  Cellular Vehicle-to-Everything (C-V2X) networks are increasingly adopted by
automotive original equipment manufacturers (OEMs). C-V2X, as defined in 3GPP
Release 14 Mode 4, allows vehicles to self-manage the network in absence of a
cellular base-station. Since C-V2X networks convey safety-critical messages, it
is crucial to assess their security posture. This work contributes a novel set
of Denial-of-Service (DoS) attacks on C-V2X networks operating in Mode 4. The
attacks are caused by adversarial resource block selection and vary in
sophistication and efficiency. In particular, we consider "oblivious"
adversaries that ignore recent transmission activity on resource blocks,
"smart" adversaries that do monitor activity on each resource block, and
"cooperative" adversaries that work together to ensure they attack different
targets. We analyze and simulate these attacks to showcase their effectiveness.
Assuming a fixed number of attackers, we show that at low vehicle density,
smart and cooperative attacks can significantly impact network performance,
while at high vehicle density, oblivious attacks are almost as effective as the
more sophisticated attacks.
