When an account doesnt exist, querying its storage slot should return bytes32 0 value. The problem is the account is not validated to be non-existing in GatewayVM::getStorage.
There are two issues here:
https://github.com/code-423n4/2024-12-unruggable/blob/9e0e908a207f088dd843fcdc3e4de5fa0edd6c5c/contracts/GatewayVM.sol#L193
If the Gateway request (ses.req) passed to fetchCallBack() is allowed to skip the set target instruction. The prover can prove a storage slot is bytes32(0) in value even though the storage slot for the given target account has non-zero values.
https://github.com/code-423n4/2024-12-unruggable/blob/9e0e908a207f088dd843fcdc3e4de5fa0edd6c5c/contracts/GatewayFetchTarget.sol#L42
Suppose this case: A client contract that derives GatewayFetchTarget has an insufficient check on the passed carry is valid. 
The prover passed in a modified ses.req which directly call READ_SLOT without SET_TARGET. GatewayVM::getStorage is invoked and because the account(target) is never validated, vm.storageRoot is 0 (same as NOT_A_CONTRACT), GatewayVM assumes this target is validated to be not a contract (either EOA or non-existent) and return bytes32(0) as the storage slot value.
Consider changing NOT_A_CONTRACT constant value to be a special hash other than 0x0.
