Umbrella Network RewardPool Exploit — Integer Underflow in `withdraw`

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: 2022-03-Umbrella_exp in the evm-hack-registry mirror. Upstream DeFiHackLabs PoC: src/test/…/Umbrella_exp.sol.

Vulnerability classes: vuln/arithmetic/underflow

Reproduction: the PoC compiles & runs in an isolated Foundry project at this project folder. Full verbose trace: output.txt. Verified vulnerable source: StakingRewards.

Key info#

TL;DR#

The vulnerable code (quoted in the PoC):

function _withdraw(uint256 amount, address user, address recipient) internal nonReentrant updateReward(user) {
    require(amount != 0, "Cannot withdraw 0");
    // not using safe math ...
    _totalSupply = _totalSupply - amount;          // ⚠️ underflows if amount > _totalSupply
    _balances[user] = _balances[user] - amount;    // ⚠️ underflows
    IERC20(stakingToken).safeTransfer(recipient, amount);
    ...
}

There is no check that the caller has actually staked amount. The attacker, who staked nothing, calls StakingRewards.withdraw(8_792_873_290_680_252_648_282) (8,792 LP). On a pre-0.8 contract 0 - amount wraps to type(uint256).max - amount + 1, so the arithmetic "succeeds", and safeTransfer(recipient, amount) hands the attacker 8,792 UniLP tokens straight out of the staking pool's balance.

After exploiting, Attacker UniLP Balance: 8792873290680252648282 — the full drain.

Root cause#

A missing-balance-check + unchecked subtraction (integer underflow) on a withdrawal path. Canonical StakingRewards (Synthetix) checks require(amount > 0) and implicitly relies on _balances[msg.sender] >= amount via SafeMath's revert. This fork dropped both the SafeMath and the balance check, so a withdraw of any amount from an un-staked account underflows the accounting and pays out real stakingToken.

Preconditions#

• None beyond the ability to call withdraw(amount) with amount exceeding your stake. The contract must be compiled pre-0.8 (or use unchecked math) so the underflow does not revert.

Diagrams#

Remediation#

1. Check _balances[msg.sender] >= amount before subtracting (or use Solidity 0.8+ checked math).
2. Use OZ SafeMath / SafeERC20 consistently; never hand-roll subtraction on balances.
3. Invariant test: withdraw must revert if amount > stake.
4. Cap withdrawal to the user's actual staked balance.

How to reproduce#

_shared/run_poc.sh 2022-03-Umbrella_exp --mt testExploit -vvvvv

• RPC: mainnet archive (block 14,421,983). Infura mainnet in foundry.toml.
• Result: [PASS] — After exploiting, Attacker UniLP Balance: 8792873290680252648282.

Reference: Umbrella Network RewardPool integer-underflow withdraw, Mar 2022 (~$700K).

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 2022-03-Umbrella_exp (evm-hack-registry mirror).
• Upstream DeFiHackLabs PoC: Umbrella_exp.sol.

Alerts & third-party analyses

• DeFiHackLabs incident explorer: search "Umbrella Network RewardPool 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.