  Blockchain technology has evolved through many changes and modifications,
such as smart-contracts since its inception in 2008. The popularity of a
blockchain system is due to the fact that it offers a significant security
advantage over other traditional systems. However, there have been many attacks
in various blockchain systems, exploiting different vulnerabilities and bugs,
which caused a significant financial loss. Therefore, it is essential to
understand how these attacks in blockchain occur, which vulnerabilities they
exploit, and what threats they expose. Another concerning issue in this domain
is the recent advancement in the quantum computing field, which imposes a
significant threat to the security aspects of many existing secure systems,
including blockchain, as they would invalidate many widely-used cryptographic
algorithms. Thus, it is important to examine how quantum computing will affect
these or other new attacks in the future. In this paper, we explore different
vulnerabilities in current blockchain systems and analyse the threats that
various theoretical and practical attacks in the blockchain expose. We then
model those attacks using Petri nets concerning current systems and future
quantum computers.
