An ERC-4626 first-depositor inflation attack exploits share-price math in an empty or nearly empty vault. An attacker deposits a small amount, donates more underlying assets directly to the vault, then benefits when a victim’s deposit is rounded down to too few—or even zero—shares. The main defenses are virtual assets and shares, a carefully designed seed deposit, and on-chain limits that let depositors reject poor share output.
How an ERC-4626 first-depositor inflation attack works
ERC-4626 standardizes how a tokenized vault exchanges underlying assets for shares through operations such as deposit, mint, withdraw, and redeem. Shares represent claims on the vault’s assets, so the conversion between assets and shares matters. With integer arithmetic, share output for a deposit is rounded down. OpenZeppelin explains the mechanics and a worked example in its ERC-4626 documentation.
- The attacker seeds the vault. Before ordinary users deposit, the attacker makes a small deposit and receives shares.
- The attacker donates assets directly. They transfer underlying tokens to the vault without receiving additional shares. The vault now holds more assets relative to its share supply, increasing the assets-per-share rate.
- A victim deposits. The vault calculates how many shares the victim receives at the manipulated rate. Because the result is rounded down, the victim may receive very few shares or zero, depending on the donation and deposit size.
- The attacker redeems. If the attacker remains the only meaningful shareholder, redeeming their shares can recover assets contributed by the victim.
This is not a special ERC-4626 transaction: it is a sequence of ordinary asset transfers and vault operations. OpenZeppelin identifies frontrunning a user’s first deposit as a typical route. The exposure is greatest when supply is empty or very small and the victim’s deposit is modest relative to the manipulated rate.
Why the attack cost is not a fixed number
The attacker’s required donation depends on the victim deposit size and the degree of dilution they are trying to cause. A single quoted cost would not describe every vault or target. OpenZeppelin’s numerical walkthrough is illustrative, not a general attack-cost estimate or evidence about how often attacks occur.
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How to protect a vault from a donation attack
Use virtual assets and shares, with a deliberate offset
OpenZeppelin’s ERC-4626 implementation uses virtual assets and shares to mitigate inflation attacks. Its _decimalsOffset() setting controls the difference between the asset and share decimal representations, affecting the virtual share scale. OpenZeppelin describes the default offset as making the modeled attack non-profitable and a larger offset as making it substantially more expensive; these statements concern that implementation’s model, not every customized vault.
Virtual balances change conversion math without requiring a real depositor to supply the virtual assets. Choose and review the offset as part of the vault’s design, including its consequences for conversions and exits. See OpenZeppelin’s implementation guidance and discussion of the defense and its trade-offs.
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Seed or initialize the vault deliberately
A non-trivial initial deposit can make rate manipulation infeasible or uneconomic for the intended deposit sizes. Define who supplies that seed, who owns or controls the resulting shares, and what happens to them. If initialization is meant to happen before public deposits, consider whether the deployment and initialization sequence prevents an uninitialized window from being exposed.
Give depositors an enforceable minimum-share bound
A caller can require a minimum number of shares and revert if the deposit would mint less. OpenZeppelin points to wrappers that check expected output as a user-side protection. A displayed estimate or warning in a frontend is not an on-chain guarantee: inspect the transaction route and confirm that a caller check is actually enforced.
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Test the vault’s actual conversion paths
Test the implementation rather than assuming that a standard interface alone settles the risk. In particular, examine share conversion and rounding on deposit and withdrawal paths around zero supply, after direct asset transfers, and when asset and share decimals differ. Review any overrides to conversion logic as well as the behavior of the underlying asset.
- Test empty-vault and near-empty-vault states.
- Include direct donations that change assets held without minting shares.
- Check the smallest deposits users are expected to make and the share amounts they receive after rounding.
- Verify that user minimum-output conditions are enforced by the contract path, not merely displayed by an interface.
- Recheck behavior after custom conversion changes and under loss or exit conditions.
Compare the defenses by their consequences
No single mitigation answers every design question. Compare the options in terms of rate manipulation, operational trust, user protection, integration behavior, and how gains or losses are allocated.
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| Defense | What it changes | Design question or trade-off |
|---|---|---|
| Virtual assets and shares / decimal offset | Changes conversion math to mitigate the manipulated-rate attack. | The offset and implementation need review. OpenZeppelin notes that virtual shares and assets capture a small part of accrued value; when the vault loses value, they can cause the first exiting user to experience smaller losses at the expense of later exiting users. |
| Seed deposit | Begins the vault with a meaningful asset and share position rather than an exposed empty state. | Decide who funds it, who holds the shares, and how initialization is ordered relative to public deposits. |
| Minimum-share check | Lets the depositor’s transaction reject an unfavorable conversion. | Protection depends on an enforced bound in the actual on-chain call path; a frontend-only estimate does not enforce one. |
Virtual offsets mitigate this particular inflation mechanism; they do not eliminate vault risk. The exact implementation, parameters, customizations, asset behavior, and transaction path still matter. A security review should examine those specifics rather than treating the presence of an offset as a blanket assurance.
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