A null byte injection vulnerability in the free5GC User Data Management (UDM) service allows an attacker to insert URL‑encoded null bytes (%00) into the supi path parameter of the Nudm_SubscriberDataManagement API. This causes Go's net/url package to reject the request with an "invalid control character in URL" error, resulting in a 500 Internal Server Error instead of a proper 400 Bad Request. The effect is a denial of service, as legitimate requests to UDM can be repeatedly denied, impacting service availability.

The vulnerable component is free5GC's UDM module. All releases prior to version 1.4.2 contain the flaw. The vulnerability lies in the path parameter handling used when UDM contacts the User Data Repository (UDR) via the Nudm_SubscriberDataManagement API. Users running any of the affected free5GC versions on Linux-based 5G core networks are at risk.

The CVSS score of 8.7 reflects a high severity for denial of service. The EPSS score of less than 1% indicates a low likelihood of widespread exploitation at present. The vulnerability is not listed in the CISA KEV catalog. A remote attacker can exploit this by sending a crafted HTTP request containing a null byte in the supi parameter to the UDM's API endpoint. The CVE description does not state whether authentication is required, so the access requirement remains uncertain; however, the mechanics of the attack do not rely on privileged input.