We present a security analysis of the SPREADS 1 system, a distributed storage service based on a centralized peer-to-peer architecture. We formally modelled the salient behavior of the actual system using ABCD, a high level specification language with a coloured Petri net semantics, which allowed the execution states of the system to be verified. We verified the behavior of the system in the presence of an external Dolev-Yao attacker, unearthing some replay attacks in the original system. Furthermore, since the implementation is also a formal model, we have been able to show that any execution of the model satisfies certain desirable security properties once these flaws are repaired.