The decentralized finance ecosystem faced a brutal September 2024, with flash loan-fueled exploits draining more than $120 million across multiple protocols. As Bitcoin traded at $65,635 and Ethereum held at $2,659, attackers exploited reentrancy vulnerabilities, manipulated oracle prices, and leveraged permissionless market registrations to siphon funds from supposedly secure smart contracts.
The Exploit Mechanics
The most devastating attack of the month targeted Penpie, a yield farming protocol built on Pendle Finance. On September 3, 2024, an attacker deployed a malicious Synthetic Yield contract and registered it on Penpie’s permissionless market. By exploiting the _harvestBatchMarketRewards function — which lacked reentrancy protection — the attacker repeatedly called the reward distribution mechanism before the contract could update its internal state.
The attacker borrowed massive amounts of wstETH, sUSDe, egETH, and rswETH through flash loans, depositing them into the malicious SY contract during the reentrancy window. This artificially inflated token balances, generating exaggerated reward claims. Within just three transactions, approximately $27 million was drained from Penpie’s smart contract system.
This pattern — combining flash loans with reentrancy vulnerabilities — has become a signature attack vector in 2024. The speed and sophistication of these exploits highlight how a single missing guard can cascade into catastrophic losses.
Affected Systems
Beyond Penpie, September 2024 saw at least ten DeFi hacks cross the $1 million threshold. Protocols across Ethereum, BNB Chain, and smaller layer-2 networks reported losses from price oracle manipulation, governance attacks, and private key compromises. The total value lost to crypto hacks and scams throughout 2024 surpassed $3 billion, according to blockchain security firm PeckShield.
The systemic risk extends beyond individual protocols. Many DeFi platforms share composability layers — meaning a vulnerability in one protocol can create cascading effects across interconnected liquidity pools, lending markets, and yield aggregators. Pendle Finance itself had to pause operations temporarily following the Penpie exploit to assess potential contagion.
The Mitigation Strategy
Preventing flash loan attacks requires a multi-layered security approach. First, protocols must implement reentrancy guards on all functions that handle token transfers and state updates. OpenZeppelin’s ReentrancyGuard modifier provides a basic but essential defense. More advanced solutions include pull-over-push payment patterns and checks-effects-interactions ordering.
Second, permissionless market registration — the mechanism the Penpie attacker exploited to register a malicious contract — requires stricter validation. Protocols should implement whitelisting for new market registrations, requiring governance approval or security audits before new SY tokens or liquidity pools can interact with core contracts.
Third, flash loan resistance mechanisms such as time-weighted average prices (TWAP) from decentralized oracles can prevent the price manipulation that often accompanies these attacks. Using Chainlink or Pyth Network price feeds instead of spot prices from a single DEX reduces attack surface significantly.
Lessons Learned
The September 2024 exploits reinforce a fundamental truth in DeFi security: no amount of code is too small to audit. The Penpie vulnerability existed in a single function — a function that should have had a reentrancy guard but did not. Comprehensive third-party audits, formal verification of critical functions, and ongoing monitoring are not optional; they are the minimum standard for any protocol handling user funds.
Furthermore, the composability that makes DeFi powerful also multiplies risk. Protocols must conduct thorough dependency audits, understanding exactly how their integrations with other platforms could be weaponized by attackers.
User Action Required
For DeFi users, the September attacks serve as a stark reminder of the risks involved in yield farming and liquidity provision. Key actions include: diversifying across multiple protocols to limit single-platform exposure, monitoring protocol governance forums for security announcements, and maintaining awareness of which underlying protocols your investments depend on. With ETH at $2,659 and the total DeFi TVL exceeding $80 billion, the stakes have never been higher. Users should verify that protocols they invest in have undergone recent security audits from reputable firms and maintain active bug bounty programs.
Disclaimer: This article is for informational purposes only and does not constitute financial or investment advice. Always conduct your own research before engaging with DeFi protocols.
Penpie getting hit through a permissionless market registration is wild. anyone could deploy a malicious SY contract and nobody reviewed it
the _harvestBatchMarketRewards function with zero reentrancy protection. in 2024. on a protocol holding millions. I cant even
Carlos R. zero reentrancy guard in 2024 is embarrassing. OpenZeppelin has had the ReentrancyGuard mixin for years. no excuse
three transactions to drain the whole thing. flash loans need to be rate limited or protocol-gated somehow because this same pattern keeps repeating
Carlos R. zero reentrancy guard in 2024 on a protocol holding millions is embarrassing. OpenZeppelin ships ReentrancyGuard as a one line import
permissionless market registration without even basic code review for new SY contracts was the real failure. Penpie optimized for speed over security and paid the price
permissionless registration was the entire point of Pendle model though. adding a review queue kills composability. the fix is circuit breakers on abnormal reward claims not gating deployments
deploy gated circuit breakers on abnormal reward claims is the right fix. you dont need to kill permissionless registration just add rate limits on withdrawals. basic stuff
reentrancy_zero_ rate limits on withdrawals would have stopped the Penpie drain cold. the attacker needed three sequential transactions, any one of them blocked kills the whole attack
deploy_gated circuit breakers on abnormal reward claims would have stopped the Penpie drain. the attacker needed three sequential txs, any one blocked kills the whole thing
wstETH, sUSDe, egETH, rswETH all borrowed in flash loans to inflate the malicious contract balances. the attacker understood Pendle internals better than the Pendle team apparently
the attacker borrowed wstETH sUSDe egETH and rswETH in flash loans. knew exactly which Pendle market had the deepest liquidity. this was weeks of recon minimum
weeks of recon is right. the attacker deployed the SY contract on August 14, tested on testnet, then waited 20 days before executing on mainnet. patient and methodical
penpie used the exact same pattern as every reentrancy exploit before it. deposit during callback, inflate balance, drain rewards. when does the pattern click
$120M in one month and the next protocol that gets hit will have the exact same vulnerability. auditors are treated as rubber stamps not gatekeepers
Penpie losing $27M to a reentrancy bug in 2024 is wild. every Solidity tutorial covers checks-effects-interactions. how do yield protocols still ship without it
rekt_archivist_ fr. penpie even had a bug bounty program. whoever audited that _harvestBatchMarketRewards function needs to find a new career
borrowing wstETH and sUSDe through flash loans to inflate reward claims was actually clever exploit design. the attack wasnt dumb luck, someone read the code carefully
$120M across September 2024 and the pattern is always identical. flash loan in, inflate balances, exploit reentrancy, swap to stables, bridge out. auditors keep flagging this and protocols keep ignoring it
natasha d. the pattern is always identical but protocols keep shipping without reentrancy guards. openzeppelin has had this solved since 2020. copy paste the modifier and move on
Penpie losing $27M because _harvestBatchMarketRewards had no reentrancy guard. OpenZeppelin ships ReentrancyGuard for free and teams still skip it
borrowing wstETH sUSDe egETH and rswETH through flash loans to inflate reward claims was surgical. the attacker studied Pendle internals for weeks
$27M theoretical damage but only $132k actual gain thanks to that MEV bot. Shows how MEV protection can work.
Penpie losing $27M to the same reentrancy bug that haunted every protocol since the DAO hack in 2016 is genuinely embarrassing. checks-effects-interactions has been a thing for a decade
borrowing wstETH sUSDe egETH and rswETH in one tx is insane leverage. permissionless market registration without rate limits on deposit size is just asking for this