BounceBit, a Binance-backed crypto project, is shutting down its blockchain after an attacker exploited a flaw that appears to match a security bug publicly identified and fixed in the software BounceBit built its network on two years ago.
- An attacker moved over 280 million BB tokens from nine accounts on BounceBit Chain between Aug. 19 and Aug. 20 without possessing their private keys or forging their signatures, BounceBit said on Friday, Aug. 20, in a postmortem.
- The project halted the network 42 minutes after the last unauthorized transfer and has now decided not to restart it.
"We have decided to permanently sunset BounceBit Chain," the team said.
BounceBit will take a snapshot from immediately before the attack and reissue BB as a BEP-20 token on BNB Chain, a network backed by Binance. The 286.5 million BB moved by the attacker won't exist in the replacement token distribution.
- BounceBit was backed by Binance Labs, the crypto exchange's former venture arm now known as YZi Labs, in April 2024. The size of that investment wasn't disclosed.
- BounceBit had separately raised $6 million in a seed round led by Blockchain Capital and Breyer Capital earlier that year.
A familiar flaw
BounceBit attributed the attack to a protocol-level authorization vulnerability inherited from the Evmos stack on which its chain was built.
According to the project's explanation, a built-in module allowed contracts to fund vesting schedules on behalf of another account. The protocol was supposed to ensure that the account being debited had authorized the operation. It didn't.
BounceBit said the relevant check was performed against the wrong principal, effectively allowing an attacker to provide an arbitrary account as the source of funds. No signature from that account was required in the contract execution path.
- That description closely tracks CVE-2024-39696, a high-severity vulnerability disclosed by Evmos on July 5, 2024.
- The National Vulnerability Database describes the Evmos flaw as allowing a user to designate a third-party account as the funder of a vesting account while authorization was checked against the contract caller instead.
- Funds could therefore be taken from an arbitrary address without its permission.
The advisory went further, warning that the flaw could potentially be used to "drain all the accounts in the chain."
Evmos said the vulnerability affected versions before v19.0.0 and was fixed in that release. Evmos published v19.0.0 on July 24, 2024, listing "precompile security fixes" among its changes.
The similarities don't establish that BounceBit simply failed to install Evmos's patch as BounceBit didn't identify the precise Evmos version or commit its fork was running when it was attacked.
- The Coinformer couldn't independently establish whether the old vulnerable code remained in BounceBit, was modified after the fork, or whether an equivalent flaw was introduced separately.
- BounceBit itself hasn't identified the vulnerability as CVE-2024-39696 in its postmortem.
But the authorization failure it describes — checking permission against one caller while debiting a separately supplied funder account — is the same core behavior described in the 2024 Evmos advisory.
The chain will not come back
BounceBit said it considered repairing the network but decided against it.
The project noted that Evmos, the stack underlying BounceBit Chain, has since been discontinued. Moving the fork onto another codebase would require what BounceBit described as a substantial rebuild, audit and revalidation rather than a routine upgrade.
There was another reason to give up the chain.
BounceBit said most of its core products already operate on BNB Chain and that an increasing share of user activity has migrated there. Maintaining a separate Layer-1 therefore no longer justified the engineering and security resources required to run it.
The replacement BB balances will be calculated from BounceBit Chain at block 20,697,260, immediately before the first unauthorized transfer. Transactions made during the roughly five-and-a-half-hour period between that snapshot and the network halt will effectively be rolled back.