Recent security findings reveal that threat actors are actively exploiting misconfigured Azure application proxies to gain unauthorized access to organizations’ internal resources.

When Azure app proxy pre-authentication is set to “Passthrough” instead of the default “Microsoft Entra ID” setting, private network resources may become unintentionally exposed to potential attackers.

Attackers Leveraging Azure App Proxy Pre-authentication

Microsoft’s Azure app proxy service enables organizations to publish on-premises applications to the public internet without opening inbound firewall ports.

This service typically leverages Microsoft Entra ID (formerly Azure Active Directory) for authentication, creating a secure access pathway for remote users.

However, security researchers at TrustedSec have discovered that when administrators configure the pre-authentication option to “Passthrough” instead of the default “Microsoft Entra ID” setting, they effectively remove the authentication barrier that protects internal resources.

“Passthrough pre-authentication is basically the equivalent of opening a port on your firewall to the private system,” researchers said.

“While the intent may have been to have specific applications accessible, other resources may be unintentionally exposed.”

In a demonstration environment, researchers configured two application URLs pointing to the same internal website:

The difference in behavior was stark. When accessing the MSENTRAID URL, all requests were protected by Microsoft Entra ID authentication, requiring proper credentials before granting access.

In contrast, requests to the PASSTHROUGH URL bypass authentication entirely, directly exposing the internal application and potentially other resources on the same server.

Real-World Attack Scenarios

Security experts have observed attackers performing forced browsing and content discovery against exposed Passthrough proxies.

By systematically probing different URL paths, attackers can identify unprotected internal resources, administrative interfaces, and potentially vulnerable endpoints.

In one documented case, attackers discovered a “/secure/” path that only used basic HTTP authentication.

Using simple brute force techniques with default credential combinations like “admin:admin” and “test:test,” they successfully gained unauthorized access to sensitive internal systems:

This vulnerability highlights ongoing challenges in securing hybrid cloud environments. While Azure app proxy offers convenience for remote access, improper configuration can lead to significant security gaps.

“Organizations need to understand that Passthrough pre-authentication removes an important security layer,” notes the research team.

“Microsoft’s own documentation warns that Passthrough doesn’t provide protection against anonymous attacks.” The vulnerability adds to a growing list of security concerns in Azure services.

Earlier this year, Orca Security researchers identified SSRF vulnerabilities in four different Azure services that allowed attackers to scan local ports and access internal endpoints

Mitigation Recommendations

Security experts recommend the following steps to protect against this vulnerability:

Review all Azure app proxy configurations and ensure pre-authentication is set to “Microsoft Entra ID” rather than “Passthrough”
Implement additional security layers for any applications that cannot use Entra ID authentication
Regularly audit exposed applications for potential security gaps
Consider implementing Web Application Firewall protection for critical applications

As cloud-based services continue to expand, organizations must remain vigilant about configuration settings that could inadvertently expose internal resources to the public internet.

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