BaseSwapCallback Exploit — Public `uniswapV3SwapCallback` Approval Drain

Source & credit. Exploit reproduction, trace data, and analysis adapted from DeFiHackLabs by SunWeb3Sec — an open registry of reproduced on-chain exploits. Standalone Foundry PoC and full write-up: 2025-10-BaseSwapCallback_exp in the evm-hack-registry mirror. Upstream DeFiHackLabs PoC: src/test/…/BaseSwapCallback_exp.sol.

Vulnerability classes: vuln/access-control/missing-auth · vuln/dependency/unsafe-external-call

Reproduction: isolated Foundry project at this project folder. Verbose run: output.txt. Victim is unverified; PoC spoofs a Uniswap V3 pool via a minimal token0() contract.

Key info#

TL;DR#

An unverified Base router-like contract exposes uniswapV3SwapCallback(int256,int256,bytes) without verifying that msg.sender is a real Uniswap V3 pool created by a trusted factory.

Its only “auth” is effectively:

1. Call msg.sender.token0() (and related pool surface) so the caller looks like a pool.
2. Decode attacker-controlled data as (payer, recipient).
3. WETH.transferFrom(payer, msg.sender, amount0Delta) when amount0Delta > 0.

Anyone who previously approved the vulnerable contract can be drained by a one-line fake pool:

function token0() external pure returns (address) { return WETH; }

function exploit(uint256 amount) external {
    bytes memory data = abi.encode(VICTIM, address(this));
    IVulnerable(VULN).uniswapV3SwapCallback(int256(amount), -1, data);
}

Live attacker used a MEV-style multicall bot and ~219 repeated ~0.253 WETH pulls (same encoding) totaling 55.51 WETH. The PoC drains the full remaining balance in a single callback.

Root cause#

Correct Uniswap V3 integrators must authenticate the callback, e.g. via CallbackValidation.verifyCallback(factory, PoolAddress.PoolKey({...})) so that msg.sender == factory.getPool(token0, token1, fee).

This contract either:

• omits factory verification entirely, or
• only checks that msg.sender responds to token0() / returns a non-zero token,

which is trivial to spoof. Combined with arbitrary payer in callback data, any residual ERC-20 allowance becomes freely stealable.

Observed pre-attack state (fork block 37,487,848):

Attack steps (PoC)#

1. Fork Base at 37487849 - 1.
2. Deploy FakePool with token0() → WETH.
3. Call uniswapV3SwapCallback(victimBal, -1, abi.encode(victim, fakePool)) from FakePool.
4. Vulnerable contract pulls full victim WETH into FakePool via transferFrom.
5. Sweep WETH to the attacker test contract.

Fix class#

• Require msg.sender is the canonical pool for the swap path (CallbackValidation / factory getPool).
• Never accept a user-controlled payer unless that user is msg.sender of the initiating swap and a reentrancy/lock flag proves an in-flight swap.
• Prefer pulling tokens in the outer function (before pool.swap) rather than in the callback from arbitrary payers.

Sources#

• CertiKAlert
• SkyLens tx

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 2025-10-BaseSwapCallback_exp (evm-hack-registry mirror).
• Upstream DeFiHackLabs PoC: BaseSwapCallback_exp.sol.
• Attack transaction: view on explorer.

Alerts & third-party analyses

• Original alert / thread: post on X.
• DeFiHackLabs incident explorer: search "BaseSwapCallback Exploit".
• Web3Sec X hacked database: search.
• Rekt leaderboard: search.
• Solodit incident search: search.

These dashboards index community alerts tweets, post-mortems, and independent write-ups. Reach them through the protocol name above to cross-check this reproduction against other analyses.