Ethereum contract loses millions due to storage collision

A security investigation by SlowMist reveals a critical flaw in an Ethereum smart contract that leads to mass theft. Attackers managed to siphon approximately 14.4 ETH by exploiting a storage collision between two parts of the code. This occurred at a time when Ethereum was trading around $1.610,

Ethereum is available on OKX and Bybit.

Storage slot collision causes million-dollar theft

The problem arises from a collision between two different functions in the ATOHook contract. The rewards mapping and the reentrancy guard from Solady (a widely used security library) use the same storage slot. This causes the sentinel value of the reentrancy guard (0xffffffffffffff) to be read simultaneously as a reward amount.

Because of this storage slot collision, an attacker can claim artificially high rewards. The getReward function is protected by a nonReentrant modifier, but the modifier actually writes the defence mechanism to the same location where the rewards are stored. This creates a dual use of the same memory location.

Attacker manages to withdraw money 200 times in a row

The attacker (identified as 0x2d2aafc193c24e59bd16139056ac9b4df4d37ad0) was able to claim rewards 200 times in succession thanks to this flaw. Each time, the inflated amount was paid out as ETH, resulting in a total loss of 14.4 ETH for the victim.

SlowMist demonstrates that this is a result of insufficient testing and architectural errors in the contract. The collision between rewards accounting and the reentrancy guard storage makes it possible to completely bypass the security mechanism and drain unlimited funds.

How such errors can be prevented

This incident underscores the importance of thorough code audits and a proper understanding of the underlying blockchain architecture. When working with well-known libraries such as Solady, it is crucial to know how they internally use storage slots. A simple configuration change or moving a single mapping could prevent such a catastrophic collision.

Developers must be aware that storage collisions are not merely a theoretical danger. They can be directly exploitable and cost millions. The victim in this case suffers the consequences of a preventable error that should have been detected through proper auditing.