free5GC UDM vulnerable to null byte injection in URL path parameters causing 500 Internal Server Error

Impact
This is an Improper Input Validation vulnerability with Denial of Service and Injection implications.

• Security Impact: A remote attacker can inject null bytes (URL-encoded as %00) into the supi path parameter of the UDM's Nudm_SubscriberDataManagement API. This causes URL parsing failure in Go's net/url package with the error "invalid control character in URL", resulting in a 500 Internal Server Error. This null byte injection vulnerability can be exploited for denial of service attacks.
• Functional Impact: When the supi parameter contains null characters, the UDM attempts to construct a URL for UDR that includes these control characters. Go's URL parser rejects them, causing the request to fail with 500 instead of properly validating input and returning 400 Bad Request.
• Affected Parties: All deployments of free5GC v4.0.1 using the UDM Nudm_SDM service with endpoints that include path parameters (e.g., /nudm-sdm/v2/{supi}/am-data).

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 requests containing null bytes in path parameters before they reach UDM.