An improper input validation and protocol compliance vulnerability in free5GC v4.0.1 allows remote attackers to cause a denial of service. The UPF incorrectly accepts a malformed PFCP Association Setup Request, violating 3GPP TS 29.244. This places the UPF in an inconsistent state where a subsequent valid PFCP Session Establishment Request triggers a cascading failure, disrupting the SMF connection and causing service degradation.