OKEx lost about $5.6M in a 51% attack on 18 Aug 2020, according to QuillMonitor (confirmed).
Digibastion is a free Web3 security site. It tracks crypto hacks, exploits and phishing from QuillMonitor and other public trackers, links every record to its source, and sends alerts by email and on Telegram. It also has security checklists, a two-minute OpSec self-check and practical guides. Teams that need more can hire Digibastion for OpSec consulting or a full-stack security review, or try Vantage (beta) to check their domain's security.