Blockchain is an innovative technology that represents a decentralized alternative to
centralized systems for recording transactions and maintaining records. With the advancement
of blockchain technology and its integration into applications, security remains one of the key
issues. This thesis, through a review of existing literature, explores the security challenges
faced by the two largest networks, Bitcoin and Ethereum. In the past, there have been many
attacks on various blockchain systems that exploited vulnerabilities and bugs, causing
significant financial losses. Therefore, it is crucial to understand how these attacks are carried
out. In this thesis, I explore the concept of blockchain and its utility. Through a detailed
analysis of the consensus protocols used by Bitcoin (Proof of Work, PoW) and Ethereum
(Proof of Stake, PoS), I examine the impact of these protocols on the security of the network
and transactions. By reviewing the literature, I have identified the most frequently mentioned
attacks that are possible on the Bitcoin and Ethereum networks, and have described them in
detail, summarising known findings. Using various sources, I have tried to assess the
feasibility of the selected attacks and the potential costs of execution. The goal of this thesis is
to contribute to a better understanding of the current vulnerabilities in blockchain networks
and the likelihood of these vulnerabilities being exploited.
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