## How to Access the User's Address (msg.sender) in Uniswap v4 Hooks One of the most common challenges when developing Uniswap v4 hooks is identifying the original end-user who initiated a transaction. For many hook applications—such as tracking loyalty points, implementing KYC checks, or enforcing user-specific permissions—knowing the end-user's address is essential. However, due to the layered architecture of Uniswap v4, the global `msg.sender` variable within a hook does not point to the user. This lesson explains why this happens and presents the standard, secure pattern for retrieving the user's address. ### The `msg.sender` Problem in a Multi-Contract Call Chain To understand the problem, we must first look at the typical transaction flow for a swap. A user rarely interacts directly with the core `PoolManager` contract. Instead, they use a peripheral or "Router" contract that simplifies the process. This creates a multi-step call chain: 1. The **User** calls a `swap` function on a **Router** contract. 2. The **Router** contract then calls the **PoolManager** contract to execute the swap logic. 3. The **PoolManager** contract, as part of its execution, calls the registered **Hook** contract. In Solidity, `msg.sender` always refers to the address of the *immediate* caller. Let's trace its value through this chain: * Inside the **Router** contract, `msg.sender` is the **User's address**. * Inside the **PoolManager** contract, `msg.sender` is the **Router's address**. * Finally, inside the **Hook** contract, `msg.sender` is the **PoolManager's address**. As you can see, by the time execution reaches your hook, the original user's address is lost if you only rely on the `msg.sender` global variable. Your hook is blind to who started the transaction. ### The Standard Solution: Using the `sender` Parameter The Uniswap v4 architecture anticipates this problem and provides a robust solution. When the `PoolManager` invokes a hook function, it passes along a special `sender` parameter. This parameter is the `msg.sender` from the `PoolManager`'s perspective—which, in our call chain, is the address of the **Router** contract. This `sender` parameter is the key. It gives the hook a direct reference to the contract that initiated the pool action. The solution is a two-part pattern: 1. **Router Implementation:** The Router contract must expose a public or external view function that returns the address of the user who called it. A common convention is a function named `getMsgSender()`. 2. **Hook Implementation:** The hook contract uses the `sender` address it receives from the `PoolManager` to make an external call to the Router's `getMsgSender()` function, thereby retrieving the original user's address. #### A Step-by-Step Example Let's walk through the `swap` transaction again, this time applying the solution: 1. **User Initiates Swap:** A user calls `swap()` on a `Router` contract. 2. **Router Calls PoolManager:** The `Router` forwards the call to the `PoolManager`. In the context of the `PoolManager`, `msg.sender` is now the `Router`'s address. 3. **PoolManager Executes Hook:** After the swap logic, the `PoolManager` calls the `afterSwap` function on your hook. It passes the `Router`'s address as the `sender` argument: `afterSwap(address sender, ...)`. 4. **Hook Retrieves User Address:** Inside your `afterSwap` function, you now have the `Router`'s address in the `sender` variable. You perform an external call like `IRouter(sender).getMsgSender()`. This call executes on the Router contract, which returns the original user's address, solving the problem. ### Implementing the Solution in Code Here are the conceptual code blocks that demonstrate this pattern. #### In the Router Contract The Router needs a simple getter function to expose its `msg.sender`. For this pattern to be widely adopted, it's beneficial for routers to implement a standardized interface. ```solidity // Inside Router.sol contract Router { /** * @notice Returns the msg.sender of the transaction originator. * @dev This function is designed to be called by a Uniswap v4 hook * to identify the end-user who initiated the action. * @return address The address of the account that called the Router. */ function getMsgSender() external view returns (address) { // In the context of a user's transaction, msg.sender is the user. // Note: A production implementation might store msg.sender in a state // variable at the start of the primary function to avoid potential // complexities with re-entrancy or multicalls. return msg.sender; } // ... other swap and liquidity logic ... } ``` #### In the Hook Contract The hook contract must perform two critical actions: first, verify that the `sender` is a trusted router, and second, call the getter function to retrieve the user's address. ```solidity // Inside MyHook.sol // An interface to ensure the sender has the required function. interface IRouter { function getMsgSender() external view returns (address); } contract MyHook { // A mapping to maintain an allow-list of trusted router addresses. mapping(address => bool) public authorizedRouters; // A hook function, e.g., afterSwap function afterSwap(address sender, PoolKey calldata key, IPoolManager.SwapCallbackData calldata data) internal override returns (bytes4) { // CRITICAL SECURITY CHECK: Ensure the sender is a known, trusted router. if (!authorizedRouters[sender]) { revert("SenderNotAuthorized"); } // Get the original user's address by calling the trusted router. address user = IRouter(sender).getMsgSender(); // Now the hook can use the 'user' address for its custom logic. // For example, log user activity, award points, etc. // logUserPoints(user, data.amount0); return MyHook.afterSwap.selector; } } ``` ### Security is Paramount: Authorizing the Sender The most critical part of this pattern is the security check: `if (!authorizedRouters[sender])`. Without this check, your hook is vulnerable. A malicious actor could deploy a contract that calls the `PoolManager` directly. In this scenario, the `PoolManager` would pass the malicious contract's address as the `sender` to your hook. If your hook blindly trusts this `sender` and calls `getMsgSender()` on it, the malicious contract could return any address it wants, effectively spoofing the user and bypassing your hook's logic. Therefore, your hook **must** maintain an allow-list or check against a trusted registry of router addresses before making any external calls. Never trust the `sender` parameter without verification.