Azure Custom Locations SSRF (CVE-2026-26135)

Overview

A critical Server-Side Request Forgery (SSRF) vulnerability, tracked as CVE-2026-26135, has been identified in the Azure Custom Locations Resource Provider. With a CVSS score of 9.6, this flaw allows an attacker with low-privilege access to send unauthorized requests from the vulnerable Azure service to internal systems.

Vulnerability Details

In simple terms, the Azure Custom Locations Resource Provider did not properly validate user-supplied URLs. An authorized attacker could exploit this to trick the service into making requests to other internal Azure services or resources that should be inaccessible. This SSRF flaw acts as a stepping stone, enabling the attacker to interact with systems behind the network perimeter as if they were the trusted resource provider itself.

Impact

The primary risk is privilege escalation within the Azure environment. A successful attack could allow a user with limited permissions to perform actions reserved for higher-privileged identities or services. This could lead to data exposure, configuration tampering, or further lateral movement within the cloud tenant. The network-based attack vector and lack of required user interaction make this vulnerability particularly dangerous for affected deployments.

Remediation and Mitigation

Microsoft has released a patch for this vulnerability. Administrators must apply updates to the Azure Custom Locations Resource Provider immediately. There is no effective workaround for this flaw; patching is the only complete remediation.

To ensure protection:

1. Update Immediately: Apply the latest security updates provided by Microsoft for Azure Custom Locations. Enable automatic updates where possible.
2. Review Access: Audit user and service principal assignments in affected subscriptions, ensuring adherence to the principle of least privilege. This can limit the pool of potential attackers.
3. Monitor: Review audit logs for anomalous outbound requests from the Resource Provider, which could indicate attempted or successful exploitation.

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Security Insight

This high-severity SSRF in a core Azure resource provider echoes a concerning trend where cloud management planes become prime targets for privilege escalation. Similar to past incidents in other platforms, it highlights how a single validation flaw in a foundational service can undermine the entire shared responsibility model, shifting significant risk to the vendor’s internal security controls. Organizations must treat provider-side patches with the same urgency as those for their own virtual machines.

Further Reading

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