Astrolab — deposit and redeem take 10 percent off the gross amount

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 58102-h-05-fee-calculation-mismatch-in-mint-deposit-redeem-and-wit. Standalone Foundry PoC and full write-up: 58102-h-05-fee-calculation-mismatch-in-mint-deposit-redeem-and-wit_exp in the evm-hack-registry mirror.

Vulnerability classes: vuln/logic/fee-calculation · vuln/defi/fee-manipulation

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/58102-h-05-fee-calculation-mismatch-in-mint-deposit-redeem-and-wit_exp.sol.

Key info#

TL;DR#

Price is 1:1 and the entry and exit fee is 10 percent, the numbers in the report.

previewMint(100) uses addBp and costs 110. That is a 10 asset fee, which is the intended charge. previewDeposit(110) uses subBp and mints 99 shares. The amount 110 already includes the fee, so the fair share count is 110 * 10000 / 11000 = 100. The depositor is short 1 share.

previewRedeem(110) uses the same subBp and pays 99 assets. The fair payout is 100. The shareholder is short 1 asset. previewWithdraw(100) uses addBp and burns 110 shares to pay 100, which is the consistent fee.

The vulnerable code#

function previewDeposit(uint256 amount) public pure returns (uint256) {
    uint256 shares = convertToShares(amount);
    return shares * (BP - FEE) / BP;
}

function previewRedeem(uint256 shares) public pure returns (uint256) {
    uint256 assets = convertToAssets(shares);
    // @> VULN: subBp of the gross amount, not amount * base / (base + fee)
    return assets * (BP - FEE) / BP;
}

subBp is amount * 9000 / 10000. The report's fix is amount * base / (base + fee).

Root cause#

mint and withdraw treat the user number as the net amount and add the fee on top. deposit and redeem treat the user number as the gross amount and then take 10 percent off it. The second pair charges the fee on a base that already contains the fee.

Preconditions#

• Share price is 1:1, as in the report's example.
• The fee is 1,000 bps, 10 percent.
• The worked amounts are 100 and 110, scaled by 1 ether.

Attack walkthrough#

1. previewMint(100 ether) returns 110 ether.
2. The fair deposit of 110 ether is 100 ether of shares.
3. A depositor pays 110 ether and is minted 99 ether of shares.
4. A shareholder redeems 110 ether of shares and is paid 99 ether of assets.
5. Each path is short 1 ether against amount * 10000 / 11000.

Control: both previews use amount * 10000 / 11000. The deposit mints 100 shares and the redeem pays 100 assets.

Diagrams#

Impact#

Every deposit and every redeem overcharges by the difference between subBp and base / (base + fee). On the report's 110-unit example that is 1 unit of shares on the way in and 1 unit of assets on the way out. The extra stays in the vault and benefits the remaining shareholders.

Remediation#

previewDeposit should return convertToShares(amount) * 10000 / 11000. previewRedeem should return convertToAssets(shares) * 10000 / 11000. Leave addBp on previewMint and previewWithdraw.

Sources#

• AuditVault finding 58102
• Pashov, Astrolab security review

Taxonomy#

• vuln/logic/fee-calculation · vuln/defi/fee-manipulation

Sources & further analysis#

Reproductions & code

• Standalone PoC + full trace: 58102-h-05-fee-calculation-mismatch-in-mint-deposit-redeem-and-wit_exp (evm-hack-registry mirror).
• AuditVault finding: 58102-h-05-fee-calculation-mismatch-in-mint-deposit-redeem-and-wit.

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.