  Telecom networks together with mobile phones must be rigorously tested for
robustness against vulnerabilities in order to guarantee availability. RRC
protocol is responsible for the management of radio resources and is among the
most important telecom protocols whose extensive testing is warranted. To that
end, we present a novel RRC fuzzer, called Berserker, for 4G and 5G.
Berserker's novelty comes from being backward and forward compatible to any
version of 4G and 5G RRC technical specifications. It is based on RRC message
format definitions in ASN.1 and additionally covers fuzz testing of another
protocol, called NAS, tunneled in RRC. Berserker uses concrete implementations
of telecom protocol stack and is unaffected by lower layer protocol handlings
like encryption and segmentation. It is also capable of evading size and type
constraints in RRC message format definitions. Berserker discovered two
previously unknown serious vulnerabilities in srsLTE -- one of which also
affects openLTE -- confirming its applicability to telecom robustness.
