Beosin’s security team said a recent surge of activity on Robinhood Chain has drawn many users to trade on Uniswap pools deployed on that network. One risk pattern highlighted in the article is straightforward: a user spots a Uniswap v4 pool showing 0% fees and a quote a few basis points better than the best market price, picks it as the best route, and then receives almost 1% fewer tokens than quoted once the trade settles. After checking on-chain data, the loss can be traced to a hidden "dark tax" inside the Hook contract, which uses afterSwapReturnsDelta privileges to intercept part of the tokens before settlement.
The piece also cited September research from liquidity aggregation infrastructure provider 0x Protocol. Across six chains, 0x analyzed 84,163 Uniswap v4 Hooks through static analysis, dynamic analysis, and observation of real executions. Only 19.4% were judged safe, 54.2% were categorized as malicious, and another 26.4% showed behavior considered suspicious. Beosin said the goal of the report is to explain how attack vectors in Uniswap v4 Hooks work so that users and developers can better understand the kinds of malicious behavior they may contain.
How the Uniswap Hook mechanism expands the attack surface
Under Uniswap v4, liquidity pools are managed through PoolManager, while Hook contracts can inject custom logic at several lifecycle points, including pool initialization, adding liquidity, removing liquidity, swaps, and donations. That logic can affect trading fees, balances before and after execution, and settlement behavior.
Beosin grouped the main security risks around several areas:
- Hooks returning abnormal balance changes;
- Hooks making external contract calls during callbacks;
- Hooks reading spot prices and using them to decide fee levels;
- Hooks storing state across pools and across users;
- Hooks exploiting callback order and transaction rollback behavior to create denial-of-service conditions.
According to the article, an attacker may first set up what appears to be a normal liquidity pool, embed hidden charges or blacklist logic in the Hook, and then attract users through a frontend, an airdrop, an aggregator, or social media. Once users start swapping or supplying liquidity, the attacker can use custom logic or external calls to extract assets from them.
Beosin said the malicious Hook patterns it currently sees most often include the following:
- charging a small extra fee on every trade;
- applying different fee rates to specific addresses;
- suddenly changing behavior when price approaches a threshold;
- preventing liquidity providers from removing liquidity;
- burning large amounts of gas in callbacks so swaps fail;
- tricking users into calling fake unlock, settle, or token transfer functions.
Examples of malicious Hooks
1. Hidden trading fees
The first example in the article shows a Hook that charges additional fees to selected addresses while exempting others. In the sample contract, named SuspiciousFeeHook, the developer uses privilegedUsers to exempt the developer, a market maker, or designated bots, while everyone else pays an extra charge.
The sample contract defines manager, feeRecipient, and a privilegedUsers mapping. In beforeSwap, the function first checks that msg.sender is the manager. If the sender is in privilegedUsers, it returns amountIn unchanged. Otherwise it calculates hiddenFee = amountIn * 30 / 10_000, sets adjustedAmountIn to amountIn - hiddenFee, and then calls IERC20(token()).transferFrom(sender, feeRecipient, hiddenFee) to move the hidden fee.
Beosin noted that in a real v4 architecture, a Hook generally cannot bypass PoolManager settlement rules at will. Even so, it can still affect final settlement through custom deltas, external token calls, or pool configuration. That is why an audit needs to trace every path that changes balances inside the Hook.
2. Dynamic pricing or fee logic
The second example, AddressBasedHook, shows the risk of address-based fees. The contract contains a blocked mapping and a customFee mapping. In beforeSwap, if blocked[sender] is true, the transaction reverts with "blocked trader." If customFee[sender] is not zero, the function returns that fee for the address. If not, it returns 10_000, which the code comments label as a default 100% fee.
This setup combines blacklist logic with different fees for different addresses. If a frontend does not fully disclose the fee structure, the conditions users see in the pool may not match the conditions they actually face when trading. Beosin said reviews and development checks must confirm that every possible returned fee value stays within protocol-allowed bounds, rather than testing only ordinary paths.
3. Reentrancy and external callbacks
The third risk area is reentrancy and callback abuse. In the sample contract ReentrantHook, the Hook stores callbackTarget and a private entered flag. The beforeSwap function first requires that entered is false, then sets entered to true, calls ICallback(callbackTarget).onSwap(sender), and finally resets entered to false.
Beosin said an external call inside a Hook can trigger many types of downstream behavior, including:
- ERC-777 or custom ERC-20 callbacks;
- ERC-721 / ERC-1155 receiver callbacks;
- lending protocol callbacks;
- aggregator callbacks;
- reinvocation of the same Hook;
- swaps in other pools.
If state updates happen in the wrong order relative to external calls, the result can be duplicated calculations, bypassed single-transaction limits, duplicate fee collection, cross-pool state contamination, or broken price and limit checks. The article stressed that although the v4 core contracts impose their own unlock and settlement constraints, that does not make arbitrary external calls inside a Hook safe. Each Hook has to define its own security boundary.
4. Oracle manipulation through spot prices
The fourth example, FragileOracleHook, focuses on the danger of treating the current pool price as a trusted oracle. The contract stores lastObservedPrice. In beforeSwap, it computes spotPrice = priceFromSqrtPrice(currentSqrtPriceX96). If spotPrice is greater than 1_100e18, it calls triggerLiquidation(), and then writes spotPrice into lastObservedPrice.
Beosin said an attacker can use a flash loan to obtain a large amount of assets within a single transaction, perform a large swap against the target pool, push the instantaneous price to an extreme, and then trigger liquidation, reward, or collateral valuation logic inside the Hook. If the Hook trusts the current pool price directly, the manipulation can be completed within one block.
5. Blocking liquidity exits
The fifth pattern is a Hook that allows liquidity in but blocks liquidity out. The sample ExitBlockingHook keeps a blockedLP mapping. Its beforeRemoveLiquidity(address provider) function reverts with "withdraw disabled" when blockedLP[provider] is true. It also includes setExitLock(address user, bool locked), callable by onlyOwner, to update blockedLP[user].
Beosin said a Hook of this type can let users add liquidity while making it impossible for them to remove it later. The article listed several common trigger conditions:
- the user address is placed on a blacklist;
- the pool reaches a certain TVL;
- the time passes a specific timestamp;
- the Hook administrator flips a hidden switch;
- the user has traded through an aggregator;
- the user has not paid an additional fee.
6. Gas griefing
The sixth category is gas griefing. Beosin said this pattern is also common in earlier smart contract attacks: the attacker makes a transaction consume abnormal amounts of gas until it fails.
In the sample GasGriefHook, beforeSwap(uint256 n) runs a loop from i = 0 to i < n and computes keccak256(abi.encodePacked(i, msg.sender)) on every iteration.
The article added that more subtle forms include:
- iterating over an ever-growing address array;
- cleaning up historical user state;
- querying balances across many pools one by one;
- performing large amounts of computation before failure;
- calling multiple malicious external contracts.
How Beosin suggests reviewing Hooks
1. Check the Hook address permission bits
The first step is to confirm which callback permissions the Hook address declares and compare those permissions with the functions that actually exist in the bytecode. If the address declares beforeSwap, afterSwap, beforeAddLiquidity, or beforeRemoveLiquidity, each matching function should be reviewed separately.
2. Pull the code and inspect implementation details
Beosin said reviewers can use a block explorer or an RPC call such as eth_getCode(hookAddress) and then check for several specific issues:
- whether the Hook is a proxy contract;
- whether an upgrade entry point exists;
- whether self-destruct logic exists;
- whether there are unknown external calls;
- whether the deployed code matches the verified source.
3. Examine the pool initialization transaction
The article said that PoolKey, currency0 / currency1, fee, tick spacing, the Hook address, the initialization address, the administrator, and the fund recipient address should all be reviewed in the pool’s initialization transaction.
4. Simulate boundary conditions
Beosin recommended simulating at least the following scenarios: a normal swap, a very small swap, a large swap, a zero-amount swap, extreme price changes, repeated swaps, adding and then immediately removing liquidity, Hook administrator changes, token transfer failures, and callback behavior from external calls.
During those simulations, reviewers should compare the amount users actually pay with the amount they actually receive, then track Hook balance changes, PoolManager deltas, event logs, and gas consumption.
Closing view from Beosin
In its conclusion, Beosin said Uniswap v4 Hooks are designed to make Uniswap more extensible, but they also introduce new attack surfaces and new ways to abuse custom logic. For ordinary users, the article said the key questions are whether a Hook has been verified, whether its source code is public, and whether its permissions are constrained.
For the broader DeFi ecosystem, the report said DeFi protocols and security firms need to do more, including building detection tools and strengthening scrutiny of liquidity pools, in order to improve the safety and reliability of on-chain trading.
The article also listed related reading under the title "Uniswap v4 Hook Analysis: Architecture, Common Vulnerabilities, and Defensive Practices."
Its disclaimer said the market carries risk and the article does not constitute investment advice. Users should decide whether the opinions, views, or conclusions in the piece fit their own circumstances and bear responsibility for their own investment decisions.