Reentrancy is a control-flow problem: an external call can hand execution to another contract before a vault has finished updating its state. The practical review is not limited to Ether withdrawals. Check same-function and cross-function entry points, token and receiver callbacks, strategy call chains, and values that other contracts can read during a transition. These are five useful inspection surfaces, not a measured ranking of the most frequent vulnerabilities.
What makes a vault reentrant?
A vault is at risk when it yields control while relevant state is unfinished and a reachable callback path can use that inconsistent state to break an invariant. Solidity’s security guidance notes that reentrancy can arise from any external contract call, not only Ether transfers: Solidity documentation, Security Considerations. A callback alone is not proof of an exploit; the key questions are what state is pending and what code can run before the transition completes.
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Five reentrancy patterns to inspect
1. Same-function reentry during withdrawal
A classic failure occurs when a vault sends assets to a recipient before reducing that recipient’s recorded claim. A contract recipient can call the withdrawal function again while the old balance is still visible, potentially withdrawing against the same claim more than once. Ethereum.org illustrates how delayed balance updates can enable repeated withdrawals: Ethereum.org smart-contract security.
Apply checks-effects-interactions (CEI): validate the request, record the reduced claim and other accounting effects, then make the external payment. The vault should not expose a balance that remains spendable after the payment has begun.
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2. Cross-function reentry through shared accounting
A callback need not repeat the original withdrawal. It may enter another public or external function that reads or changes the same shares, assets, debt, or reward state. For example, one entry point might be guarded while another can still manipulate the accounting on which the first relies. OWASP distinguishes cross-function reentrancy from same-function recursion in its smart-contract reentrancy guidance: OWASP SCWE-046.
Review every reachable entry point that touches the affected state, not just the function where the external call occurs. A guard on one function does not automatically cover sibling functions.
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3. Token and receiver callback reentry
Token transfers, receiver hooks, and other callback interfaces can execute code during what may look like a routine asset operation. A token contract or recipient should therefore be treated as code that can call back into the vault. This applies even when the vault never sends Ether. Solidity’s warning about external calls and OWASP’s reentrancy categories cover these callback-capable interactions (Solidity; OWASP).
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A vault may call a strategy, DEX, or another module that subsequently reaches a dependent contract or a vault entry point. The security boundary is the full call graph, not just the vault’s local function. State may be distributed among the vault, strategy, and integrations, so a locally sensible ordering can still leave another contract able to observe or exploit a partial transition. Solidity advises considering multi-contract situations; OWASP specifically identifies vault-to-strategy-to-DEX flows as a review surface (Solidity; OWASP).
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5. Read-only reentrancy and transient views
A callback may query a view function while the vault is partway through a state transition. The view itself may not write to the vault, but an integrator, oracle consumer, or other dependent protocol may act on the transient value it returns. A view can therefore matter to system-wide invariants even when it cannot mutate the vault directly.
OpenZeppelin’s Very Liquid Vaults audit discusses the limits of guarding view functions and describes use of nonReentrant and nonReentrantView where possible, alongside constraints that remain: OpenZeppelin Very Liquid Vaults audit. View protections need special care when state-changing functions call view functions internally.
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How to choose and combine defenses
Defenses address different parts of the problem. CEI prevents exposure of a vulnerable intermediate state by completing accounting effects before interaction. A mutex-style reentrancy guard rejects recursive entry to covered functions. Read-only protections aim to limit what callers can observe during a transition, but they must account for internal call patterns and dependent contracts.
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|---|---|---|
| Checks-effects-interactions | Updates relevant accounting before control leaves the vault. | All state needed to preserve the invariant is updated before every callback-capable interaction. |
| Reentrancy guard or mutex | Rejects recursive entry into functions covered by the guard. | Every relevant entry point and callback path is covered; internal calls and inheritance structure remain compatible. |
| Read-only reentrancy protection | Limits exposure of transient values through views during a state transition. | Dependent contracts cannot act on an unsafe intermediate view, and internal view use remains valid. |
These measures are complementary rather than interchangeable. A guard can block a route without correcting stale accounting; CEI can protect an accounting sequence without addressing every transient value read elsewhere. Assess coverage against the vault’s actual call graph and invariants, as emphasized by Solidity and the OpenZeppelin audit (Solidity; OpenZeppelin).
A practical review workflow
- Map external call sites. Include asset transfers, token hooks, receiver callbacks, strategy calls, DEX interactions, and calls to dependent modules.
- Mark the state at each handoff. For each call, record which balances, shares, liabilities, rewards, or other fields have changed and which updates are still pending.
- Trace reachable entry points. Enumerate same-function and cross-function routes that can touch that state, including routes reached through other contracts.
- Write the invariants. State what must remain true, such as a correct share claim or consistency between assets and liabilities, throughout the transition—not only after the transaction finishes.
- Test callback sequences. Exercise the relevant entry points and state transitions with callback-capable dependencies, and check whether any sequence violates an invariant or exposes an unsafe value to an integrator.
- Check the deployed version. Confirm that the code and configuration being reviewed match the actual deployment. A finding or mitigation in one version does not establish the state of another.
Ethereum.org recommends documenting critical security properties and using automated property testing; those tests complement, rather than replace, manual call-graph and integration review: Ethereum.org smart-contract security.
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