Virtual Shares

Virtual shares are a defensive accounting technique used in ERC-4626 vaults to neutralize the inflation attack. Instead of computing the share/asset exchange rate from the vault's real balances alone, the implementation adds a fixed number of non-existent ("virtual") shares and assets to both sides of the conversion. OpenZeppelin's reference implementation adds +1 virtual asset and +10**offset virtual shares to every convertToShares and convertToAssets calculation.

Why they work

The inflation attack relies on a first depositor being able to hold a tiny totalSupply (often a single share) while donating a large amount of the underlying asset to skew the exchange rate. Virtual shares mean the vault behaves as if a well-capitalized "virtual" depositor were always present, so the empty-vault state can never be driven to an extreme ratio by one attacker. To move the rate meaningfully, an attacker would have to donate an amount scaled by 10**offset, which quickly costs far more than the attack can return.

Limitations

Virtual shares reduce the profitability of an inflation attack rather than mathematically eliminating it. With only the +1 virtual asset and no decimal offset, a determined attacker can still dilute victims (see OpenZeppelin issue #5223). The technique also skims a small amount of yield toward the virtual position and can distort withdrawals when the vault is operating at a loss. The recommended posture is to combine a sufficient virtual offset with internal balance tracking and a zero-share deposit revert.