free5GC UDM incorrectly returns 500 for empty supi path parameter in PATCH sdm-subscriptions reques

Impact
This is an Improper Error Handling vulnerability with Information Exposure implications, combined with an HTTP Method Translation issue.

• Security Impact: The UDM incorrectly converts a downstream 400 Bad Request (from UDR) into a 500 Internal Server Error when handling PATCH requests with an empty supi path parameter. Additionally, the UDM incorrectly translates the PATCH method to PUT when forwarding to UDR, indicating a deeper architectural issue. This leaks internal error handling behavior and makes it difficult for clients to distinguish between client-side errors and server-side failures.
• Functional Impact: When a client sends a PATCH request with an empty supi (e.g., double slashes // in URL path), the UDM forwards a PUT request to UDR with the malformed path, which correctly returns 400. However, UDM propagates this as 500 SYSTEM_FAILURE instead of returning the appropriate 400 error to the client. This violates REST API best practices for PATCH operations and may indicate improper HTTP method handling.
• Affected Parties: All deployments of free5GC v4.0.1 using the UDM Nudm_SDM service with PATCH operations on sdm-subscriptions endpoint.

Patches
Yes, the issue has been patched.
The fix is implemented in PR free5gc/udm#79.
Users should upgrade to the next release of free5GC that includes this commit.

Workarounds
There is no direct workaround at the application level. The recommendation is to apply the provided patch or implement API gateway-level validation to reject PATCH requests with empty path parameters before they reach UDM.