Bypass Azure Web App Authentication with Path Traversal

What this lab covers

You enumerate Azure web applications in a subscription, identify one that serves content based on a path derived from user input, and use that to reach material the application's authentication was supposed to protect.

The interesting question is where authorisation actually happens. Platform-level authentication is convenient and correct for many purposes, and it operates on the request rather than on the resource, which is a distinction worth internalising.

It assumes no prior Azure experience and suits web testers moving into cloud work.

How authentication bypass by traversal works

Azure App Service can enforce authentication before a request reaches application code, an arrangement often called Easy Auth. It evaluates the incoming request, and if a rule requires authentication for a path, it challenges the caller.

Application code may then serve content based on a parameter, and if that parameter is used to build a filesystem path without constraint, the response can include files outside the intended directory. The platform saw a request for an allowed path and the application returned something else.

Configuration nuance widens this. Rules that apply to specific paths rather than to everything, and settings that allow unauthenticated requests to proceed to the application, both create conditions where some content is protected and some is not. The attacker's task is to find the route the rule did not anticipate.

How to detect this in your own environment

Enable App Service diagnostic and HTTP logging, then look for traversal sequences and encoded variants in request paths and parameters. Unsophisticated attempts are visible immediately.

Alert on responses returning file content for requests that did not carry a session, since the mismatch between authentication state and content served is the actual anomaly.

Review requests reaching the application that the platform did not authenticate. If your configuration permits unauthenticated requests through to code, that population is worth quantifying.

How to prevent it

Enforce authorisation in the application as well as at the platform, so a request that arrives by an unexpected path is still checked before content is served. Defense in depth matters here because the two layers see different things.

Resolve requested paths and verify they fall inside the permitted directory rather than filtering input for traversal sequences, which requires anticipating every encoding.

Configure platform authentication to require it for all paths by default and add exceptions explicitly, rather than protecting selected paths and leaving the remainder open.

Real-world context

Path traversal remains a persistent flaw class and appears regularly in advisories for widely deployed software. It is common in cloud-hosted applications for the same reason it is common everywhere, which is that it is easy to fix incorrectly.

The cloud-specific lesson is about layered platform features. Managed platforms provide authentication, network rules and other protections that are genuinely valuable, and each operates on a particular view of the request. Understanding what each layer evaluates is what prevents assuming coverage that does not exist.

The pattern maps to MITRE ATT&CK T1190 Exploit Public-Facing Application and T1083 File and Directory Discovery.

Applying this at work

• Check whether your App Service authentication settings require authentication for all paths or only some.
• Confirm your applications enforce their own authorisation rather than relying entirely on the platform layer.
• Review code paths that build filesystem paths from user input and ensure they validate the resolved path.
• Enable HTTP logging on App Service and check whether traversal attempts would be visible to you.
• Test your own applications for content reachable by unexpected paths, which takes minutes with a proxy.

Frequently asked questions

Does Azure App Service authentication protect everything?

It protects the requests it evaluates according to your configuration. If rules apply to selected paths, or unauthenticated requests are allowed through to application code, some content may be reachable without authentication.

Why does path traversal bypass authentication here?

Because the platform check is performed on the request as presented, while the application decides which file to return afterwards. A request for an allowed path can result in a file outside the intended directory being served.

Where should authorisation be enforced?

In both places. Platform authentication is a useful outer layer, and the application should still verify that the caller is permitted to receive the specific resource being returned.

Do I need my own Azure account to practice this?

No. The lab runs in a live Azure environment that Pwned Labs provisions for you, so there is nothing to set up and no risk to your own tenant.

Which certification covers this in depth?

The Microsoft Cloud Red Team Professional (MCRTP) covers Entra ID, Azure and Microsoft 365 attack paths in full, including token abuse, phishing tradecraft and privilege escalation, and is assessed hands-on in a live tenant.