NowSwap DEX on Ethereum was exploited on September 15, 2021 when K constant value mismatch (updated in 2 of 3 code locations during upgrade) allowed attacker to swap 1 wei for 98% of pool reserves.

Nowswap, a decentralized exchange on Ethereum, was attacked by a flash loan. The attacker emptied Nowswap’s liquidity pool. The liquidity pool was reduced from US$1,069,197 to US$24.15. The attacker made a profit of 536,000 USDT and 158 WETH. A total of more than 1 million US dollars. The attacker used the K value verification vulnerability in the Nowswap USDT/WETH transaction pair contract to perform multiple exchanges, and each exchange obtained multiple times the normal due assets, until the assets in the trading pair pool were exhausted.