MPRO Staking Proxy unwrapWETH Flash-Loan Exploit (Base)

Incident Overview & TL;DR

• Protocol: MPRO Double Reward Auto Stake on Base (chainid 8453)
• Primary victim system: MPRO staking proxy 0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802 fronting an implementation MPRORewardStake at 0xd971fD39D9714d5eb1B54B931790170A0630f131.
• Exploit style: ACT-style single-transaction exploit by an unprivileged adversary using a flash loan and a protocol bug.
• Root-cause category: protocol_bug (missing access control on a value-bearing withdrawal function).

In a single adversary-crafted transaction on Base, the attacker uses a Balancer Vault WETH flash loan to temporarily inflate the MPRO staking proxy's WETH balance and then calls a publicly callable unwrapWETH(uint256,address) function on the proxy. Because the implementation behind the proxy has no access control on this function and forwards ETH to an arbitrary recipient, the call converts both the pre-existing reward WETH pot and the flash-loaned WETH into native ETH, sends that ETH to an attacker-controlled helper contract, repays the flash loan, and leaves the residual ETH as attacker profit.

The underlying MPRORewardStake implementation exposes unwrapWETH(uint256,address) as an externally callable function that (a) checks only that the requested amount is positive and that the contract's WETH balance is at least that amount, (b) calls WETH9.withdraw(amount), and (c) transfers the resulting ETH to an arbitrary recipient address. With the staking proxy holding a shared reward pot in WETH, any unprivileged caller with temporary WETH liquidity (via a flash loan) can drain that pot to an arbitrary address.

ACT Opportunity and Exploit Predicate

Pre-state (sigma_B) and block

• Block height B: Base block 30210274.
• Pre-state sigma_B definition:
  • The MPRO staking proxy 0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802 is configured as a TransparentUpgradeableProxy delegating to MPRORewardStake at 0xd971fD39D9714d5eb1B54B931790170A0630f131.
  • The proxy holds an existing rewardToken (WETH) balance of 3.981326901636573675 WETH.
  • WETH9 is deployed at 0x4200000000000000000000000000000000000006 on Base with standard deposit/withdraw semantics.
  • Balancer Vault is deployed at 0xBA12222222228d8Ba445958a75a0704d566BF2C8 and exposes permissionless flash loans in WETH.
  • The adversary controls:
    • EOA 0x5CC162c556092fE1d993b95D1b9E9CE58a11dBC9, and
    • Helper contract 0x0c6A8c285d696d4D9b8dD4079a72a6460A4dA05F.

This pre-state is reconstructed from seed metadata, traces, prestate balance diffs, and contract source/decompilation.

Transaction sequence b

There is a single adversary-crafted transaction b that realizes the exploit from sigma_B:

• Tx: 0xac6f716c57bbb1a4c1e92f0a9531019ea2ecfcaea67794bbd27115d400ae9b41
• Chain: Base (8453)
• Type: Standard EIP-1559 transaction
• From: EOA 0x5CC162c556092fE1d993b95D1b9E9CE58a11dBC9
• To: Helper contract 0x0c6A8c285d696d4D9b8dD4079a72a6460A4dA05F
• High-level behavior:
  • Requests a Balancer Vault flash loan of 100852657473363426325 WETH.
  • Sends the borrowed WETH to the MPRO staking proxy.
  • Calls unwrapWETH(104833984375000000000, 0x0c6A8c285d696d4D9b8dD4079a72a6460A4dA05F) via the proxy (delegatecall into MPRORewardStake).
  • Unwraps a total of 104.833984375 WETH (which includes the pre-existing 3.981326901636573675 WETH reward pot) into ETH.
  • Uses 100852657473363426325 wei of ETH to re-mint WETH and repay the flash loan.
  • Forwards the remaining ETH back to the attacker EOA as profit.

Snippet – Seed transaction trace for exploit tx 0xac6f… (cast run -vvvvv)

0x0c6A8c285d696d4D9b8dD4079a72a6460A4dA05F::flashLoanCallback(...)
  ├─ BalancerVault.flashLoan(..., WETH9, 100852657473363426325, ...)
  │   ├─ WETH9.transfer(0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802, 100852657473363426325)
  │   └─ ...
  ├─ 0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802::unwrapWETH(104833984375000000000, 0x0c6A8c2...)
  │   ├─ 0xd971fD39D9714d5eb1B54B931790170A0630f131::unwrapWETH(104833984375000000000, 0x0c6A8c2...) [delegatecall]
  │   │   ├─ WETH9.balanceOf(0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802)
  │   │   ├─ WETH9.withdraw(104833984375000000000)
  │   │   └─ ETH sent to 0x0c6A8c2...
  ├─ WETH9.deposit{value: 100852657473363426325}()
  ├─ WETH9.transfer(0xBA12222222228d8Ba445958a75a0704d566BF2C8, 100852657473363426325)
  └─ ETH residual sent to 0x5CC162c556092fE1d993b95D1b9E9CE58a11dBC9

Exploit predicate and profit

• Predicate type: profit
• Reference asset: ETH
• Adversary address: 0x5CC162c556092fE1d993b95D1b9E9CE58a11dBC9
• Adversary profit: 3.980180099247068721 ETH (3,980,180,099,247,068,721 wei)
• System-level ETH drained from WETH9: 3.981326901636573675 ETH (3,981,326,901,636,573,675 wei)

The profit computation is grounded in the prestate-tracer balance diff for the exploit tx.

Snippet – Native balance deltas for exploit tx (prestate tracer)

{
  "native_balance_deltas": [
    {
      "address": "0x4200000000000000000000000000000000000006",
      "delta_wei": "-3981326901636573675"
    },
    {
      "address": "0x5cc162c556092fe1d993b95d1b9e9ce58a11dbc9",
      "delta_wei": "3980180099247068721"
    },
    {
      "address": "0x4200000000000000000000000000000000000011",
      "delta_wei": "1146620000000000"
    },
    {
      "address": "0x420000000000000000000000000000000000001a",
      "delta_wei": "17596017272"
    },
    {
      "address": "0x4200000000000000000000000000000000000019",
      "delta_wei": "164793487682"
    }
  ]
}

The attacker EOA gains 3.980180099247068721 ETH, WETH9's ETH backing decreases by 3.981326901636573675 ETH, and the small difference is accounted for by system fee-collector addresses.

Key Background

The following context is needed to understand the incident:

1. MPRO staking architecture on Base

   • The staking system uses a TransparentUpgradeableProxy at 0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802 that delegates to MPRORewardStake at 0xd971fD39D9714d5eb1B54B931790170A0630f131.
   • The implementation manages staking rewards in WETH (WETH9). Over time, admin operations and user staking activity accumulate a shared WETH reward pot on the proxy.

2. External dependencies: WETH9 and Balancer Vault

   • WETH9 at 0x4200000000000000000000000000000000000006 is a standard wrapped-ETH contract with deposit() and withdraw(uint256) that burns WETH and releases the same amount of native ETH to the caller.
   • Balancer Vault at 0xBA12222222228d8Ba445958a75a0704d566BF2C8 provides permissionless flash loans in WETH on Base, allowing an unprivileged contract to borrow and repay WETH within a single transaction.

These components together make it possible for an attacker to temporarily swell the proxy's WETH balance and then unwrap that balance into ETH.

Vulnerability & Root Cause Analysis

Vulnerability summary

The core vulnerability is that MPRORewardStake exposes a publicly callable unwrapWETH(uint256,address) function that performs only a balance check and then unconditionally unwraps WETH and sends the resulting ETH to an arbitrary recipient address. There is no access control tying this operation to stakers, admins, or protocol-controlled recipients.

Detailed root cause

The decompiled MPRORewardStake implementation at 0xd971fD39D9714d5eb1B54B931790170A0630f131 contains the following unwrapWETH logic:

Snippet – Decompiled unwrapWETH implementation (MPRORewardStake)

function unwrapWETH(uint256 amount, address recipient) public returns (bool) {
    require(recipient == address(recipient));
    require(amount > 0);
    require(address(recipient) != address(0));
    require(address(rewardToken) != address(0));

    // Check contract's WETH balance
    uint256 bal = rewardToken.balanceOf(address(this));
    require(bal >= amount, "MPRORewardStake: Insufficient WETH balance");

    // Unwrap WETH to ETH
    rewardToken.withdraw(amount); // WETH9.withdraw(amount)

    // Send ETH to arbitrary recipient
    (bool success, ) = recipient.transfer(amount);
    require(success, "MPRORewardStake: ETH transfer failed");

    return true;
}

Key properties:

• Externally callable: There is no onlyOwner, onlyStaker, or similar modifier; any caller can invoke unwrapWETH through the proxy.
• Balance-based gating only: The function checks only that rewardToken.balanceOf(address(this)) >= amount. It does not distinguish between long-term staking rewards and transient balances (such as flash-loaned WETH).
• Arbitrary recipient: The ETH is sent to a caller-specified recipient address with no restriction that it be the proxy owner, a staking rewards contract, or any other protocol-controlled account.

As a result:

• Any unprivileged caller can cause the proxy to unwrap all WETH currently held by the contract, as long as the requested amount is less than or equal to the current WETH balance.
• Because the contract's WETH balance is the sum of the pre-existing reward pot and any temporary inflows, a flash loan can be used to pass the balance check for a large amount that includes both the reward pot and the flash-loaned WETH.
• The withdraw call converts this entire amount to ETH; the attacker can then repay the flash loan out of the same ETH and keep the residual as profit.
• The staking reward pot is not segregated or reserved; it is simply part of rewardToken.balanceOf(address(this)) and is therefore fully exposed to this mechanism.

Vulnerable components

• MPRO staking proxy 0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802 – a TransparentUpgradeableProxy front-end that routes calls (including unwrapWETH) to the implementation.
• MPRORewardStake implementation 0xd971fD39D9714d5eb1B54B931790170A0630f131 – specifically the unwrapWETH(uint256,address) function.

Exploit preconditions

For the exploit to succeed, all of the following must hold:

1. Positive WETH reward balance on the proxy – Prior to block 30210274, the staking proxy must hold a positive rewardToken balance in WETH. In this incident, the relevant pot is 3.981326901636573675 WETH.
2. Vulnerable implementation active – The proxy's implementation must be MPRORewardStake at 0xd971fD39D9714d5eb1B54B931790170A0630f131 with the vulnerable unwrapWETH logic.
3. Access to temporary WETH liquidity – The attacker must have access to a flash loan or equivalent mechanism to temporarily add enough WETH to the proxy so that rewardToken.balanceOf(address(this)) is at least the desired amount for unwrap.
4. Publicly callable unwrapWETH – The unwrapWETH(uint256,address) function must be callable by arbitrary addresses without role checks, and the recipient parameter must be unconstrained.

Security principles violated

• Missing access control on a withdrawal primitive: A function that moves value out of the protocol (from WETH balance to external ETH recipient) is callable by anyone.
• Failure to segregate reward balances from transient balances: The implementation treats total WETH balance as withdrawable rewards, even when part of that balance is flash-loaned capital controlled by an attacker.
• Implicit trust in raw token balances: The design assumes that rewardToken.balanceOf(address(this)) reflects legitimate, protocol-owned reward funds, which is not true under flash-loan adversaries.

Adversary Flow Analysis

Adversary strategy summary

The adversary executes a single transaction that:

1. Uses a Balancer Vault flash loan to borrow a large amount of WETH.
2. Transfers the borrowed WETH to the MPRO staking proxy to inflate its WETH balance.
3. Calls the vulnerable unwrapWETH(uint256,address) function on the proxy with an amount that includes both the existing reward pot and the flash-loaned WETH.
4. Receives the ETH produced by WETH9.withdraw(amount) at a helper contract.
5. Repays the flash loan in WETH re-minted from ETH.
6. Keeps the residual ETH as profit, funded entirely by the destruction of the staking proxy's WETH reward pot.

Adversary-related accounts

Adversary cluster:

• EOA 0x5CC162c556092fE1d993b95D1b9E9CE58a11dBC9
  • Sender of the exploit transaction 0xac6f716c….
  • Final recipient of the residual ETH profit in the balance diffs.
• Helper contract 0x0c6A8c285d696d4D9b8dD4079a72a6460A4dA05F
  • Deployed by the attacker EOA.
  • Receives the Balancer flash loan.
  • Forwards WETH to the staking proxy and invokes unwrapWETH via the proxy.
  • Routes the residual ETH back to the attacker EOA.

Victim-related contracts:

• MPRO staking proxy – 0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802
• MPRORewardStake implementation – 0xd971fD39D9714d5eb1B54B931790170A0630f131
• WETH9 – 0x4200000000000000000000000000000000000006
• Balancer Vault – 0xBA12222222228d8Ba445958a75a0704d566BF2C8

Lifecycle stages

1. Reward pot accumulation on the staking proxy

• Over prior blocks, admin address 0x29eb782b8707227fac7620ee7b3ab8c6a34f074b and user activity configure the staking system and fund the reward pot.
• By sigma_B (just before block 30210274), the staking proxy holds exactly 3.981326901636573675 WETH as rewardToken.
• This amount corresponds to the difference between the total amount unwrapped (104.833984375 WETH) and the flash-loaned amount (100.852657473363426325 WETH).

This pre-existing reward pot becomes the funding source for the attacker's profit when combined with the flash-loaned WETH.

2. Adversary helper deployment and setup

• Tx: 0x3acfcb1d8fef75bdaf88c9f8a49043937d5ce664631f9b5e1e5f0880b6cc2a77
• Block: 26181727 on Base
• From: 0x5CC162c556092fE1d993b95D1b9E9CE58a11dBC9
• To: (contract creation)
• Outcome: Deploys the helper contract 0x0c6A8c285d696d4D9b8dD4079a72a6460A4dA05F, which exposes methods that orchestrate the Balancer flash-loan and unwrapWETH call path.

This establishes a reusable entry point the attacker later uses in the exploit transaction.

3. Exploit execution via flash loan and unwrapWETH

• Tx: 0xac6f716c57bbb1a4c1e92f0a9531019ea2ecfcaea67794bbd27115d400ae9b41
• Block: 30210274
• From: 0x5CC162c556092fE1d993b95D1b9E9CE58a11dBC9
• To: 0x0c6A8c285d696d4D9b8dD4079a72a6460A4dA05F

Flow:

1. Helper calls Balancer Vault to flash-loan 100852657473363426325 WETH (token 0x4200…0006).
2. Balancer Vault transfers the borrowed WETH to the helper, which in turn transfers the same amount of WETH to the MPRO staking proxy.
3. The helper calls the proxy's unwrapWETH(uint256,address) function with amount = 104833984375000000000 and recipient = 0x0c6A8c2….
4. The proxy delegates to MPRORewardStake.unwrapWETH, which:
   • Checks that the contract's WETH balance is at least 104.833984375 WETH (which holds because the balance is the sum of the pre-existing 3.981326901636573675 WETH reward pot and the 100.852657473363426325 WETH flash loan),
   • Calls WETH9.withdraw(104833984375000000000), burning that much WETH and causing 104.833984375 ETH to be sent to the proxy,
   • Forwards the ETH to the helper contract.
5. The helper re-deposits 100852657473363426325 wei of ETH into WETH via WETH9.deposit and transfers exactly that amount of WETH back to Balancer Vault, fully repaying the flash loan.
6. The remaining 3.981326901636573675 ETH is forwarded to the attacker EOA, which realizes a net profit of 3.980180099247068721 ETH after protocol-level fees.

4. Post-incident behavior change (patched implementation)

After the exploit, the proxy is upgraded to a new implementation at 0x8127D4532D0DA08C2DB6c36e18160d8343265b93.

• Tx: 0xf91d6c36def77da30431d6648e77fdd2c17ef51d7da9fdfcc95efdae0291083a
• From: EOA 0xFF8DaC673883f2BC40454c940f5f07DEe6842424
• To: MPRO staking proxy 0x8bEfC1d90d03011a7d0b35B3a00eC50f8E014802
• Call: unwrapWETH(1000, 0xFF8DaC673883f2BC40454c940f5f07DEe6842424) via the proxy.

Snippet – Post-incident unwrapWETH attempt (debug_traceTransaction callTracer)

{
  "from": "0xff8dac673883f2bc40454c940f5f07dee6842424",
  "to": "0x8befc1d90d03011a7d0b35b3a00ec50f8e014802",
  "input": "0xe16d9ce5...000003e8...",
  "error": "execution reverted",
  "revertReason": "MPRORewardStake: Amount must be 0",
  "calls": [
    {
      "type": "DELEGATECALL",
      "to": "0x8127d4532d0da08c2db6c36e18160d8343265b93",
      "revertReason": "MPRORewardStake: Amount must be 0"
    }
  ]
}

This shows that in the patched implementation, unwrapWETH reverts when called with a nonzero amount, effectively disabling the vulnerable withdrawal path that was abused in the incident.

Impact & Losses

System-level loss

• Token: ETH (backing WETH9)
• Total amount removed from WETH9's ETH balance: 3.981326901636573675 ETH

During the exploit transaction:

• The MPRO staking proxy's entire WETH reward pot of 3.981326901636573675 WETH is consumed as part of the 104.833984375 WETH unwrapped via unwrapWETH.
• WETH9.withdraw converts this WETH into ETH, which is then split between repaying the flash loan and the attacker profit.
• The flash-loaned principal is fully repaid in WETH, so Balancer Vault's WETH holdings are restored; the net funding for the attack comes from the WETH pot backing MPRO staking rewards.

Adversary profit

• Attacker EOA: 0x5CC162c556092fE1d993b95D1b9E9CE58a11dBC9
• Net native balance increase: 3.980180099247068721 ETH

The difference between WETH9's ETH loss and the attacker's ETH gain is paid to Base system fee-collector addresses and does not represent further protocol-user loss.

Scope of this report

This report focuses on protocol-level impact:

• The destruction of the shared MPRO staking reward pot held in WETH on the proxy.
• The attacker's ETH profit realized from that pot.

It does not attempt to apportion the drained amount across individual staker wallets or external protocols; that per-staker attribution is out of scope for this ACT-focused root-cause report.

All Relevant Transactions

• Exploit transaction (attacker-crafted):
  • Chain: Base (8453)
  • Tx: 0xac6f716c57bbb1a4c1e92f0a9531019ea2ecfcaea67794bbd27115d400ae9b41
• Related transaction – helper deployment:
  • Chain: Base (8453)
  • Tx: 0x3acfcb1d8fef75bdaf88c9f8a49043937d5ce664631f9b5e1e5f0880b6cc2a77
• Related transaction – post-incident unwrap attempt (patched revert):
  • Chain: Base (8453)
  • Tx: 0xf91d6c36def77da30431d6648e77fdd2c17ef51d7da9fdfcc95efdae0291083a

References

• Exploit transaction metadata and trace – Seed metadata and full call trace for Base tx 0xac6f716c57bbb1a4c1e92f0a9531019ea2ecfcaea67794bbd27115d400ae9b41 (cast run with verbose call tree).
• Prestate native balance diff – Prestate-tracer native balance deltas for the exploit transaction, used to quantify WETH9's ETH loss and the attacker's ETH profit.
• MPRORewardStake implementation source/decompilation – Source for MPRODoubleRewardAutoStake and decompiled MPRORewardStake at 0xd971fD39D9714d5eb1B54B931790170A0630f131, including the unwrapWETH(uint256,address) logic.
• Post-incident unwrap revert trace – Debug-trace callTracer output for Base tx 0xf91d6c36def77da30431d6648e77fdd2c17ef51d7da9fdfcc95efdae0291083a, demonstrating the patched implementation's Amount must be 0 revert.