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A critical vulnerability in Microsoft Azure’s API Connection allowed attackers to breach resources in various Azure tenants globally. Gulbrandsrud discovered the flaw that earned him a $40,000 bounty and a chance to present at Black Hat. This flaw exploited Azure’s shared API Management setup, allowing unauthorized access to Key Vaults, Azure SQL databases, and third-party services like Jira and Salesforce.
The vulnerability involved Azure’s shared APIM instance, where all API Connections are hosted, leading to security risks that go beyond tenant boundaries.
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Azure’s Default API Connection Vulnerability:
The vulnerability was found in how Azure Resource Manager (ARM) handled the DynamicInvoke endpoint, which processes API Connection requests using highly privileged tokens.
When ARM receives a DynamicInvoke request, it constructs URLs using the pattern /apim/[ConnectorType]/[ConnectionId]/[Action-Endpoint] with elevated authentication tokens.
Gulbrandsrud found that attackers could exploit a custom Logic App connector with a weak path parameter to perform path traversal attacks.
The researcher demonstrated this by defining a simple endpoint with a {path} parameter, then supplying malicious input like ../../../../[VictimConnectorType]/[VictimConnectionID]/[action].
When ARM processed this request, URL normalization resulted in direct access to victim connections.
The attack was demonstrated against an Azure Key Vault connection:
Mitigation:
Microsoft acknowledged the vulnerability within three days of the April 7, 2025 disclosure and applied fixes within a week. The first fix was to create a blacklist for path parameters to block ../ sequences and their URL-encoded versions.
Gulbrandsrud mentioned that this solution might not be enough, and suggested alternatives like path normalization techniques or altering the API connection paths.
The vulnerability required Contributor-level privileges to the attacking tenant’s API Connection, limiting the attack surface to privileged users.
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