ACryptoS, a DeFi yield aggregator whose vaults held close to half a billion dollars in 2021, has shut down after an attacker and a crowd of copycats drained about $665,000 from 77 of its vaults on five networks on Sunday, Nov. 8. The team disabled deposits, left withdrawals open and said it no longer has the resources to keep the protocol running. Users with funds still in ACryptoS vaults are being told to take them out.
What ACryptoS said
“ACryptoS vaults were exploited. We’ve deployed an emergency fix: deposits are disabled and withdrawals remain open. We no longer have the resources to maintain ACryptoS, so it is now deprecated. Please withdraw ASAP,” the team said on Sunday.
About 90 minutes later it published a detailed post-mortem. It states plainly that no fix or reopening of deposits is planned. The project also listed five exploiter addresses for exchanges and bridges to flag, and offered the attackers a deal: return the funds, keep a 10% whitehat bounty, and face “no further action.”
A missing length check
The bug sat in ACryptoS’s concentrated-liquidity vaults, built on Uniswap V3, SushiSwap V3, PancakeSwap v3 and Biswap v3 pools. According to the post-mortem, the vaults’ depositMulti function never checked that the list of deposit amounts had one entry per token in the vault.
When an attacker passed a single amount into a two-token vault, the contract valued both the deposit and the vault’s existing holdings using only the first token. Everything the vault held in the second token was ignored. In a vault whose position sat mostly in that second token, a tiny deposit could be worth several times the vault’s apparent value and mint most of its shares. A withdrawal straight afterwards paid those shares out across both tokens.
The team reproduced one of the trades on a Polygon fork. A 10 USDC deposit into a USDC/USDT0 vault minted 66.84% of the share supply. The round trip returned about 9.99 USDC plus 48,634 USDT0. Each round took a fixed fraction of whatever was left, which is why that vault lost 67%, 67% and then 90% of its remaining balance in successive hits.
ACryptoS said pool prices were not manipulated. It also said the flaw had been in these vaults since they were first deployed, and no recent upgrade introduced it.
Where the $665,000 came from
The ACryptoS exploit post-mortem breaks the losses down by chain, priced at about 17:00 UTC on Nov. 8:
- Arbitrum: 28 of 40 vaults, $230,213
- BNB Chain: 12 of 33 vaults, $190,845
- Polygon: 11 of 16 vaults, $134,686
- Base: 13 of 18 vaults, $69,813
- Optimism: 13 of 16 vaults, $39,871
- Linea: 0 of 3 vaults
That comes to about $665,429 from 77 of 126 vaults. The total leaves out roughly 19,400 ACS, the project’s own token, taken from a BNB Chain vault that the team did not price. The losses fell on ordinary depositors, whose shares were diluted by the over-minted ones.
Copycats and a slow alarm
The timeline is the most uncomfortable part of the report. The first exploit hit an Arbitrum WBTC/WETH vault at 02:02 UTC. From about 09:30 UTC, more than 100 copycat addresses, many of them EIP-7702 smart wallets, were running the same one-shot trade on every chain. In all, 135 addresses took part. ACryptoS counted 28 addresses that each hit at least three chains with identical rounds, taking $243,000 between them, which it said suggests a shared script.
The exploit was first reported to the project on Telegram at 09:34 UTC. The developer was notified at 14:19 UTC. Once that happened, the fix moved fast: one beacon upgrade per vault family closed the first chain, Polygon, in about 80 minutes and every chain within 2 hours and 21 minutes. The Polygon upgrade landed 12 seconds after the last exploit there. Deposits on all 126 vaults have been rejected since 16:40 UTC.
That left close to five hours between the first report and the developer being told. The post-mortem does not explain the gap.
Where the money is now
According to the post-mortem, most of the proceeds were swapped into ETH, WETH or WBNB or bridged to other chains, with Mayan, Across, Relay, Squid and LayerZero among the routes used. ACryptoS said freezable stablecoins left in exploiter wallets amount to under $1,000. Three wallets still held sizeable balances when the report was written: one with 50.82 WETH (about $113,000) on Ethereum, and two on BNB Chain with 116.5 WBNB (about $81,000) and 78.2 WBNB (about $54,000).
The project’s best hope is the bounty offer or a voluntary return. It has not reported any recovered funds.
A long fade before the end
ACryptoS was once a sizeable name. DefiLlama data shows its total value locked peaking at about $482 million in May 2021. By this month the tracker put it near $10.5 million, and that figure had not yet reflected Sunday’s losses or withdrawals when AltcoinReporter checked on Monday.
The case is a familiar one: an old bug in contracts that had been running for years, found and used at scale within hours once someone spotted it. Polkadot’s dotUSD incident, covered here last week, showed a related problem: a fix existed for four months before attackers used the bug.
FAQ
Can ACryptoS users still withdraw? Yes. ACryptoS said withdrawals remain open on all vaults while deposits are disabled, and it urged users to withdraw as soon as possible because the protocol is deprecated.
Were all ACryptoS vaults affected? The bug affected every vault in the concentrated-liquidity family. Attackers hit 77 of the 126 vaults across Arbitrum, BNB Chain, Polygon, Base and Optimism. None of the three Linea vaults was exploited.
Will ACryptoS reopen? No. The post-mortem says no fix or reopening of deposits is planned.
Disclaimer: This article is for informational purposes only and does not constitute financial advice.
