The Fifth Generation (5G) mobile networks are designed to provide a large range of services with stringent requirements in robustness and security. Thus, it is important to ensure that these networks fulfill these requirements and are resilient against attacks. To meet this challenge, experimental testbeds and tools are required to test and evaluate the security of 5G networks. This work presents an experimentation testbed and support tools for generating and injecting on the fly 5G packets to realize multiple security assessing tasks in particular fuzzing operations for vulnerabilities discovery. Our tool is implemented and tested within a controlled testbed environment built on top of a 5G standalone core server provided by a hardware base station (gNb) and uses an Software Defined Radio(SDR) card for radio transmission. We validate our testbed and the developed tools by successfully injecting at the 5G air interface and the network control levels different messages including RRC and NGAP/NAS over already established communications.