Reentrancy is one of the oldest and best-known security problems in smart contracts. It was at the heart of The DAO hack on Ethereum in 2016, and developers still design against it today. On October 21, 2026, Aave Labs proposed an upgrade called Aave V3.8, focused on adding a reentrancy guard to Aave V3. That makes it a good moment to explain what reentrancy is, why it matters and what users can learn from it.
What reentrancy means
Smart contracts often call other contracts. For example, a lending pool may send tokens to a user, or trigger a callback that lets another contract run its own code in the middle of a transaction.
Reentrancy happens when that outside code calls back into the original contract before the first operation has finished. If the original contract has not yet updated its records, it may act on outdated information.
A simple analogy: imagine a cashier who hands you cash first and only afterwards writes the withdrawal in the ledger. If you could ask for another withdrawal before the first one is written down, the cashier would still see your old balance and pay you again.
How a classic reentrancy attack works
In simplified form, a reentrancy attack follows these steps:
- The attacker deploys a malicious contract and deposits funds into a vulnerable protocol.
- The attacker asks to withdraw.
- The protocol sends the funds before reducing the recorded balance.
- Receiving the funds triggers code in the attacker's contract, which immediately requests another withdrawal.
- Because the balance has not been updated yet, the protocol pays out again, and the loop repeats until the pool is drained or the transaction runs out of gas.
There are variations. In cross-function reentrancy, the attacker reenters a different function that relies on the same data. In read-only reentrancy, another protocol reads a price or balance from a contract while it is in an inconsistent state mid-transaction, and makes a wrong decision based on it.
How developers defend against reentrancy
Common protections include:
- Checks-Effects-Interactions: verify conditions first, update internal records second, and only then call external contracts or send tokens.
- Reentrancy guards: a "lock" that is set when a function starts and cleared when it ends. If anything tries to enter again while the lock is set, the call fails.
- Pull-based payments: users withdraw what they are owed in a separate step, instead of the contract pushing funds during complex operations.
- Audits and formal verification: independent reviews and mathematical checks of the contract logic.
What Aave V3.8 proposes
According to the Aave governance proposal ([ARFC] Aave V3.8) published by Aave Labs on October 21, 2026:
- V3.8 adds a reentrancy guard to Pool operations during external callbacks, to keep Pool accounting consistent when one transaction interacts with the Pool more than once.
- The guard uses transient storage, introduced in Ethereum by EIP-1153. Transient storage only lasts for one transaction, which makes this kind of lock much cheaper in gas than older designs.
- Aave Labs describes the change as minor and preventive, protecting against "theoretical reentrancy exploits." The proposal does not point to any active exploit.
- The rollout targets active Aave V3 instances on Ethereum (Core and Lido), Plasma, Avalanche, BNB, Monad, Base, Arbitrum, Mantle, X Layer, Polygon, Linea, Optimism, Gnosis, MegaETH and Celo.
- The code will be audited by Certora and third parties before a formal Aave Improvement Proposal (AIP) is submitted.
The proposal is at the early ARFC stage. Next come community feedback, a Snapshot vote and, after audits, an on-chain governance vote. Until then, nothing has been deployed.
Why gas costs used to matter
Reentrancy guards are not new. But a traditional lock writes to permanent contract storage twice per call, which costs gas on every transaction. For a protocol as heavily used as Aave, that adds up. Transient storage removes most of that overhead, which is why the proposal says gas is "no longer the concern it once was." This is a good example of how Ethereum upgrades make stronger security cheaper for applications.
What this means for DeFi users
Reentrancy is mostly a developer problem, but users can still lower their exposure:
- Prefer protocols with a long track record, public audits and active bug bounty programs.
- Read upgrade proposals or summaries for protocols where you hold large positions, so you know what is changing and when.
- Be careful with new forks and copycat contracts, which often skip audits.
- Limit token approvals to what you need, and revoke old approvals you no longer use.
- Spread risk instead of keeping all funds in one contract.
Key takeaways
- Reentrancy happens when external code calls back into a contract before it finishes updating its records.
- The main defenses are updating state before external calls, reentrancy guards and thorough audits.
- Aave V3.8 is a proposed, preventive upgrade that adds a low-cost guard using transient storage. It is not a response to a known exploit and is not live yet.
Understanding how protocols protect themselves helps you judge where to trade, lend and swap. When you swap supported assets on AviorSwap, which lists 17+ networks, the same habits apply: check what you sign and keep security first. This article is for educational purposes only.