulnerability to achieve privilege escalation, potentially gaining access to internal Azure services and sensitive resources that would otherwise be inaccessible.

Affected Products

• Microsoft Azure Custom Locations Resource Provider

Discovery Timeline

• April 3, 2026 - CVE-2026-26135 published to NVD
• April 6, 2026 - Last updated in NVD database

Technical Details for CVE-2026-26135

Vulnerability Analysis

This vulnerability is classified as CWE-918 (Server-Side Request Forgery). SSRF vulnerabilities occur when an application can be tricked into making requests to unintended locations, often internal services or resources. In the context of Azure Custom Locations Resource Provider, the vulnerability allows an authenticated user to manipulate the RP into making server-side requests that can be used to access internal Azure infrastructure or metadata services that should not be directly accessible.

The Azure Custom Locations Resource Provider is responsible for managing custom location resources that enable Azure Arc-enabled services. When the RP processes certain requests, it fails to properly validate or sanitize the destination of outbound requests, allowing an attacker to redirect these requests to arbitrary endpoints including internal Azure services, metadata endpoints, or other sensitive infrastructure.

Root Cause

The root cause of this vulnerability lies in insufficient input validation within the Azure Custom Locations Resource Provider when handling user-controlled data that influences server-side request destinations. The RP does not adequately validate or restrict the target of outbound HTTP requests, allowing attackers to specify internal or otherwise restricted endpoints as request targets. This enables access to Azure Instance Metadata Service (IMDS), internal APIs, or other cloud infrastructure components.

Attack Vector

The attack vector is network-based, requiring the attacker to have valid Azure credentials with at least low-level privileges. The attacker can craft malicious requests to the Azure Custom Locations Resource Provider API that include attacker-controlled URLs or parameters. When the RP processes these requests, it makes server-side connections to the specified targets, potentially exposing internal service responses, tokens, or metadata to the attacker.

The exploitation does not require user interaction and can be performed with low attack complexity once authenticated. A successful attack could result in access to sensitive credentials, internal service tokens, or the ability to interact with internal Azure infrastructure, leading to privilege escalation within the Azure environment.

Detection Methods for CVE-2026-26135

Indicators of Compromise

• Unusual outbound requests from Azure Custom Locations Resource Provider to internal IP ranges (e.g., 169.254.169.254 for IMDS)
• Unexpected API calls to Custom Locations RP with suspicious URL parameters or payloads
• Authentication logs showing privilege escalation activities following interaction with the Resource Provider
• Anomalous access patterns to Azure management plane APIs from unexpected sources

Detection Strategies

• Monitor Azure Activity Logs for unusual Custom Locations Resource Provider operations
• Implement network monitoring to detect requests to metadata service endpoints (169.254.169.254)
• Configure Azure Defender for Cloud to alert on suspicious resource provider activities
• Review Azure Resource Manager logs for anomalous API patterns involving custom locations

Monitoring Recommendations

• Enable detailed logging for Azure Custom Locations Resource Provider operations
• Configure alerts for any access attempts to Azure Instance Metadata Service from unexpected sources
• Implement continuous monitoring of privileged operations in Azure environments
• Utilize Microsoft Defender for Cloud security recommendations and alerts

How to Mitigate CVE-2026-26135

Immediate Actions Required

• Review the Microsoft Security Response Center advisory for specific remediation guidance
• Audit Azure Custom Locations Resource Provider configurations and access permissions
• Review and restrict permissions for users and service principals with access to Custom Locations RP
• Monitor for any suspicious activity in Azure Activity Logs related to Custom Locations operations

Patch Information

Microsoft has addressed this vulnerability through their standard security update process. Organizations should review the Microsoft Security Update Guide for CVE-2026-26135 for specific patch information and remediation steps. As this is a cloud service vulnerability, Microsoft typically applies patches to Azure services automatically; however, customers should verify their environments are protected by following the guidance in the advisory.

Workarounds

• Implement strict network segmentation and access controls for Azure Custom Locations resources
• Apply the principle of least privilege for all identities accessing the Custom Locations Resource Provider
• Enable Azure Private Link where possible to reduce exposure of internal endpoints
• Consider temporarily restricting access to Custom Locations RP functionality until patches are confirmed applied

# Review Custom Locations Resource Provider permissions using Azure CLI
az role assignment list --scope "/subscriptions/{subscription-id}" --query "[?contains(roleDefinitionName, 'Custom Locations')]"

# Enable enhanced diagnostic logging for Custom Locations
az monitor diagnostic-settings create --name "CustomLocationsAudit" --resource "/subscriptions/{subscription-id}/providers/Microsoft.ExtendedLocation" --logs '[{"category": "Administrative", "enabled": true}]'