Page 3 of Satoshi Nakamoto’s whitepaper, Bitcoin: A Peer-to-Peer Electronic Cash System, states the following:
“If a majority of CPU power is controlled by honest nodes, the honest chain will grow the fastest and outpace any competing chains.”
The “honest[y]” of more than half of miners is a core requirement for the security of Bitcoin and any proof-of-work cryptocurrencies based on Bitcoin. Honest action, in this context, means following the behavior described in the Bitcoin white paper. This is sometimes described as a “security risk” or “attack vector,” but is more accurately described as a known limitation to the proof-of-work model.