Harmonix — withdraw burns the rounded-down share count

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 57884-c-02-users-can-exploit-rounding-to-withdraw-excess-assets-fr. Standalone Foundry PoC and full write-up: 57884-c-02-users-can-exploit-rounding-to-withdraw-excess-assets-fr_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/arithmetic/rounding-direction · vuln/arithmetic/rounding · vuln/logic/incorrect-calculation

Reproduction: a self-contained Foundry PoC that compiles and runs offline with only forge-std — no fork and no RPC. Trace: output.txt. PoC: test/57884-c-02-users-can-exploit-rounding-to-withdraw-excess-assets-fr_exp.sol.

Key info#

TL;DR#

The fund holds 1000 ether of assets and 10 shares. One share is worth 100 ether. The caller holds 5 shares and withdraws 199 ether.

convertToShares computes assets * supply / totalAssets, which is 199 ether * 10 / 1000 ether = 1 share after truncation. The caller receives 199 ether and burns 1 share. The extra over that share's 100 ether is 99 ether. The other holder still has 5 shares.

Rounding the burn up gives ceil(199 ether * 10 / 1000 ether) = 2 shares, worth 200 ether. The same 199 ether payout then has extra 0.

The vulnerable code#

function convertToShares(uint256 assets) public view returns (uint256) {
    if (roundUp) return (assets * totalSupply + totalAssets - 1) / totalAssets;
    return assets * totalSupply / totalAssets;
}

function withdraw(uint256 assets, address receiver) external returns (uint256 shares) {
    // @> VULN: convertToShares rounds down. withdraw must round the burned shares up.
    shares = convertToShares(assets);

The audited body is uint256 shares = convertToShares(assets); return _withdraw(shares, receiver, owner); in fundContract.sol at commit f02157ab. roundUp is the control. It is false on the vulnerable fund.

Root cause#

withdraw specifies the assets that leave and derives the shares that burn. Rounding that division down burns too few shares for the assets paid. ERC-4626 rounds this conversion up so the vault does not pay assets that the burned shares do not cover.

Preconditions#

• totalAssets is 1000 ether and totalSupply is 10.
• The caller owns 5 shares. Another address owns the other 5.
• The withdrawal amount is 199 ether, which does not divide the share price evenly.

Attack walkthrough#

1. Preview: shares round to 1, and that share is worth 100 ether.
2. withdraw(199 ether) burns 1 share and transfers 199 ether.
3. Extra is 199 ether - 100 ether = 99 ether.
4. The caller has 4 shares left. The other holder still has 5. Assets left are 801 ether for 9 shares.

Control: the same withdrawal burns 2 shares. Two shares were worth 200 ether. Received 199 ether does not exceed that, so the extra is 0.

Diagrams#

Impact#

The caller takes 99 ether more than the burned share was worth. The other holder keeps the same 5 shares against a smaller asset balance. Rounding the burn the other way makes the extra 0 on this same withdrawal.

Remediation#

In withdraw, convert assets to shares with a round-up: (assets * supply + totalAssets - 1) / totalAssets.

Sources#

• AuditVault finding 57884
• Shieldify Harmonix review
• harmonixfi/core-contracts@f02157ab fundContract.sol

Taxonomy#

• severity/high
• vuln/arithmetic/rounding-direction · vuln/arithmetic/rounding · vuln/logic/incorrect-calculation

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 57884-c-02-users-can-exploit-rounding-to-withdraw-excess-assets-fr_exp (evm-hack-registry mirror).
• AuditVault finding: 57884-c-02-users-can-exploit-rounding-to-withdraw-excess-assets-fr.

Alerts & third-party analyses

• 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.