What Happens When Someone Controls Over Half a Blockchain's Computing Power

A blockchain’s security fundamentally depends on its decentralization. When a single entity or coordinated group manages to control more than 50% of a network’s mining hashrate or computational resources, it creates a critical vulnerability known as a 51% attack. This threshold gives attackers unprecedented power to manipulate the network according to their interests.

The Mechanics of Network Compromise

Once malicious actors seize majority control over mining operations, they gain the ability to dominate transaction validation. They can selectively choose which transactions to include in blocks, determine the order these transactions appear, and most alarmingly, reverse previously confirmed transactions. This capability enables double-spending—the same cryptocurrency can be sent to different recipients, with attackers retaining their original funds after the reversal.

Real-World Consequences

Consider Bitcoin as an example: if attackers commanded the majority of its hashrate, they wouldn’t just disrupt transaction flow. The implications extend far beyond temporary network delays. By controlling the consensus mechanism, bad actors can execute denial of service attacks that effectively lock legitimate users out of the network. The attack surface expands further when attackers attempt to unilaterally alter block rewards, create tokens from nothing, or drain existing cryptocurrency holdings from network participants.

Why This Remains a Systemic Risk

The 51% attack represents one of blockchain technology’s foundational security challenges. While most mature networks like Bitcoin maintain sufficient decentralization that such attacks remain economically impractical, smaller or emerging blockchains remain vulnerable. Understanding this attack vector is essential for evaluating any cryptocurrency network’s true security posture and long-term viability.