Blockchain and cryptocurrency networks, particularly those based on Proof-of-Work (PoW) mechanisms, face numerous security vulnerabilities, including 51% attacks, Sybil attacks, Replay attacks, Routing attacks, Eclipse attacks, Phishing attacks, Distributed Denial of Service (DDoS) attacks, collision attacks, injection attacks, and more. Among these, the 51% and Sybil attacks pose significant threats (Anita, N., & Vijayalakshmi, M. (2019). Blockchain security attack: A brief survey. Paper presented at the 10th International Conference on Computing, Communication and Networking Technologies (ICCCNT), Kanpur, India. https://doi.org/10.1109/ICCCNT45670.2019.8944615 ; Crypto.com. (2022). What is consensus? A beginner’s guide. Retrieved February 2024, from https://crypto.com ; Dwivedi, K., Agrawal, A., Bhatia, A., & Tiwari, K. (2024). Novel classification of attacks on blockchain layers: Vulnerabilities, attacks, mitigations, and research directions. Retrieved from https://arxiv.org/abs/2404.18090 ). A 51% attack occurs when a single entity gains majority computational control, enabling actions like double spending and transaction censorship, thereby undermining the integrity and trust of the network. Sybil attacks involve an adversary creating multiple network nodes to disrupt or influence operations, eroding user confidence and trust in the network’s reliability. This chapter provides an in-depth analysis of these attacks and their impact on trust within the blockchain ecosystem. We explore how security challenges can severely harm trust by compromising data integrity, causing financial losses, and diminishing user confidence. To rebuild and enhance trust, we review current mitigation strategies, including decentralizing mining resources, implementing alternative consensus mechanisms, incurring costs for node creation, and utilizing trusted certifications. Additionally, we discuss the importance of vigilant coding practices, updated network protocols, user education, and sometimes drastic measures like hard forks. Furthermore, this chapter emphasizes that protecting against these attacks and restoring trust requires a holistic approach involving technical solutions and community engagement. As the cryptocurrency space continues to grow, the sophistication of attacks and security measures also evolves. We will conclude by examining ongoing and emerging security challenges and their implications for the future trust and security of blockchain networks.