DeFi User Loses $50.4M in One Swap as MEV Bots and Protocol Failures Collide

Table of Contents A DeFi user suffered approximately $50.4 million in losses from a single swap on the Aave platform. The user exchanged aEthUSDT for aEthAAVE through a CoW Swap widget and received only $36,000 in return. Both Aave and CoW Swap have released detailed post-mortem reports on the incident. The reports cite a combination of user error, illiquid markets, and multiple technical failures. MEV bots also exploited the situation, extracting tens of millions in profit from the DeFi trade. The user manually confirmed a “High price impact (99.9%)” warning before completing the DeFi swap. Aave’s report confirmed this warning was clearly visible within the interface. The trade proceeded regardless, setting the stage for what followed. CoW Swap’s report identified multiple system-level failures that escalated the outcome. A legacy hardcoded gas ceiling rejected better quotes that could have routed the trade efficiently. The winning solver also failed to execute the trade on-chain as intended. Together, these two failures severely limited the options available for completing the swap. 🚨THE BIGGEST SINGLE TRANSACTION LOSS IN DEFI? A POST-MORTEM OF HOW A USER LOST ~$50M IN ONE CLICK Recently, a DeFi user swapped $50.4 million of aEthUSDT into just $36,000 of aEthAAVE on the Aave platform using a CoW Swap widget. Both protocols have now released reports on… pic.twitter.com/NsbDYUUttV — Coin Bureau (@coinbureau) March 15, 2026 Further complicating matters, a suspected mempool leak exposed the private transaction to public view. This meant any observer, including automated MEV bots, could see the order before confirmation. The exposure proved costly, as it directly opened the door for a targeted attack. Because better routes were blocked, the trade was pushed through a SushiSwap AAVE/WETH pool. That pool held only about $73,000 in total liquidity at the time of the swap. Routing a $50 million order through such a thin market caused extreme price slippage. The user ultimately received a fraction of what the trade should have returned. Once the transaction leaked to the public mempool, an MEV bot quickly identified the opportunity. The bot front-ran the trade by buying available AAVE before the user’s order confirmed. This action drove the price of AAVE sharply higher, hurting the user’s final settlement. The bot then sold its AAVE position immediately after the user’s trade was filled. This sandwich attack netted the bot an estimated $9.9 million in profit. @CoWSwap’s report identified the mempool leak as a central factor enabling this attack on the DeFi user. To guarantee the correct block sequence, the MEV bot paid Titan Builder directly. The block builder extracted roughly $34 million in ETH for facilitating the arrangement. This coordination between the bot and the builder was key to the attack’s execution. In response, @CoWSwap has patched its legacy gas limits to prevent similar routing failures. @aave is deploying “Aave Shield,” which will automatically block swaps with a price impact above 25% by default. Both protocols are now working to prevent this type of loss from recurring across DeFi.