ACT Opportunity and System State

ACT Metadata

• Report title: H2O Helper-Token Reward Drain on BSC
• Protocol name: H2O (BSC)
• ACT flag: This incident is modeled as an ACT (Adversarial Contract Threat) opportunity.
• Root cause category: protocol_bug

Pre-state σ_B and Block Height

The ACT opportunity is evaluated at block height 47454937 on BSC (chainid 56), immediately before exploit transaction 0x994abe7906a4a955c103071221e5eaa734a30dccdcdaac63496ece2b698a0fc3. The pre-state σ_B is the publicly reconstructible BSC state before this tx, including balances and allowances for:

• H2O token: 0xe9c4d4f095c7943a9ef5ec01afd1385d011855a1
• H2O/USDT PancakePair: 0x42717781d93197247907f82482ae1d35d7bc101b
• USDT PancakeV3Pool (flash-loan pool): 0x4f31fa980a675570939b737ebdde0471a4be40eb
• Helper contract: 0x03ca8b574dd4250576f7bccc5707e6214e8c6e0d
• Helper tokens: 0x6AC860AE21993d65b790a95Cfc1A3A4b42dd0ce3 and 0xD061A395190581cb677b5FfFF1dc38448D4976c8
• BEP20 USDT: 0x55d398326f99059ff775485246999027b3197955
• Adversary EOA: 0x8842dd26fd301c74afc4df12e9cdabd9db107d1e

This state is reconstructed and cross-checked using:

• Seed metadata and balance diffs for the exploit tx (including prestate-based ERC20 balance deltas for USDT and H2O).
• Additional prestate-based balance diffs for the cash-out txs 0x33cfa125... and 0xa0688f0f....
• On-chain BNB/USDT price data at the exploit block obtained from a PancakeSwap v2 WBNB/USDT pair.
• Verified H2O source code and decompiled helper/helpertoken contracts.

Transaction Sequence b (Adversary-Crafted)

The ACT sequence b consists of three adversary-crafted transactions on BSC:

1. 0x994abe7906a4a955c103071221e5eaa734a30dccdcdaac63496ece2b698a0fc3

   • Type: adversary-crafted exploit aggregation.
   • Behavior: helper contract borrows 100,000 USDT from PancakeV3Pool 0x4f31fa98..., swaps USDT into H2O on pair 0x42717781..., repeatedly triggers H2O._calulate via pair-initiated transfers to move H2O from the token contract into the pair and helper, then exits the flash loan with a net 22,688.210530453207954293 USDT increase on helper 0x03ca8b57..., while paying a 50 USDT flash-loan fee to the pool holder.

2. 0x33cfa1257d85bcd77206438d5d7efdf22e6cea3c306fce646fa9aa7594b964ec

   • Type: adversary-crafted cash-out.
   • Behavior: adversary EOA 0x8842dd26... calls helper.bfbaa190(USDT), which reads USDT.balanceOf(address(this)) on 0x03ca8b57... and transfers that entire USDT balance to msg.sender. Prestate-based ERC20 balance diffs show helper USDT delta -22,770.891449589918819435 and EOA USDT delta +22,770.891449589918819435.

3. 0xa0688f0f02aad1ecdbdf653c407ac9c5757a99444ee9beab7b3a60eecc59e1ae

   • Type: adversary-crafted noop cash-out attempt.
   • Behavior: the EOA repeats bfbaa190(USDT) after the helper’s USDT balance is already zero; prestate-based ERC20 diffs show zero USDT movement for both the helper and the EOA, so only gas is spent.

Exploit Predicate and Profit

The exploit predicate is expressed as a profit opportunity in USDT:

• Reference asset: USDT (0x55d398326f99059ff775485246999027b3197955).
• Adversary primary address: EOA 0x8842dd26fd301c74afc4df12e9cdabd9db107d1e.
• Gross ERC20 profit (from exploit tx): helper-contract USDT delta +22,688.210530453207954293.
• Flash-loan fee: 50 USDT paid from the H2O/USDT pair to the flash-loan pool holder, included in the pair’s USDT loss.
• Gas cost across the three sequence-b txs:
  • 0x994abe79...: -0.004001803 BNB (EOA).
  • 0x33cfa125...: -0.0000518 BNB (EOA).
  • 0xa0688f0f...: -0.0000339 BNB (EOA).
  • Total: -0.004087503 BNB.

Using the on-chain BNB/USDT price at block 47454937:

{
  "pair_address": "0x16b9a82891338f9ba80e2d6970fdda79d1eb0dae",
  "token0": "0x55d398326f99059ff775485246999027b3197955",
  "token1": "0xbb4cdb9cbd36b01bd1cbaebf2de08d9173bc095c",
  "reserve0": "11578991082411373539449813",
  "reserve1": "19952960489260653037466",
  "price_bnb_in_usdt": 580.3144394859886
}

Snippet: On-chain PancakeSwap v2 WBNB/USDT reserves and derived BNB price in USDT at the exploit block.

The BNB gas expenditure corresponds to approximately 2.372037012342296860 USDT. Combining the gross USDT gain and gas-equivalent cost yields a net adversary profit of:

• Net USDT profit: 22,685.838493440865657433 USDT.

This satisfies the ACT profit predicate in a single reference asset, computed entirely from prestate-based balance diffs and on-chain DEX pricing data.

Key Background

H2O is a BEP20-like token on BSC with symbol H2O and total supply 200,000,000 * 10**18. The contract introduces two helper tokens:

• _o2: 0x6AC860AE21993d65b790a95Cfc1A3A4b42dd0ce3
• _h2: 0xD061A395190581cb677b5FfFF1dc38448D4976c8

The H2O/USDT trading pair is created on deployment using PancakeSwap’s factory, and the contract’s own balance (address(this)) holds half of the initial H2O supply, effectively acting as a treasury.

Whenever the H2O/USDT pair 0x42717781... calls H2O.transfer(to, amount), H2O’s transfer implementation invokes _calulate(to, amount) to run helper-token-based reward logic. The _calulate function:

• Computes h2obalance = balanceOf(address(this)), the remaining H2O held by the token contract itself.
• Selects a small integer rate in {1, 2, 3, 4, 5} depending on h2obalance buckets.
• Derives a pseudo-random bit random = getRandomOnchain() % 2, where getRandomOnchain() uses keccak256(block.timestamp, msg.sender, blockhash(block.number-1)).
• Mints amount * rate / 100 helper tokens to the transfer recipient on _h2 or _o2 depending on random.
• Reads h2balance = IBEP20(_h2).balanceOf(to) and o2balance = IBEP20(_o2).balanceOf(to); when they exceed thresholds, burns some helper tokens from the recipient and transfers H2O from the token contract’s own balance (address(this)) to the recipient, capped only by the current h2obalance.

Critically, there is no per-recipient or per-transaction cap on how many times this reward branch can execute within a single transaction. A DEX pair, acting as msg.sender, can repeatedly call transfer(to=helper) in one flash-loan callback, causing _calulate to fire many times for the same helper address and drain a large amount of H2O from the token contract.

The adversary’s helper contract 0x03ca8b57... orchestrates the attack:

• It borrows USDT via a flash loan from PancakeV3Pool 0x4f31fa98....
• Swaps USDT into H2O via PancakeRouter 0x10ED43C718714eb63d5aA57B78B54704E256024E into the H2O/USDT pair.
• Repeatedly sends H2O between itself and the pair, ensuring that each pair-to-helper transfer invokes _calulate(to=helper) and fuels further reward emissions.
• Uses helper tokens as counters to trigger H2O transfers from the token contract’s treasury to the helper.
• Exposes a simple public cash-out function bfbaa190(USDT) that moves all USDT held by the helper contract to msg.sender.

H2O Contract Snippet (transfer and _calulate)

function transfer(address to, uint256 amount) public virtual override returns (bool) {
    address owner = msg.sender;
    _spendAllowance(owner, to, 0);
    _transfer(owner, to, amount);
    if (pair == msg.sender) {
        _calulate(to, amount);
    }
    return true;
}

function _calulate(address to, uint256 amount) internal {
    uint256 h2obalance = balanceOf(address(this));
    uint256 rate = 0;
    if (h2obalance <= 20_000_000 * 10**18) {
        rate = 1;
    } else if (h2obalance <= 40_000_000 * 10**18) {
        rate = 2;
    } else if (h2obalance <= 60_000_000 * 10**18) {
        rate = 3;
    } else if (h2obalance <= 80_000_000 * 10**18) {
        rate = 4;
    } else if (h2obalance <= 100_000_000 * 10**18) {
        rate = 5;
    }

    uint256 random = getRandomOnchain() % 2;
    if (random == 1) {
        IBEP20(_h2).mint(to, amount * rate / 100);
    } else if (random == 0) {
        IBEP20(_o2).mint(to, amount * rate / 100);
    }
    uint256 h2balance = IBEP20(_h2).balanceOf(to);
    uint256 o2balance = IBEP20(_o2).balanceOf(to);

    if (h2balance >= 10 * 10**18 && o2balance >= 5 * 10**18) {
        if (h2balance / 2 >= o2balance) {
            IBEP20(_o2).burn(to, o2balance);
            IBEP20(_h2).burn(to, o2balance * 2);
            uint256 amountto = o2balance;
            if (amountto >= h2obalance) {
                amountto = h2obalance;
            }
            _transfer(address(this), to, amountto);
        } else if (h2balance / 2 < o2balance) {
            IBEP20(_o2).burn(to, h2balance / 2);
            IBEP20(_h2).burn(to, h2balance);
            uint256 amountto = h2balance / 2;
            if (amountto >= h2obalance) {
                amountto = h2obalance;
            }
            _transfer(address(this), to, amountto);
        }
    }
}

Snippet: H2O token’s transfer and _calulate logic, showing how pair-initiated transfers can repeatedly move H2O from the token contract’s own balance to an attacker-controlled recipient.

Vulnerability & Root Cause Analysis

Vulnerability Brief

H2O’s helper-token reward logic allows an unbounded number of _calulate() invocations on the same recipient within a single transaction. When driven by a DEX pair, this logic can be abused to transfer large amounts of H2O from the token contract’s own balance to an attacker-controlled address. The mechanism relies on helper tokens and pseudo-randomness from getRandomOnchain, but it lacks any per-address or per-transaction cap or invariant that prevents the contract’s treasury from being drained during a single flash-loan cycle.

Detailed Root Cause

From the H2O source:

• transfer() calls _transfer(owner, to, amount) and, if msg.sender equals the H2O/USDT pair, calls _calulate(to, amount).
• _calulate() mints helper tokens _h2 or _o2 to the recipient based on a pseudo-random bit and the transfer amount. It then uses helper-token balances as thresholds to decide when to transfer H2O from the contract’s treasury to the recipient via _transfer(address(this), to, amountto).
• The amount amountto is bounded only by the current h2obalance and o2/h2 balances; there is no global limit on total H2O that can be emitted to a single recipient in one transaction.

An adversary can therefore:

• Use a helper contract to orchestrate repeated H2O transfers between itself and the pair within one flash-loan callback, causing many pair-to-helper transfers and hence many _calulate() calls for the same helper address.
• Accumulate helper-token balances sufficient to repeatedly trigger reward branches.
• Drain a large amount of H2O from address(this) into the helper and the pair in a single exploit tx.

The vulnerability is aggravated by:

• The use of block-dependent pseudo-randomness (block.timestamp, blockhash) that can be influenced by transaction ordering and miner/validator behavior.
• The fact that DEX pairs can be adversarially driven via complex swap paths and flash loans, executing arbitrary internal transfer patterns within a single transaction.

Vulnerable Components

• H2O token contract 0xe9c4d4f095c7943a9ef5ec01afd1385d011855a1, specifically:
  • transfer(address,address,uint256)
  • _calulate(address,uint256)
• Helper tokens 0x6AC860AE21993d65b790a95Cfc1A3A4b42dd0ce3 and 0xD061A395190581cb677b5FfFF1dc38448D4976c8, whose mint and burn functions act as counters inside _calulate.
• Helper contract 0x03ca8b574dd4250576f7bccc5707e6214e8c6e0d, whose ecf8ecb0 function orchestrates flash loans, swaps, and looping transfers that drive _calulate, and whose bfbaa190 function enables USDT cash-out.

Exploit Conditions

For the exploit to succeed, the following conditions hold at σ_B:

• Existence of the H2O/USDT pair 0x42717781... that routes H2O transfers with msg.sender == pair and to == helper.
• The H2O token contract’s own balance (address(this)) holds sufficient H2O to support large outgoing reward transfers without immediately reaching zero.
• The adversary can borrow USDT via a flash loan from PancakeV3Pool 0x4f31fa98... and use PancakeRouter’s swapExactTokensForTokensSupportingFeeOnTransferTokens to enter/exit the H2O/USDT pool.
• The helper contract can repeatedly send H2O to the pair within a single flash-loan callback, causing the pair to send H2O back to the helper many times and thus invoke _calulate(to=helper) multiple times in one transaction.
• There is no per-address reward cap or global rate limit in _calulate, and no restriction preventing the helper contract or the pair from being the primary reward beneficiary.

Security Principles Violated

• The contract lacks an invariant ensuring that the token contract’s own balance cannot be drained by reward mechanics triggered via DEX interactions.
• Economically significant transfers from the treasury are governed by unbounded, pseudo-random logic (_calulate + getRandomOnchain) without enforced caps.
• The design fails to account for adversarially-driven DEX pairs and flash-loan-based loops that can trigger reward logic arbitrarily many times within a single transaction.

Adversary Flow Analysis

Adversary Strategy Summary

The adversary’s strategy is to:

1. Deploy a helper contract that can orchestrate flash loans, swaps, and looping transfers and expose a cash-out method to withdraw USDT.
2. Use a flash-loan transaction to borrow USDT, enter the H2O/USDT pool, and repeatedly trigger H2O’s _calulate reward logic via pair-initiated transfers to an attacker-controlled helper contract.
3. Convert the resulting H2O and pool imbalance into USDT profit held by the helper.
4. Call bfbaa190(USDT) to transfer all USDT from the helper to the EOA, and optionally repeat cash-out when no USDT remains (incurring only gas).

This lifecycle is entirely permissionless and uses public DEX, flash-loan, and token interfaces; no privileged roles or non-standard assumptions are required.

Adversary-Related Accounts

Identified adversary cluster:

• EOA 0x8842dd26fd301c74afc4df12e9cdabd9db107d1e (BSC, chainid 56)

  • Sender of all attacker-crafted sequence-b txs.
  • Deployer of helper contract 0x03ca8b57... (via tx 0xe57e600c...).
  • Final USDT profit recipient in cash-out tx 0x33cfa125....

• Helper contract 0x03ca8b574dd4250576f7bccc5707e6214e8c6e0d (BSC, chainid 56)

  • Deployed by 0x8842dd26....
  • Executes the ecf8ecb0 exploit logic, holding intermediate USDT profit after tx 0x994abe79....
  • Transfers all USDT to the EOA via bfbaa190(USDT) in tx 0x33cfa125....

Victim candidates:

• H2O token contract 0xe9c4d4f095c7943a9ef5ec01afd1385d011855a1 (verified source).
• H2O/USDT PancakePair 0x42717781d93197247907f82482ae1d35d7bc101b.
• USDT PancakeV3Pool flash-loan pool 0x4f31fa980a675570939b737ebdde0471a4be40eb.

Helper Contract Cash-Out Snippet

function Unresolved_bfbaa190(uint256 arg0, address arg1) public payable {
    // ...
    (bool success, bytes memory ret0) = address(arg1).Unresolved_70a08231(address(this)); // balanceOf(this)
    // ...
    (bool success, bytes memory ret1) = address(arg1).Unresolved_a9059cbb(msg.sender); // transfer(msg.sender, balance)
    // ...
}

Snippet: Decompiled helper-contract bfbaa190 function (selector 0xbfbaa190), reading the helper’s full token balance and transferring it to msg.sender, used to cash out USDT to the adversary EOA.

Lifecycle Stages

1. Adversary deployment and setup

   • Tx: 0xe57e600cf8909a37833855005e5b2491414650b863329e13275806f940d17a7f (block 47454856, mechanism deploy).
   • EOA 0x8842dd26... deploys helper contract 0x03ca8b57..., configuring references to:
     • USDT 0x55d398326f99059fF775485246999027B3197955
     • PancakeV3Pool 0x4f31fa980a675570939b737ebdde0471a4be40eb
     • PancakeRouter 0x10ED43C7...
     • H2O token 0xe9c4d4f09...
     • Helper tokens 0x6AC860AE... and 0xD061A395....
   • Evidence: helper contract decompiled source and txlist confirming deployment by the EOA.

2. Exploit execution via flash loan and helper-token rewards

   • Tx: 0x994abe7906a4a955c103071221e5eaa734a30dccdcdaac63496ece2b698a0fc3 (block 47454937, mechanism flashloan+swap).

   • Inside this tx, helper contract 0x03ca8b57...:

     • Borrows 100,000 USDT from PancakeV3Pool 0x4f31fa98....
     • Swaps USDT to H2O via PancakeRouter 0x10ED43C7... into pair 0x42717781....
     • Repeatedly cycles H2O between itself and the pair, so that each pair-initiated transfer to the helper invokes H2O._calulate(to=helper) and causes H2O transfers from address(this) to the helper and the pair.
     • Ends with the helper holding 22,688.210530453207954293 more USDT than it started with, while the pair has lost 22,738.210530453207954293 USDT and gained 83,280,442,437,787,583,376,702,261 H2O; the flash-loan pool holder receives a 50 USDT fee.

   • Supporting ERC20 balance diffs (exploit tx prestate):

{
  "erc20_balance_deltas": [
    {
      "token": "0x55d398326f99059ff775485246999027b3197955",
      "holder": "0x03ca8b574dd4250576f7bccc5707e6214e8c6e0d",
      "delta": "22688210530453207954293"
    },
    {
      "token": "0x55d398326f99059ff775485246999027b3197955",
      "holder": "0x42717781d93197247907f82482ae1d35d7bc101b",
      "delta": "-22738210530453207954293"
    },
    {
      "token": "0xe9c4d4f095c7943a9ef5ec01afd1385d011855a1",
      "holder": "0x42717781d93197247907f82482ae1d35d7bc101b",
      "delta": "83280442437787583376702261"
    },
    {
      "token": "0xe9c4d4f095c7943a9ef5ec01afd1385d011855a1",
      "holder": "0xe9c4d4f095c7943a9ef5ec01afd1385d011855a1",
      "delta": "-83280442437787583376702261"
    }
  ]
}

Snippet: Prestate-based ERC20 balance deltas for exploit tx 0x994abe79..., showing USDT and H2O movements between the helper, pair, and token contract.

3. Cash-out of USDT to adversary EOA

   • Txs:

     • 0x33cfa1257d85bcd77206438d5d7efdf22e6cea3c306fce646fa9aa7594b964ec (block 47454972, mechanism transfer).
     • 0xa0688f0f02aad1ecdbdf653c407ac9c5757a99444ee9beab7b3a60eecc59e1ae (block 47455274, mechanism transfer).

   • In 0x33cfa125..., EOA 0x8842dd26... calls bfbaa190(USDT):

{
  "erc20_balance_deltas": [
    {
      "token": "0x55d398326f99059ff775485246999027b3197955",
      "holder": "0x8842dd26fd301c74afc4df12e9cdabd9db107d1e",
      "delta": "22770891449589918819435"
    },
    {
      "token": "0x55d398326f99059ff775485246999027b3197955",
      "holder": "0x03ca8b574dd4250576f7bccc5707e6214e8c6e0d",
      "delta": "-22770891449589918819435"
    }
  ]
}

• This moves all USDT from the helper to the EOA.
• In 0xa0688f0f..., the EOA repeats bfbaa190(USDT); prestate-based ERC20 balance diffs show zero USDT delta for both the helper and EOA, confirming no additional USDT is moved and only gas is spent.

Impact & Losses

Total Loss Overview

• Asset: USDT (BEP20).
• Gross profit to adversary cluster: 22,688.210530453207954293 USDT.
• Net profit after gas, using on-chain BNB/USDT pricing: 22,685.838493440865657433 USDT.

Detailed Impacts

From the exploit tx prestate-based balance diffs:

• H2O/USDT pair 0x42717781... loses 22,738.210530453207954293 USDT.

  • Of this, 50 USDT is paid as a flash-loan fee to the USDT PancakeV3Pool holder.
  • The remaining 22,688.210530453207954293 USDT becomes adversary profit held by helper contract 0x03ca8b57... before cash-out.

• H2O balances:

  • H2O token contract 0xe9c4d4f09... loses 83,280,442,437,787,583,376,702,261 H2O from its own treasury balance (address(this)).
  • H2O/USDT pair gains the same amount of H2O, leaving the pool heavily imbalanced with significantly more H2O and significantly less USDT backing.

Consequences:

• Liquidity providers in the H2O/USDT pool suffer a loss because the pool’s USDT reserves are drained while H2O reserves increase, exposing them to unfavorable post-exploit pricing and reduced liquidity.
• The H2O contract’s internal balance is depleted, undermining any implicit or explicit reliance on the treasury for buybacks, stability, or other tokenomics, and harming H2O holders.
• The adversary-related cluster {0x8842dd26..., 0x03ca8b57...} realizes a fee-adjusted net profit of approximately 22,685.8385 USDT, fully supported by prestate-based on-chain balance diffs and DEX pricing.

All Relevant Transactions

For completeness, the following BSC transactions are relevant to this incident:

• 0x3b0891a4eb65d916bb0069c69a51d9ff165bf69f83358e37523d0c275f2739bd — related (failed/earlier attempt).
• 0x729c502a7dfd5332a9bdbcacec97137899ecc82c17d0797b9686a7f9f6005cb7 — related (smaller-scale attempt without durable profit).
• 0x994abe7906a4a955c103071221e5eaa734a30dccdcdaac63496ece2b698a0fc3 — attacker-crafted exploit aggregation tx.
• 0xd97694e02eb94f48887308a945a7e58b62bd6f20b28aaaf2978090e5535f3a8e — related (failed exploit attempt).
• 0xe57e600cf8909a37833855005e5b2491414650b863329e13275806f940d17a7f — related (helper contract deployment).
• 0x33cfa1257d85bcd77206438d5d7efdf22e6cea3c306fce646fa9aa7594b964ec — attacker-crafted USDT cash-out to EOA.
• 0xa0688f0f02aad1ecdbdf653c407ac9c5757a99444ee9beab7b3a60eecc59e1ae — attacker-crafted noop cash-out attempt (gas only).

References

• [1] H2O token Contract.sol source (verified on-chain contract source).
• [2] Helper contract 0x03ca8b57... decompiled source and ABI (Heimdall decompilation).
• [3] Helper tokens 0x6AC860AE... and 0xD061A395... decompiled sources and ABIs (Heimdall decompilation).
• [4] Exploit tx 0x994abe79... cast trace and prestate-based balance diff, used to reconstruct the flash-loan exploit flow and exact ERC20 deltas.
• [5] Cash-out txs 0x33cfa125... and 0xa0688f0f... prestate-based balance diffs, used to reconstruct USDT movements between helper and EOA.
• [6] On-chain BNB/USDT price at the exploit block, used to convert BNB gas costs into USDT and compute fee-adjusted net profit.