CVE-2026-53551: free5GC AUSF: null byte injection in supiOrSuci causes HTTP 500 internal service failure

The free5GC AUSF (Authentication Server Function) does not validate the supiOrSuci field in UE authentication requests. Null bytes (\x00) and other control characters pass through JSON parsing unchanged and are forwarded to the UDM in an unescaped URL path. This causes Go’s net/url.Parse() to fail, returning HTTP 500 “System failure” and leaking internal stack traces. An unauthenticated attacker can trigger this at scale—4.1% of special_chars mutations produce HTTP 500—causing denial of service for all subscribers attempting authentication through the affected AUSF. CWE-20 (Improper Input Validation).