> For the complete documentation index, see [llms.txt](https://reports.immunefi.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://reports.immunefi.com/acre/34999-sc-low-the-tbtc-in-the-mezoallocator-itself-is-not-considered-in-the-withdrawal-function.md). # #34999 \[SC-Low] The tBTC in the MezoAllocator itself is not considered in the withdrawal function **Submitted on Sep 2nd 2024 at 14:22:04 UTC by @dash for** [**Audit Comp | Acre**](https://immunefi.com/audit-competition/boost-acre) * **Report ID:** #34999 * **Report Type:** Smart Contract * **Report severity:** Low * **Target:** * **Impacts:** * Smart contract unable to operate due to lack of token funds ## Description ## Brief/Intro \`tBTC\` deposited into \`StBTC\` is routed through the \`MezoAllocator\` to the \`Portal\`. When \`depositors\` want to \`withdraw\` \`tBTC\` from \`StBTC\`, and if \`StBTC\` lacks sufficient \`tBTC\`, it \`withdraws\` the necessary amount from the \`Portal\` via the \`MezoAllocator\`. However, \`tBTC\` held in the \`MezoAllocator\` itself but not yet \`deposited\` to the \`Portal\` is not accounted for, which can lead to an \`arithmetic underflow\` during the \`withdrawal\` process. ## Vulnerability Details The \`total assets\` of \`StBTC\` consist of three parts: its \`balance\`, the \`total assets\` of the \`MezoAllocator\`, and \`total debts\`. \`\`\` function totalAssets() public view override returns (uint256) { return IERC20(asset()).balanceOf(address(this)) + dispatcher.totalAssets() + totalDebt; } \`\`\` The \`MezoAllocator\`'s \`total assets\` include its \`balance\` and the amount \`deposited\` in the \`Portal\`. \`\`\` function totalAssets() external view returns (uint256) { return depositBalance + tbtc.balanceOf(address(this)); } \`\`\` During a \`withdrawal\`, if \`StBTC\`'s current \`tBTC\` \`balance\` is insufficient, it attempts to \`withdraw\` the required amount from the \`Portal\`. \`\`\` function withdraw( uint256 assets, address receiver, address owner ) public override returns (uint256) { uint256 currentAssetsBalance = IERC20(asset()).balanceOf(address(this)); uint256 assetsWithFees = assets + \_feeOnRaw(assets, exitFeeBasisPoints); if (assetsWithFees > currentAssetsBalance) { dispatcher.withdraw(assetsWithFees - currentAssetsBalance); } return super.withdraw(assets, receiver, owner); } \`\`\` The \`withdraw\` function assumes that the requested amount is less than the amount \`deposited\` in the \`Portal\`. However, \`tBTC\` that hasn’t been \`deposited\` into the \`Portal\` yet—such as donated tokens or rewards—is not considered. As a result, the requested withdrawal amount could exceed the \`Portal\`'s \`deposits\` but still be less than the \`total assets\` of the \`MezoAllocator\`. \`\`\` function withdraw(uint256 amount) external { if (msg.sender != address(stbtc)) revert CallerNotStbtc(); if (amount < depositBalance) { mezoPortal.withdrawPartially( address(tbtc), depositId, uint96(amount) ); } else { mezoPortal.withdraw(address(tbtc), depositId); } depositBalance -= uint96(amount); // @audit: underflow when amount > depositBalance tbtc.safeTransfer(address(stbtc), amount); } \`\`\` This discrepancy can cause a \`withdrawal\` failure due to \`underflow\`, effectively locking users' funds until the next \`allocation\` by \`MezoAllocator\` \`maintainers\`. ## Impact Details The impact is clear, as it can lead to a denial of service (DoS) and lock users' funds. ## References ## Recommendation \`\`\` function withdraw(uint256 amount) external { if (msg.sender != address(stbtc)) revert CallerNotStbtc(); if (amount < depositBalance) { mezoPortal.withdrawPartially( address(tbtc), depositId, uint96(amount) ); } else { mezoPortal.withdraw(address(tbtc), depositId); } - depositBalance -= uint96(amount); * depositBalance = depositBalance > uint96(amount) ? depositBalance - uint96(amount) : 0; tbtc.safeTransfer(address(stbtc), amount); } \`\`\` ## Proof of Concept ## Proof of Concept Add below test to the \`stBTC.test.ts\`: \`\`\` describe("withdraw from portal", () => { it("should withdraw from the dispatcher", async () => { const assets = to1e18(100) await tbtc.mint(depositor1.address, assets) await tbtc.connect(depositor1).approve(await stbtc.getAddress(), assets) await stbtc.connect(depositor1).deposit(assets, depositor1.address) ``` await mezoAllocator.connect(maintainer).allocate() /** * some tbtc are not deposited to the Portal */ await tbtc.mint(await mezoAllocator.getAddress(), to1e18(1)) const shares = await stbtc.balanceOf(depositor1.address) /** * reverted with below error * Error: VM Exception while processing transaction: reverted with panic code 0x11 (Arithmetic operation overflowed outside of an unchecked block) */ await stbtc.connect(depositor1).redeem(shares, depositor1.address, depositor1.address) ``` }) }) \`\`\` --- # Agent Instructions This documentation is published with GitBook. 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