How should teams secure file path handling in .NET applications that load assemblies from user-influenced input?

Why path validation has to be done before the load

Assembly loading is a trust boundary, not just a filesystem operation. Once .NET resolves a path and loads a DLL, the application is no longer reasoning about a file name, it is executing code. That means validation has to happen on the exact path the runtime will use, before any load or probe step can be influenced by attacker-controlled input.

The safest pattern is to define an allowlisted base directory and compare the fully resolved target against that boundary after canonicalisation. Treat relative paths, mixed separators, alternate encodings, and traversal segments as untrusted until the resolved result is proven to stay inside the intended location. The point is to control where execution can come from, not just to reject obvious "../" strings.

A useful reference point for the broader input-handling discipline is the OWASP Cheat Sheet Series, especially where it reinforces canonicalisation, input validation, and safe file handling.

What usually breaks in real implementations

The common failure is validating the raw string and trusting that the final path is equivalent. In practice, path parsing, symlinks, UNC paths, drive-relative paths, and case or separator differences can cause the resolved location to differ from the developer's expectation. If the check happens before resolution, a path can appear safe and still point outside the intended boundary.

Another weak point is treating upload handling and assembly loading as separate concerns. If a user can influence a file name, upload location, or extraction target, then a later load step can turn a storage issue into code execution. That is especially dangerous when the application loads assemblies by convention from writable directories, temporary folders, or any location shared with user content.

For teams that need a stricter control baseline around file trust and execution boundaries, the NIST SP 800-53 Rev 5 Security and Privacy Controls helps anchor path handling to access control, configuration management, and integrity expectations.

Practical controls that reduce the blast radius

Keep executable assemblies in a directory that users cannot write to, and keep user uploads in a separate location that the runtime never scans for code. Resolve the candidate path, confirm it stays under the approved root, and reject anything that escapes that boundary. If the application must load plugins or extensions, prefer explicit registration over implicit discovery, because discovery patterns are easier to subvert.

When the file source is external or the workflow includes third-party components, the risk is not just path traversal but untrusted code introduction. If a path bug can place a DLL where the loader expects it, the operational consequence is arbitrary code execution under the application identity. A broader governance lens is provided by the NIST Cybersecurity Framework 2.0, which ties protection and recovery to asset trust and secure configuration.

Where this issue intersects with software supply chain and packaged components, the NHI discussion is less relevant than the code-loading boundary itself; the better practitioner focus is on preventing a writable path from becoming a code execution path.

Risk and Threat Considerations

Path handling bugs become materially more dangerous when the application later loads assemblies from the same filesystem surface. An attacker does not need a perfect exploit chain if they can steer a trusted loader toward a malicious DLL, especially in applications that accept uploads, cache plugins, or extract archives into predictable locations.

Failure mechanism: The application validates a string, but the runtime loads the resolved file after traversal, symlink, or directory boundary manipulation has redirected the path outside the intended safe area. If the target file is a DLL, that becomes a code execution path rather than a simple file write issue.

Impact: The compromise can extend from local file access to arbitrary code execution, persistence, data theft, and lateral movement under the application's privileges. The security impact is highest when the load location is writable, shared, or reused across deployments.

Practitioner Guidance

What to verify: Verify the resolved canonical path, not the user-supplied string, and confirm the final location is inside a non-writable application-controlled directory. If your loader accepts plugins, verify that the allowlist is explicit and that no fallback search path can be influenced by user content.

Common mistake: Teams often harden upload validation but forget that a later assembly load turns storage into execution. Treat any directory that can receive user-controlled files as hostile unless you can prove the runtime will never load code from it.

Practitioner takeaway: The decisive control is not rejecting suspicious path text, it is ensuring the runtime can only load assemblies from locations the user cannot influence.

Related resources from NHI Mgmt Group

• How should .NET teams prevent path traversal when user input is used to build file paths?
• How should Rails teams prevent path traversal when building file paths from user input?
• How should security teams prevent path traversal in file download features that accept user input?
• How should security teams prevent XSS in Django applications that render user-controlled input?